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2.2 979 This is the Nearest One Head P U Z Z L E R Before this vending machine will deliver its product, it conducts several tests on the coins being inserted How can it determine what material the coins are made of without damaging them and without making the customer wait a long time for the results? (George Semple) c h a p t e r Faraday’s Law Chapter Outline 31.1 31.2 31.3 31.4 Faraday’s Law of Induction Motional emf Lenz’s Law Induced emf and Electric Fields 31.5 (Optional) Generators and Motors 31.6 (Optional) Eddy Currents 31.7 Maxwell’s Wonderful Equations 979 980 CHAPTER 31 Faraday’s Law T he focus of our studies in electricity and magnetism so far has been the electric ﬁelds produced by stationary charges and the magnetic ﬁelds produced by moving charges This chapter deals with electric ﬁelds produced by changing magnetic ﬁelds Experiments conducted by Michael Faraday in England in 1831 and independently by Joseph Henry in the United States that same year showed that an emf can be induced in a circuit by a changing magnetic ﬁeld As we shall see, an emf (and therefore a current as well) can be induced in many ways — for instance, by moving a closed loop of wire into a region where a magnetic ﬁeld exists The results of these experiments led to a very basic and important law of electromagnetism known as Faraday’s law of induction This law states that the magnitude of the emf induced in a circuit equals the time rate of change of the magnetic ﬂux through the circuit With the treatment of Faraday’s law, we complete our introduction to the fundamental laws of electromagnetism These laws can be summarized in a set of four equations called Maxwell’s equations Together with the Lorentz force law, which we discuss brieﬂy, they represent a complete theory for describing the interaction of charged objects Maxwell’s equations relate electric and magnetic ﬁelds to each other and to their ultimate source, namely, electric charges 31.1 12.6 & 12.7 A demonstration of electromagnetic induction A changing potential difference is applied to the lower coil An emf is induced in the upper coil as indicated by the illuminated lamp What happens to the lamp’s intensity as the upper coil is moved over the vertical tube? (Courtesy of Central Scientiﬁc Company) FARADAY’S LAW OF INDUCTION To see how an emf can be induced by a changing magnetic ﬁeld, let us consider a loop of wire connected to a galvanometer, as illustrated in Figure 31.1 When a magnet is moved toward the loop, the galvanometer needle deﬂects in one direction, arbitrarily shown to the right in Figure 31.1a When the magnet is moved away from the loop, the needle deﬂects in the opposite direction, as shown in Figure 31.1c When the magnet is held stationary relative to the loop (Fig 31.1b), no deﬂection is observed Finally, if the magnet is held stationary and the loop is moved either toward or away from it, the needle deﬂects From these observations, we conclude that the loop “knows” that the magnet is moving relative to it because it experiences a change in magnetic ﬁeld Thus, it seems that a relationship exists between current and changing magnetic ﬁeld These results are quite remarkable in view of the fact that a current is set up even though no batteries are present in the circuit! We call such a current an induced current and say that it is produced by an induced emf Now let us describe an experiment conducted by Faraday and illustrated in Figure 31.2 A primary coil is connected to a switch and a battery The coil is wrapped around a ring, and a current in the coil produces a magnetic ﬁeld when the switch is closed A secondary coil also is wrapped around the ring and is connected to a galvanometer No battery is present in the secondary circuit, and the secondary coil is not connected to the primary coil Any current detected in the secondary circuit must be induced by some external agent Initially, you might guess that no current is ever detected in the secondary circuit However, something quite amazing happens when the switch in the primary A physicist named J D Colladon was the ﬁrst to perform the moving-magnet experiment To minimize the effect of the changing magnetic ﬁeld on his galvanometer, he placed the meter in an adjacent room Thus, as he moved the magnet in the loop, he could not see the meter needle deﬂecting By the time he returned next door to read the galvanometer, the needle was back to zero because he had stopped moving the magnet Unfortunately for Colladon, there must be relative motion between the loop and the magnet for an induced emf and a corresponding induced current to be observed Thus, physics students learn Faraday’s law of induction rather than “Colladon’s law of induction.” 981 31.1 Faraday’s Law of Induction N S N S N S Galvanometer (a) Galvanometer (b) Galvanometer (c) Figure 31.1 (a) When a magnet is moved toward a loop of wire connected to a galvanometer, the galvanometer deﬂects as shown, indicating that a current is induced in the loop (b) When the magnet is held stationary, there is no induced current in the loop, even when the magnet is inside the loop (c) When the magnet is moved away from the loop, the galvanometer deﬂects in the opposite direction, indicating that the induced current is opposite that shown in part (a) Changing the direction of the magnet’s motion changes the direction of the current induced by that motion circuit is either suddenly closed or suddenly opened At the instant the switch is closed, the galvanometer needle deﬂects in one direction and then returns to zero At the instant the switch is opened, the needle deﬂects in the opposite direction and again returns to zero Finally, the galvanometer reads zero when there is either a steady current or no current in the primary circuit The key to under- Michael Faraday Switch + – Galvanometer Battery Primary coil Secondary coil Figure 31.2 Faraday’s experiment When the switch in the primary circuit is closed, the galvanometer in the secondary circuit deﬂects momentarily The emf induced in the secondary circuit is caused by the changing magnetic ﬁeld through the secondary coil (1791 – 1867) Faraday, a British physicist and chemist, is often regarded as the greatest experimental scientist of the 1800s His many contributions to the study of electricity include the invention of the electric motor, electric generator, and transformer, as well as the discovery of electromagnetic induction and the laws of electrolysis Greatly inﬂuenced by religion, he refused to work on the development of poison gas for the British military (By kind permission of the President and Council of the Royal Society) 982 CHAPTER 31 Faraday’s Law standing what happens in this experiment is to ﬁrst note that when the switch is closed, the current in the primary circuit produces a magnetic ﬁeld in the region of the circuit, and it is this magnetic ﬁeld that penetrates the secondary circuit Furthermore, when the switch is closed, the magnetic ﬁeld produced by the current in the primary circuit changes from zero to some value over some ﬁnite time, and it is this changing ﬁeld that induces a current in the secondary circuit As a result of these observations, Faraday concluded that an electric current can be induced in a circuit (the secondary circuit in our setup) by a changing magnetic ﬁeld The induced current exists for only a short time while the magnetic ﬁeld through the secondary coil is changing Once the magnetic ﬁeld reaches a steady value, the current in the secondary coil disappears In effect, the secondary circuit behaves as though a source of emf were connected to it for a short time It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic ﬁeld The experiments shown in Figures 31.1 and 31.2 have one thing in common: In each case, an emf is induced in the circuit when the magnetic ﬂux through the circuit changes with time In general, the emf induced in a circuit is directly proportional to the time rate of change of the magnetic ﬂux through the circuit This statement, known as Faraday’s law of induction, can be written ϭ Ϫ ddt⌽B Faraday’s law (31.1) where ⌽B ϭ ͵B ؒ dA is the magnetic ﬂux through the circuit (see Section 30.5) If the circuit is a coil consisting of N loops all of the same area and if ⌽B is the ﬂux through one loop, an emf is induced in every loop; thus, the total induced emf in the coil is given by the expression ϭ ϪN d ⌽B dt (31.2) The negative sign in Equations 31.1 and 31.2 is of important physical signiﬁcance, which we shall discuss in Section 31.3 Suppose that a loop enclosing an area A lies in a uniform magnetic ﬁeld B, as shown in Figure 31.3 The magnetic ﬂux through the loop is equal to BA cos ; B A θ Figure 31.3 A conducting loop that encloses an area A in the presence of a uniform magnetic ﬁeld B The angle between B and the normal to the loop is 983 31.1 Faraday’s Law of Induction hence, the induced emf can be expressed as ϭ Ϫ dtd (BA cos ) QuickLab (31.3) From this expression, we see that an emf can be induced in the circuit in several ways: • • • • The magnitude of B can change with time The area enclosed by the loop can change with time The angle between B and the normal to the loop can change with time Any combination of the above can occur Quick Quiz 31.1 Equation 31.3 can be used to calculate the emf induced when the north pole of a magnet is moved toward a loop of wire, along the axis perpendicular to the plane of the loop passing through its center What changes are necessary in the equation when the south pole is moved toward the loop? A cassette tape is made up of tiny particles of metal oxide attached to a long plastic strip A current in a small conducting loop magnetizes the particles in a pattern related to the music being recorded During playback, the tape is moved past a second small loop (inside the playback head) and induces a current that is then ampliﬁed Pull a strip of tape out of a cassette (one that you don’t mind recording over) and see if it is attracted or repelled by a refrigerator magnet If you don’t have a cassette, try this with an old ﬂoppy disk you are ready to trash Some Applications of Faraday’s Law The ground fault interrupter (GFI) is an interesting safety device that protects users of electrical appliances against electric shock Its operation makes use of Faraday’s law In the GFI shown in Figure 31.4, wire leads from the wall outlet to the appliance to be protected, and wire leads from the appliance back to the wall outlet An iron ring surrounds the two wires, and a sensing coil is wrapped around part of the ring Because the currents in the wires are in opposite directions, the net magnetic ﬂux through the sensing coil due to the currents is zero However, if the return current in wire changes, the net magnetic ﬂux through the sensing coil is no longer zero (This can happen, for example, if the appliance gets wet, enabling current to leak to ground.) Because household current is alternating (meaning that its direction keeps reversing), the magnetic ﬂux through the sensing coil changes with time, inducing an emf in the coil This induced emf is used to trigger a circuit breaker, which stops the current before it is able to reach a harmful level Another interesting application of Faraday’s law is the production of sound in an electric guitar (Fig 31.5) The coil in this case, called the pickup coil, is placed near the vibrating guitar string, which is made of a metal that can be magnetized A permanent magnet inside the coil magnetizes the portion of the string nearest Alternating current Circuit breaker Sensing coil Iron ring Figure 31.4 Essential components of a ground fault interrupter This electric range cooks food on the basis of the principle of induction An oscillating current is passed through a coil placed below the cooking surface, which is made of a special glass The current produces an oscillating magnetic ﬁeld, which induces a current in the cooking utensil Because the cooking utensil has some electrical resistance, the electrical energy associated with the induced current is transformed to internal energy, causing the utensil and its contents to become hot (Courtesy of Corning, Inc.) 984 CHAPTER 31 Faraday’s Law Pickup coil Magnet N S N S Magnetized portion of string To amplifier Guitar string (a) (b) Figure 31.5 (a) In an electric guitar, a vibrating string induces an emf in a pickup coil (b) The circles beneath the metallic strings of this electric guitar detect the notes being played and send this information through an ampliﬁer and into speakers (A switch on the guitar allows the musician to select which set of six is used.) How does a guitar “pickup” sense what music is being played? (b, Charles D Winters) the coil When the string vibrates at some frequency, its magnetized segment produces a changing magnetic ﬂux through the coil The changing ﬂux induces an emf in the coil that is fed to an ampliﬁer The output of the ampliﬁer is sent to the loudspeakers, which produce the sound waves we hear EXAMPLE 31.1 One Way to Induce an emf in a Coil A coil consists of 200 turns of wire having a total resistance of 2.0 ⍀ Each turn is a square of side 18 cm, and a uniform magnetic ﬁeld directed perpendicular to the plane of the coil is turned on If the ﬁeld changes linearly from to 0.50 T in 0.80 s, what is the magnitude of the induced emf in the coil while the ﬁeld is changing? The area of one turn of the coil is (0.18 m)2 ϭ 0.032 m2 The magnetic ﬂux through the coil at t ϭ is zero because B ϭ at that time At t ϭ 0.80 s, the magnetic ﬂux through one turn is ⌽B ϭ BA ϭ (0.50 T)(0.032 m2 ) ϭ 0.016 T и m2 Therefore, the magnitude of the induced emf Solution EXAMPLE 31.2 is, from Equation 31.2, ͉ ͉ ϭ 200(0.016 Tиm2 Ϫ Tиm2) N ⌬⌽B ϭ ⌬t 0.80 s ϭ 4.1 Tиm2/s ϭ 4.1 V You should be able to show that T и m2/s ϭ V Exercise What is the magnitude of the induced current in the coil while the ﬁeld is changing? Answer 2.0 A An Exponentially Decaying B Field A loop of wire enclosing an area A is placed in a region where the magnetic ﬁeld is perpendicular to the plane of the loop The magnitude of B varies in time according to the expression B ϭ B maxeϪat, where a is some constant That is, at t ϭ the ﬁeld is B max , and for t Ͼ 0, the ﬁeld decreases exponen- tially (Fig 31.6) Find the induced emf in the loop as a function of time Solution Because B is perpendicular to the plane of the loop, the magnetic ﬂux through the loop at time t Ͼ is 985 31.2 Motional EMF ⌽B ϭ BA cos ϭ ABmaxeϪat B Because AB max and a are constants, the induced emf calculated from Equation 31.1 is Bmax B ϭ Ϫ d⌽ dt t ϭ ϪABmax d Ϫat e ϭ aABmaxeϪat dt This expression indicates that the induced emf decays exponentially in time Note that the maximum emf occurs at t ϭ 0, where max ϭ aABmax The plot of versus t is similar to the B-versus-t curve shown in Figure 31.6 Figure 31.6 Exponential decrease in the magnitude of the magnetic ﬁeld with time The induced emf and induced current vary with time in the same way CONCEPTUAL EXAMPLE 31.3 What Is Connected to What? Two bulbs are connected to opposite sides of a loop of wire, as shown in Figure 31.7 A decreasing magnetic ﬁeld (conﬁned to the circular area shown in the ﬁgure) induces an emf in the loop that causes the two bulbs to light What happens to the brightness of the bulbs when the switch is closed? Solution Bulb glows brighter, and bulb goes out Once the switch is closed, bulb is in the large loop consisting of the wire to which it is attached and the wire connected to the switch Because the changing magnetic ﬂux is completely enclosed within this loop, a current exists in bulb Bulb now glows brighter than before the switch was closed because it is now the only resistance in the loop As a result, the current in bulb is greater than when bulb was also in the loop Once the switch is closed, bulb is in the loop consisting of the wires attached to it and those connected to the switch There is no changing magnetic ﬂux through this loop and hence no induced emf Exercise What would happen if the switch were in a wire located to the left of bulb 1? Answer Bulb would go out, and bulb would glow brighter Bulb × × × × × × Bulb × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × Switch × × × × × × Figure 31.7 31.2 MOTIONAL EMF In Examples 31.1 and 31.2, we considered cases in which an emf is induced in a stationary circuit placed in a magnetic ﬁeld when the ﬁeld changes with time In this section we describe what is called motional emf, which is the emf induced in a conductor moving through a constant magnetic ﬁeld The straight conductor of length ᐉ shown in Figure 31.8 is moving through a uniform magnetic ﬁeld directed into the page For simplicity, we assume that the conductor is moving in a direction perpendicular to the ﬁeld with constant veloc- 986 CHAPTER 31 ᐉ Bin × × × × × × × × × × × × × × FB × + + – × × × − − v × × Figure 31.8 A straight electrical conductor of length ᐉ moving with a velocity v through a uniform magnetic ﬁeld B directed perpendicular to v A potential difference ⌬V ϭ Bᐉv is maintained between the ends of the conductor Faraday’s Law ity under the inﬂuence of some external agent The electrons in the conductor experience a force FB ϭ q v ؋ B that is directed along the length ᐉ, perpendicular to both v and B (Eq 29.1) Under the inﬂuence of this force, the electrons move to the lower end of the conductor and accumulate there, leaving a net positive charge at the upper end As a result of this charge separation, an electric ﬁeld is produced inside the conductor The charges accumulate at both ends until the downward magnetic force qvB is balanced by the upward electric force q E At this point, electrons stop moving The condition for equilibrium requires that q E ϭ q vB E ϭ vB or The electric ﬁeld produced in the conductor (once the electrons stop moving and E is constant) is related to the potential difference across the ends of the conductor according to the relationship ⌬V ϭ Eᐉ (Eq 25.6) Thus, ⌬V ϭ Eᐉ ϭ Bᐉv (31.4) where the upper end is at a higher electric potential than the lower end Thus, a potential difference is maintained between the ends of the conductor as long as the conductor continues to move through the uniform magnetic ﬁeld If the direction of the motion is reversed, the polarity of the potential difference also is reversed A more interesting situation occurs when the moving conductor is part of a closed conducting path This situation is particularly useful for illustrating how a changing magnetic ﬂux causes an induced current in a closed circuit Consider a circuit consisting of a conducting bar of length ᐉ sliding along two ﬁxed parallel conducting rails, as shown in Figure 31.9a For simplicity, we assume that the bar has zero resistance and that the stationary part of the circuit has a resistance R A uniform and constant magnetic ﬁeld B is applied perpendicular to the plane of the circuit As the bar is pulled to the right with a velocity v, under the inﬂuence of an applied force Fapp , free charges in the bar experience a magnetic force directed along the length of the bar This force sets up an induced current because the charges are free to move in the closed conducting path In this case, the rate of change of magnetic ﬂux through the loop and the corresponding induced motional emf across the moving bar are proportional to the change in area of the loop As we shall see, if the bar is pulled to the right with a constant velocity, the work done by the applied force appears as internal energy in the resistor R (see Section 27.6) Because the area enclosed by the circuit at any instant is ᐉx, where x is the width of the circuit at any instant, the magnetic ﬂux through that area is ⌽B ϭ Bᐉx Using Faraday’s law, and noting that x changes with time at a rate dx/dt ϭ v, we ﬁnd that the induced motional emf is ϭ Ϫ ddt⌽B Motional emf ϭϪ d dx (Bᐉx) ϭ ϪBᐉ dt dt ϭ ϪBᐉv (31.5) Because the resistance of the circuit is R, the magnitude of the induced current is Iϭ ͉ ͉ R ϭ Bᐉv R The equivalent circuit diagram for this example is shown in Figure 31.9b (31.6) 987 31.2 Motional EMF Let us examine the system using energy considerations Because no battery is in the circuit, we might wonder about the origin of the induced current and the electrical energy in the system We can understand the source of this current and energy by noting that the applied force does work on the conducting bar, thereby moving charges through a magnetic ﬁeld Their movement through the ﬁeld causes the charges to move along the bar with some average drift velocity, and hence a current is established Because energy must be conserved, the work done by the applied force on the bar during some time interval must equal the electrical energy supplied by the induced emf during that same interval Furthermore, if the bar moves with constant speed, the work done on it must equal the energy delivered to the resistor during this time interval As it moves through the uniform magnetic ﬁeld B, the bar experiences a magnetic force FB of magnitude I ᐉB (see Section 29.2) The direction of this force is opposite the motion of the bar, to the left in Figure 31.9a Because the bar moves with constant velocity, the applied force must be equal in magnitude and opposite in direction to the magnetic force, or to the right in Figure 31.9a (If FB acted in the direction of motion, it would cause the bar to accelerate Such a situation would violate the principle of conservation of energy.) Using Equation 31.6 and the fact that F app ϭ IᐉB, we ﬁnd that the power delivered by the applied force is B ᐉ 2v ᏼ ϭ F appv ϭ (IᐉB)v ϭ ϭ R R × × × × × × × × × × × × × × × × × FB v I Fapp × x (a) I R ε= B ᐉv (31.7) (b) Figure 31.9 (a) A conducting bar sliding with a velocity v along two conducting rails under the action of an applied force Fapp The magnetic force FB opposes the motion, and a counterclockwise current I is induced in the loop (b) The equivalent circuit diagram for the setup shown in part (a) Quick Quiz 31.2 As an airplane ﬂies from Los Angeles to Seattle, it passes through the Earth’s magnetic ﬁeld As a result, a motional emf is developed between the wingtips Which wingtip is positively charged? Motional emf Induced in a Rotating Bar A conducting bar of length ᐉ rotates with a constant angular speed about a pivot at one end A uniform magnetic ﬁeld B is directed perpendicular to the plane of rotation, as shown in Figure 31.10 Find the motional emf induced between the ends of the bar × Consider a segment of the bar of length dr having a velocity v According to Equation 31.5, the magnitude of the emf induced in this segment is d ϭ Bv dr Because every segment of the bar is moving perpendicular to B, an emf d of the same form is generated across each Summing the emfs induced across all segments, which are in series, gives the total emf between the ends of × Bin × × × × × v Solution × × From Equation 27.23, we see that this power is equal to the rate at which energy is delivered to the resistor I 2R, as we would expect It is also equal to the power I supplied by the motional emf This example is a clear demonstration of the conversion of mechanical energy ﬁrst to electrical energy and ﬁnally to internal energy in the resistor EXAMPLE 31.4 × R ᐉ Bin × Figure 31.10 × × × × × × × × × dr ᐉ × × × × × r × × × × × × × × × × × × × × × × × × O A conducting bar rotating around a pivot at one end in a uniform magnetic ﬁeld that is perpendicular to the plane of rotation A motional emf is induced across the ends of the bar 988 CHAPTER 31 the bar: ϭ Faraday’s Law through the relationship v ϭ r Therefore, because B and are constants, we ﬁnd that ͵ Bv dr ϭB To integrate this expression, we must note that the linear speed of an element is related to the angular speed EXAMPLE 31.5 Solution The induced current is counterclockwise, and the magnetic force is FB ϭ ϪIᐉB, where the negative sign denotes that the force is to the left and retards the motion This is the only horizontal force acting on the bar, and hence Newton’s second law applied to motion in the horizontal direction gives dv F x ϭ ma ϭ m ϭ ϪIᐉB dt From Equation 31.6, we know that I ϭ Bᐉv/R , and so we can write this expression as dv ϪB ᐉ ϭ v mR ln v vi 2 B ᐉ r dr ϭ This expression indicates that the velocity of the bar decreases exponentially with time under the action of the magnetic retarding force Exercise Find expressions for the induced current and the magnitude of the induced emf as functions of time for the bar in this example Bᐉvi Ϫt / e ; ϭ Bᐉvi eϪt / (They both deR crease exponentially with time.) Answer Iϭ Integrating this equation using the initial condition that v ϭ v i at t ϭ 0, we ﬁnd that vi ᐉ v ϭ vieϪt / dv B ᐉ2 ϭϪ v dt R v ͵ that the velocity can be expressed in the exponential form dv B ᐉ2 ϭϪ dt v mR ͵ v dr ϭ B Magnetic Force Acting on a Sliding Bar The conducting bar illustrated in Figure 31.11, of mass m and length ᐉ, moves on two frictionless parallel rails in the presence of a uniform magnetic ﬁeld directed into the page The bar is given an initial velocity vi to the right and is released at t ϭ Find the velocity of the bar as a function of time m ͵ ϭϪ ͵ t dt B ᐉ2 t tϭϪ mR 31.3 × B × in × × × × × × × × × × × × × × × × × × × R× × × × × × × × × × × × × × × × × × × × FB vi I where the constant ϭ mR/B ᐉ From this result, we see 12.7 ᐉ × Figure 31.11 A conducting bar of length ᐉ sliding on two ﬁxed conducting rails is given an initial velocity vi to the right LENZ’S LAW Faraday’s law (Eq 31.1) indicates that the induced emf and the change in ﬂux have opposite algebraic signs This has a very real physical interpretation that has come to be known as Lenz’s law2: Developed by the German physicist Heinrich Lenz (1804 – 1865) 1028 CHAPTER 32 Inductance stored in the inductor, which requires the presence of moving charges In Figure 32.15a, all of the energy is stored as electric potential energy in the capacitor at t ϭ In Figure 32.15b, which is one fourth of a period later, all of the energy is stored as magnetic energy 21 LI 2max in the inductor, where I max is the maximum current in the circuit In Figure 32.15c, the energy in the LC circuit is stored completely in the capacitor, with the polarity of the plates now opposite what it was in Figure 32.15a In parts d and e the system returns to the initial conﬁguration over the second half of the cycle At times other than those shown in the ﬁgure, part of the energy is stored in the electric ﬁeld of the capacitor and part is stored in the magnetic ﬁeld of the inductor In the analogous mechanical oscillation, part of the energy is potential energy in the spring and part is kinetic energy of the block Let us consider some arbitrary time t after the switch is closed, so that the capacitor has a charge Q Ͻ Q max and the current is I Ͻ I max At this time, both elements store energy, but the sum of the two energies must equal the total initial energy U stored in the fully charged capacitor at t ϭ 0: U ϭ UC ϩ UL ϭ Total energy stored in an LC circuit Q2 ϩ LI 2C (32.18) Because we have assumed the circuit resistance to be zero, no energy is transformed to internal energy, and hence the total energy must remain constant in time This means that dU/dt ϭ Therefore, by differentiating Equation 32.18 with respect to time while noting that Q and I vary with time, we obtain The total energy in an ideal LC circuit remains constant; dU/dt ϭ dU d ϭ dt dt Q2 Q dQ dI ϩ LI ϭ ϩ LI ϭ0 2C C dt dt (32.19) We can reduce this to a differential equation in one variable by remembering that the current in the circuit is equal to the rate at which the charge on the capacitor changes: I ϭ dQ /dt From this, it follows that dI/dt ϭ d 2Q /dt Substitution of these relationships into Equation 32.19 gives Q d 2Q ϩL ϭ0 C dt d 2Q ϭϪ Q dt LC (32.20) We can solve for Q by noting that this expression is of the same form as the analogous Equations 13.16 and 13.17 for a block – spring system: d 2x k ϭϪ x ϭ Ϫ 2x dt m where k is the spring constant, m is the mass of the block, and ϭ !k/m The solution of this equation has the general form x ϭ A cos(t ϩ ) where is the angular frequency of the simple harmonic motion, A is the amplitude of motion (the maximum value of x), and is the phase constant; the values of A and depend on the initial conditions Because it is of the same form as the differential equation of the simple harmonic oscillator, we see that Equation 32.20 has the solution Charge versus time for an ideal LC circuit Q ϭ Q max cos(t ϩ ) (32.21) 1029 32.5 Oscillations in an LC Circuit where Q max is the maximum charge of the capacitor and the angular frequency is ϭ !LC (32.22) Angular frequency of oscillation Note that the angular frequency of the oscillations depends solely on the inductance and capacitance of the circuit This is the natural frequency of oscillation of the LC circuit Because Q varies sinusoidally, the current in the circuit also varies sinusoidally We can easily show this by differentiating Equation 32.21 with respect to time: Iϭ dQ ϭ Ϫ Q max sin (t ϩ ) dt (32.23) To determine the value of the phase angle , we examine the initial conditions, which in our situation require that at t ϭ 0, I ϭ and Q ϭ Q max Setting I ϭ at t ϭ in Equation 32.23, we have Current versus time for an ideal LC current Q Q max ϭ Ϫ Q max sin t which shows that ϭ This value for also is consistent with Equation 32.21 and with the condition that Q ϭ Q max at t ϭ Therefore, in our case, the expressions for Q and I are Q ϭ Q max cos t I ϭ Ϫ Q max sin t ϭ ϪI max sin t I I max (32.24) t (32.25) Graphs of Q versus t and I versus t are shown in Figure 32.16 Note that the charge on the capacitor oscillates between the extreme values Q max and ϪQ max , and that the current oscillates between I max and ϪI max Furthermore, the current is 90° out of phase with the charge That is, when the charge is a maximum, the current is zero, and when the charge is zero, the current has its maximum value T T 3T 2T Figure 32.16 Graphs of charge versus time and current versus time for a resistanceless, nonradiating LC circuit Note that Q and I are 90° out of phase with each other Quick Quiz 32.5 What is the relationship between the amplitudes of the two curves in Figure 32.16? UC Q 2max 2C t Let us return to the energy discussion of the LC circuit Substituting Equations 32.24 and 32.25 in Equation 32.18, we ﬁnd that the total energy is U ϭ UC ϩ UL ϭ Q 2max LI 2max cos2 t ϩ sin2 t 2C (32.26) This expression contains all of the features described qualitatively at the beginning of this section It shows that the energy of the LC circuit continuously oscillates between energy stored in the electric ﬁeld of the capacitor and energy stored in the magnetic ﬁeld of the inductor When the energy stored in the capacitor has its maximum value Q 2max /2C , the energy stored in the inductor is zero When the energy stored in the inductor has its maximum value 21 LI 2max , the energy stored in the capacitor is zero Plots of the time variations of UC and UL are shown in Figure 32.17 The sum U C ϩ U L is a constant and equal to the total energy Q 2max /2C or LI 2max /2 Analytical veriﬁcation of this is straightforward The amplitudes of the two graphs in Figure 32.17 must be equal because the maximum energy stored in the capacitor UL L I 2max t T Figure 32.17 T 3T T Plots of UC versus t and UL versus t for a resistanceless, nonradiating LC circuit The sum of the two curves is a constant and equal to the total energy stored in the circuit 1030 CHAPTER 32 Inductance (when I ϭ 0) must equal the maximum energy stored in the inductor (when Q ϭ 0) This is mathematically expressed as Q 2max LI 2max ϭ 2C Using this expression in Equation 32.26 for the total energy gives Uϭ Q 2max Q2 (cos2 t ϩ sin2 t) ϭ max 2C 2C (32.27) because cos2 t ϩ sin2 t ϭ In our idealized situation, the oscillations in the circuit persist indeﬁnitely; however, we remember that the total energy U of the circuit remains constant only if energy transfers and transformations are neglected In actual circuits, there is always some resistance, and hence energy is transformed to internal energy We mentioned at the beginning of this section that we are also ignoring radiation from the circuit In reality, radiation is inevitable in this type of circuit, and the total energy in the circuit continuously decreases as a result of this process EXAMPLE 32.7 An Oscillatory LC Circuit In Figure 32.18, the capacitor is initially charged when switch S1 is open and S2 is closed Switch S1 is then thrown closed at the same instant that S2 is opened, so that the capacitor is connected directly across the inductor (a) Find the frequency of oscillation of the circuit Solution The initial charge on the capacitor equals the maximum charge, and because C ϭ Q / , we have Solution Q max ϭ C fϭ 2 Using Equation 32.22 gives for the frequency ϭ ϭ (b) What are the maximum values of charge on the capacitor and current in the circuit? 1.08 ϫ 10 Ϫ10 C From Equation 32.25, we can see how the maximum current is related to the maximum charge: 2 !LC 2[(2.81 ϫ 10 Ϫ3 H)(9.00 ϫ 10 Ϫ12 F)]1/2 ϭ 1.00 ϫ ϭ (9.00 ϫ 10 Ϫ12 F )(12.0 V ) ϭ 10 I max ϭ Q max ϭ 2 f Q max ϭ (2 ϫ 10 sϪ1 )(1.08 ϫ 10 Ϫ10 C) Hz ϭ 6.79 ϫ 10 Ϫ4 A (c) Determine the charge and current as functions of time ε = 12.0 V Solution Equations 32.24 and 32.25 give the following expressions for the time variation of Q and I : S2 Q ϭ Q max cos t 9.00 pF ϭ (1.08 ϫ 10 Ϫ10 C) cos[(2 ϫ 10 rad/s)t] I ϭ ϪI max sin t S1 ϭ (Ϫ6.79 ϫ 10 Ϫ4 A) sin[(2 ϫ 10 rad/s)t ] 2.81 mH Figure 32.18 First the capacitor is fully charged with the switch S1 open and S closed Then, S1 is thrown closed at the same time that S is thrown open Exercise Answer What is the total energy stored in the circuit? 6.48 ϫ 10Ϫ10 J 1031 32.6 The RLC Circuit Optional Section 32.6 13.7 THE RLC CIRCUIT We now turn our attention to a more realistic circuit consisting of an inductor, a capacitor, and a resistor connected in series, as shown in Figure 32.19 We let the resistance of the resistor represent all of the resistance in the circuit We assume that the capacitor has an initial charge Q max before the switch is closed Once the switch is thrown closed and a current is established, the total energy stored in the capacitor and inductor at any time is given, as before, by Equation 32.18 However, the total energy is no longer constant, as it was in the LC circuit, because the resistor causes transformation to internal energy Because the rate of energy transformation to internal energy within a resistor is I 2R, we have dU ϭ ϪI 2R dt R + C – Q max L S Figure 32.19 A series RLC circuit The capacitor has a charge Q max at t ϭ 0, the instant at which the switch is thrown closed where the negative sign signiﬁes that the energy U of the circuit is decreasing in time Substituting this result into Equation 32.19 gives LI dI Q dQ ϩ ϭ ϪI 2R dt C dt (32.28) To convert this equation into a form that allows us to compare the electrical oscillations with their mechanical analog, we ﬁrst use the fact that I ϭ dQ /dt and move all terms to the left-hand side to obtain LI d 2Q Q ϩ I ϩ I 2R ϭ dt C Now we divide through by I : L L d 2Q Q ϩ ϩ IR ϭ dt C d 2Q dQ Q ϩR ϩ ϭ0 dt dt C (32.29) The RLC circuit is analogous to the damped harmonic oscillator discussed in Section 13.6 and illustrated in Figure 32.20 The equation of motion for this mechanical system is, from Equation 13.32, m d 2x dx ϩb ϩ kx ϭ dt dt (32.30) Comparing Equations 32.29 and 32.30, we see that Q corresponds to the position x of the block at any instant, L to the mass m of the block, R to the damping coefﬁcient b, and C to 1/k, where k is the force constant of the spring These and other relationships are listed in Table 32.1 Because the analytical solution of Equation 32.29 is cumbersome, we give only a qualitative description of the circuit behavior In the simplest case, when R ϭ 0, Equation 32.29 reduces to that of a simple LC circuit, as expected, and the charge and the current oscillate sinusoidally in time This is equivalent to removal of all damping in the mechanical oscillator When R is small, a situation analogous to light damping in the mechanical oscillator, the solution of Equation 32.29 is Q ϭ Q maxe ϪRt /2L cos d t (32.31) m Figure 32.20 A block – spring system moving in a viscous medium with damped harmonic motion is analogous to an RLC circuit 1032 CHAPTER 32 Inductance TABLE 32.1 Analogies Between Electrical and Mechanical Systems One-Dimensional Mechanical System Electric Circuit Q4x I vx ⌬V F x R4b Charge Current Potential difference Resistance Capacitance C 1/k Inductance L4m Current ϭ time derivative of charge Iϭ Velocity ϭ time derivative of position Acceleration ϭ second time derivative of position Kinetic energy of moving mass Potential energy stored in a spring Rate of energy loss due to friction dQ dx vx ϭ dt dt Rate of change of current ϭ second time derivative of charge Energy in inductor dv x dI d 2Q d 2x ax ϭ ϭ ϭ dt dt dt dt Energy in capacitor U C ϭ 12 U L ϭ 12 LI K ϭ 12 mv Rate of energy loss due to resistance RLC circuit Displacement Velocity Force Viscous damping coefﬁcient (k ϭ spring constant) Mass L Q2 U ϭ 12 kx C I 2R bv d 2Q dQ Q d 2x dx ϩR ϩ ϭ04m ϩb ϩ kx ϭ dt dt C dt dt where d ϭ ΄ LC1 Ϫ 2LR ΅ Damped mass on a spring 1/2 (32.32) is the angular frequency at which the circuit oscillates That is, the value of the charge on the capacitor undergoes a damped harmonic oscillation in analogy with a mass – spring system moving in a viscous medium From Equation 32.32, we see that, when R V !4L/C (so that the second term in the brackets is much smaller than the ﬁrst), the frequency d of the damped oscillator is close to that of the undamped oscillator, 1/!LC Because I ϭ dQ /dt, it follows that the current also undergoes damped harmonic oscillation A plot of the charge versus time for the damped oscillator is shown in Figure 32.21a Note that the maximum value of Q decreases after each oscillation, just as the amplitude of a damped block – spring system decreases in time Quick Quiz 32.6 Figure 32.21a has two dashed blue lines that form an “envelope” around the curve What is the equation for the upper dashed line? When we consider larger values of R , we ﬁnd that the oscillations damp out more rapidly; in fact, there exists a critical resistance value R c ϭ !4L/C above which no oscillations occur A system with R ϭ R c is said to be critically damped When R exceeds R c , the system is said to be overdamped (Fig 32.22) 1033 Summary Q Q Q max Q max R > 4L/C t t (a) Figure 32.22 (b) Plot of Q versus t for an overdamped RLC circuit, which occurs for values of R Ͼ !4L/C Figure 32.21 (a) Charge versus time for a damped RLC circuit The charge decays in this way when R V ! 4L/C The Q -versus-t curve represents a plot of Equation 32.31 (b) Oscilloscope pattern showing the decay in the oscillations of an RLC circuit The parameters used were R ϭ 75 ⍀, L ϭ 10 mH, and C ϭ 0.19 F SUMMARY When the current in a coil changes with time, an emf is induced in the coil according to Faraday’s law The self-induced emf is L ϭ ϪL dI dt (32.1) where L is the inductance of the coil Inductance is a measure of how much opposition an electrical device offers to a change in current passing through the device Inductance has the SI unit of henry (H), where H ϭ V и s/A The inductance of any coil is Lϭ N ⌽B I (32.2) where ⌽B is the magnetic ﬂux through the coil and N is the total number of turns The inductance of a device depends on its geometry For example, the inductance of an air-core solenoid is Lϭ N 2A ᐉ (32.4) where A is the cross-sectional area, and ᐉ is the length of the solenoid If a resistor and inductor are connected in series to a battery of emf , and if a switch in the circuit is thrown closed at t ϭ 0, then the current in the circuit varies in time according to the expression Iϭ R (1 Ϫ e Ϫt / ) (32.7) where ϭ L/R is the time constant of the RL circuit That is, the current increases to an equilibrium value of /R after a time that is long compared with If the battery in the circuit is replaced by a resistanceless wire, the current decays exponentially with time according to the expression Iϭ where R e Ϫt / /R is the initial current in the circuit (32.10) 1034 CHAPTER 32 Inductance The energy stored in the magnetic ﬁeld of an inductor carrying a current I is U ϭ 12 LI (32.12) This energy is the magnetic counterpart to the energy stored in the electric ﬁeld of a charged capacitor The energy density at a point where the magnetic ﬁeld is B is uB ϭ B2 20 (32.14) The mutual inductance of a system of two coils is given by N 2⌽12 N ⌽ ϭ M 21 ϭ 21 ϭ M I1 I2 M 12 ϭ (32.15) This mutual inductance allows us to relate the induced emf in a coil to the changing source current in a nearby coil using the relationships 2 ϭ ϪM 12 dI dt and 1 ϭ ϪM 21 dI dt (32.16, 32.17) In an LC circuit that has zero resistance and does not radiate electromagnetically (an idealization), the values of the charge on the capacitor and the current in the circuit vary in time according to the expressions Iϭ Q ϭ Q max cos (t ϩ ) (32.21) dQ ϭ Ϫ Q max sin(t ϩ ) dt (32.23) where Q max is the maximum charge on the capacitor, is a phase constant, and is the angular frequency of oscillation: ϭ !LC (32.22) The energy in an LC circuit continuously transfers between energy stored in the capacitor and energy stored in the inductor The total energy of the LC circuit at any time t is Q2 LI 2max (32.26) U ϭ U C ϩ U L ϭ max cos2 t ϩ sin2 t 2C At t ϭ 0, all of the energy is stored in the electric ﬁeld of the capacitor (U ϭ Q 2max/2C ) Eventually, all of this energy is transferred to the inductor (U ϭ LI 2max /2) However, the total energy remains constant because energy transformations are neglected in the ideal LC circuit QUESTIONS Why is the induced emf that appears in an inductor called a “counter” or “back” emf? The current in a circuit containing a coil, resistor, and battery reaches a constant value Does the coil have an inductance? Does the coil affect the value of the current? What parameters affect the inductance of a coil? Does the inductance of a coil depend on the current in the coil? How can a long piece of wire be wound on a spool so that the wire has a negligible self-inductance? A long, ﬁne wire is wound as a solenoid with a selfinductance L If it is connected across the terminals of a battery, how does the maximum current depend on L ? For the series RL circuit shown in Figure Q32.6, can the back emf ever be greater than the battery emf? Explain Problems R ε L Switch Figure Q32.6 Consider this thesis: “Joseph Henry, America’s ﬁrst professional physicist, changed the view of the Universe during a school vacation at the Albany Academy in 1830 Before that time, one could think of the Universe as consisting of just one thing: matter In Henry’s experiment, after a battery is removed from a coil, the energy that keeps the current ﬂowing for a while does not belong to any piece of matter This energy belongs to the magnetic ﬁeld surrounding the coil With Henry’s discovery of self-induction, Nature forced us to admit that the Universe consists of ﬁelds as well as matter.” What in your view constitutes the Universe? Argue for your answer 1035 Discuss the similarities and differences between the energy stored in the electric ﬁeld of a charged capacitor and the energy stored in the magnetic ﬁeld of a currentcarrying coil What is the inductance of two inductors connected in series? Does it matter if they are solenoids or toroids? 10 The centers of two circular loops are separated by a ﬁxed distance For what relative orientation of the loops is their mutual inductance a maximum? a minimum? Explain 11 Two solenoids are connected in series so that each carries the same current at any instant Is mutual induction present? Explain 12 In the LC circuit shown in Figure 32.15, the charge on the capacitor is sometimes zero, even though current is in the circuit How is this possible? 13 If the resistance of the wires in an LC circuit were not zero, would the oscillations persist? Explain 14 How can you tell whether an RLC circuit is overdamped or underdamped? 15 What is the signiﬁcance of critical damping in an RLC circuit? 16 Can an object exert a force on itself? When a coil induces an emf in itself, does it exert a force on itself? PROBLEMS 1, 2, = straightforward, intermediate, challenging = full solution available in the Student Solutions Manual and Study Guide WEB = solution posted at http://www.saunderscollege.com/physics/ = Computer useful in solving problem = Interactive Physics = paired numerical/symbolic problems Section 32.1 Self-Inductance A coil has an inductance of 3.00 mH, and the current through it changes from 0.200 A to 1.50 A in a time of 0.200 s Find the magnitude of the average induced emf in the coil during this time A coiled telephone cord forms a spiral with 70 turns, a diameter of 1.30 cm, and an unstretched length of 60.0 cm Determine the self-inductance of one conductor in the unstretched cord A 2.00-H inductor carries a steady current of 0.500 A When the switch in the circuit is thrown open, the current is effectively zero in 10.0 ms What is the average induced emf in the inductor during this time? A small air-core solenoid has a length of 4.00 cm and a radius of 0.250 cm If the inductance is to be 0.060 mH, how many turns per centimeter are required? Calculate the magnetic ﬂux through the area enclosed by a 300-turn, 7.20-mH coil when the current in the coil is 10.0 mA The current in a solenoid is increasing at a rate of 10.0 A/s The cross-sectional area of the solenoid is cm2, and there are 300 turns on its 15.0-cm length What is the induced emf opposing the increasing current? WEB A 10.0-mH inductor carries a current I ϭ I max sin t, with I max ϭ 5.00 A and /2 ϭ 60.0 Hz What is the back emf as a function of time? An emf of 24.0 mV is induced in a 500-turn coil at an instant when the current is 4.00 A and is changing at the rate of 10.0 A/s What is the magnetic ﬂux through each turn of the coil? An inductor in the form of a solenoid contains 420 turns, is 16.0 cm in length, and has a cross-sectional area of 3.00 cm2 What uniform rate of decrease of current through the inductor induces an emf of 175 V? 10 An inductor in the form of a solenoid contains N turns, has length ᐉ, and has cross-sectional area A What uniform rate of decrease of current through the inductor induces an emf ? 11 The current in a 90.0-mH inductor changes with time as I ϭ t Ϫ 6.00t (in SI units) Find the magnitude of the induced emf at (a) t ϭ 1.00 s and (b) t ϭ 4.00 s (c) At what time is the emf zero? 12 A 40.0-mA current is carried by a uniformly wound aircore solenoid with 450 turns, a 15.0-mm diameter, and 12.0-cm length Compute (a) the magnetic ﬁeld inside the solenoid, (b) the magnetic ﬂux through each turn, 1036 CHAPTER 32 Inductance and (c) the inductance of the solenoid (d) Which of these quantities depends on the current? 13 A solenoid has 120 turns uniformly wrapped around a wooden core, which has a diameter of 10.0 mm and a length of 9.00 cm (a) Calculate the inductance of the solenoid (b) The wooden core is replaced with a soft iron rod that has the same dimensions but a magnetic permeability m ϭ 800 What is the new inductance? 14 A toroid has a major radius R and a minor radius r, and it is tightly wound with N turns of wire, as shown in Figure P32.14 If R W r, the magnetic ﬁeld within the region of the torus, of cross-sectional area A ϭ r 2, is essentially that of a long solenoid that has been bent into a large circle of radius R Using the uniform ﬁeld of a long solenoid, show that the self-inductance of such a toroid is approximately the inductive time constant of the circuit? (b) Calculate the current in the circuit 250 s after the switch is closed (c) What is the value of the ﬁnal steady-state current? (d) How long does it take the current to reach 80.0% of its maximum value? S ε R L Х 0N 2A/2R Figure P32.19 (An exact expression for the inductance of a toroid with a rectangular cross-section is derived in Problem 64.) R Area A Figure P32.14 15 An emf self-induced in a solenoid of inductance L changes in time as ϭ 0e Ϫkt Find the total charge that passes through the solenoid, if the charge is ﬁnite Section 32.2 RL Circuits 16 Calculate the resistance in an RL circuit in which L ϭ 2.50 H and the current increases to 90.0% of its ﬁnal value in 3.00 s 17 A 12.0-V battery is connected into a series circuit containing a 10.0-⍀ resistor and a 2.00-H inductor How long will it take the current to reach (a) 50.0% and (b) 90.0% of its ﬁnal value? 18 Show that I ϭ I 0e Ϫt / is a solution of the differential equation dI IR ϩ L ϭ0 dt where ϭ L /R and I is the current at t ϭ 19 Consider the circuit in Figure P32.19, taking ϭ 6.00 V, L ϭ 8.00 mH, and R ϭ 4.00 ⍀ (a) What is Problems 19, 20, 21, and 24 20 In the circuit shown in Figure P32.19, let L ϭ 7.00 H, R ϭ 9.00 ⍀, and ϭ 120 V What is the self-induced emf 0.200 s after the switch is closed? 21 For the RL circuit shown in Figure P32.19, let L ϭ 3.00 H, R ϭ 8.00 ⍀, and ϭ 36.0 V (a) Calculate the ratio of the potential difference across the resistor to that across the inductor when I ϭ 2.00 A (b) Calculate the voltage across the inductor when I ϭ 4.50 A 22 A 12.0-V battery is connected in series with a resistor and an inductor The circuit has a time constant of 500 s, and the maximum current is 200 mA What is the value of the inductance? 23 An inductor that has an inductance of 15.0 H and a resistance of 30.0 ⍀ is connected across a 100-V battery What is the rate of increase of the current (a) at t ϭ and (b) at t ϭ 1.50 s? 24 When the switch in Figure P32.19 is thrown closed, the current takes 3.00 ms to reach 98.0% of its ﬁnal value If R ϭ 10.0 ⍀, what is the inductance? 25 The switch in Figure P32.25 is closed at time t ϭ Find the current in the inductor and the current through the switch as functions of time thereafter WEB r L 4.00 Ω 8.00 Ω 4.00 Ω 10.0 V 1.00 H S Figure P32.25 26 A series RL circuit with L ϭ 3.00 H and a series RC circuit with C ϭ 3.00 F have equal time constants If the two circuits contain the same resistance R, (a) what is the value of R and (b) what is the time constant? 1037 Problems 27 A current pulse is fed to the partial circuit shown in Figure P32.27 The current begins at zero, then becomes 10.0 A between t ϭ and t ϭ 200 s, and then is zero once again Determine the current in the inductor as a function of time A B S ε I (t ) 10.0 A 200 µ s L R I (t ) 100 Ω Figure P32.29 10.0 mH single ideal inductor having 1/L eq ϭ 1/L ϩ 1/L (c) Now consider two inductors L and L that have nonzero internal resistances R and R , respectively Assume that they are still far apart so that their magnetic ﬁelds not inﬂuence each other If these inductors are connected in series, show that they are equivalent to a single inductor having L eq ϭ L ϩ L and R eq ϭ R ϩ R (d) If these same inductors are now connected in parallel, is it necessarily true that they are equivalent to a single ideal inductor having 1/L eq ϭ 1/L ϩ 1/L and 1/R eq ϭ 1/R ϩ 1/R ? Explain your answer Figure P32.27 28 One application of an RL circuit is the generation of time-varying high voltage from a low-voltage source, as shown in Figure P32.28 (a) What is the current in the circuit a long time after the switch has been in position A? (b) Now the switch is thrown quickly from A to B Compute the initial voltage across each resistor and the inductor (c) How much time elapses before the voltage across the inductor drops to 12.0 V? Section 32.3 Energy in a Magnetic Field A S B 12.0 V 200 Ω 2.00 H 12.0 Ω Figure P32.28 WEB 29 A 140-mH inductor and a 4.90-⍀ resistor are connected with a switch to a 6.00-V battery, as shown in Figure P32.29 (a) If the switch is thrown to the left (connecting the battery), how much time elapses before the current reaches 220 mA? (b) What is the current in the inductor 10.0 s after the switch is closed? (c) Now the switch is quickly thrown from A to B How much time elapses before the current falls to 160 mA? 30 Consider two ideal inductors, L and L , that have zero internal resistance and are far apart, so that their magnetic ﬁelds not inﬂuence each other (a) If these inductors are connected in series, show that they are equivalent to a single ideal inductor having L eq ϭ L ϩ L (b) If these same two inductors are connected in parallel, show that they are equivalent to a WEB 31 Calculate the energy associated with the magnetic ﬁeld of a 200-turn solenoid in which a current of 1.75 A produces a ﬂux of 3.70 ϫ 10Ϫ4 Tи m2 in each turn 32 The magnetic ﬁeld inside a superconducting solenoid is 4.50 T The solenoid has an inner diameter of 6.20 cm and a length of 26.0 cm Determine (a) the magnetic energy density in the ﬁeld and (b) the energy stored in the magnetic ﬁeld within the solenoid 33 An air-core solenoid with 68 turns is 8.00 cm long and has a diameter of 1.20 cm How much energy is stored in its magnetic ﬁeld when it carries a current of 0.770 A? 34 At t ϭ 0, an emf of 500 V is applied to a coil that has an inductance of 0.800 H and a resistance of 30.0 ⍀ (a) Find the energy stored in the magnetic ﬁeld when the current reaches half its maximum value (b) After the emf is connected, how long does it take the current to reach this value? 35 On a clear day there is a 100-V/m vertical electric ﬁeld near the Earth’s surface At the same place, the Earth’s magnetic ﬁeld has a magnitude of 0.500 ϫ 10Ϫ4 T Compute the energy densities of the two ﬁelds 36 An RL circuit in which L ϭ 4.00 H and R ϭ 5.00 ⍀ is connected to a 22.0-V battery at t ϭ (a) What energy is stored in the inductor when the current is 0.500 A? (b) At what rate is energy being stored in the inductor when I ϭ 1.00 A? (c) What power is being delivered to the circuit by the battery when I ϭ 0.500 A? 37 A 10.0-V battery, a 5.00-⍀ resistor, and a 10.0-H inductor are connected in series After the current in the circuit 1038 CHAPTER 32 Inductance has reached its maximum value, calculate (a) the power being supplied by the battery, (b) the power being delivered to the resistor, (c) the power being delivered to the inductor, and (d) the energy stored in the magnetic ﬁeld of the inductor 38 A uniform electric ﬁeld with a magnitude of 680 kV/m throughout a cylindrical volume results in a total energy of 3.40 J What magnetic ﬁeld over this same region stores the same total energy? 39 Assume that the magnitude of the magnetic ﬁeld outside a sphere of radius R is B ϭ B 0(R/r)2, where B0 is a constant Determine the total energy stored in the magnetic ﬁeld outside the sphere and evaluate your result for B ϭ 5.00 ϫ 10 Ϫ5 T and R ϭ 6.00 ϫ 10 m, values appropriate for the Earth’s magnetic ﬁeld h ϭ 0.400 mm, w ϭ 1.30 mm, and L ϭ 2.70 mm, what is their mutual inductance? 47 Two inductors having self-inductances L and L are connected in parallel, as shown in Figure P32.47a The mutual inductance between the two inductors is M Determine the equivalent self-inductance L eq for the system (Fig P32.47b) I (t ) I (t ) L1 M Leq (a) (b) Figure P32.47 Section 32.4 Mutual Inductance 40 Two coils are close to each other The ﬁrst coil carries a time-varying current given by I(t ) ϭ (5.00 A) e Ϫ0.025 0t sin(377t ) At t ϭ 0.800 s, the voltage measured across the second coil is Ϫ 3.20 V What is the mutual inductance of the coils? 41 Two coils, held in ﬁxed positions, have a mutual inductance of 100 H What is the peak voltage in one when a sinusoidal current given by I(t ) ϭ (10.0 A) sin(1 000t ) ﬂows in the other? 42 An emf of 96.0 mV is induced in the windings of a coil when the current in a nearby coil is increasing at the rate of 1.20 A/s What is the mutual inductance of the two coils? 43 Two solenoids A and B, spaced close to each other and sharing the same cylindrical axis, have 400 and 700 turns, respectively A current of 3.50 A in coil A produces an average ﬂux of 300 T и m2 through each turn of A and a ﬂux of 90.0 T и m2 through each turn of B (a) Calculate the mutual inductance of the two solenoids (b) What is the self-inductance of A? (c) What emf is induced in B when the current in A increases at the rate of 0.500 A/s? 44 A 70-turn solenoid is 5.00 cm long and 1.00 cm in diameter and carries a 2.00-A current A single loop of wire, 3.00 cm in diameter, is held so that the plane of the loop is perpendicular to the long axis of the solenoid, as illustrated in Figure P31.18 (page 1004) What is the mutual inductance of the two if the plane of the loop passes through the solenoid 2.50 cm from one end? 45 Two single-turn circular loops of wire have radii R and r, with R W r The loops lie in the same plane and are concentric (a) Show that the mutual inductance of the pair is M ϭ 0r 2/2R (Hint: Assume that the larger loop carries a current I and compute the resulting ﬂux through the smaller loop.) (b) Evaluate M for r ϭ 2.00 cm and R ϭ 20.0 cm 46 On a printed circuit board, a relatively long straight conductor and a conducting rectangular loop lie in the same plane, as shown in Figure P31.9 (page 1003) If L2 Section 32.5 Oscillations in an LC Circuit 48 A 1.00-F capacitor is charged by a 40.0-V power supply The fully-charged capacitor is then discharged through a 10.0-mH inductor Find the maximum current in the resulting oscillations 49 An LC circuit consists of a 20.0-mH inductor and a 0.500-F capacitor If the maximum instantaneous current is 0.100 A, what is the greatest potential difference across the capacitor? 50 In the circuit shown in Figure P32.50, ϭ 50.0 V, R ϭ 250 ⍀, and C ϭ 0.500 F The switch S is closed for a long time, and no voltage is measured across the capacitor After the switch is opened, the voltage across the capacitor reaches a maximum value of 150 V What is the inductance L ? R ε L C S Figure P32.50 51 A ﬁxed inductance L ϭ 1.05 H is used in series with a variable capacitor in the tuning section of a radio What capacitance tunes the circuit to the signal from a station broadcasting at 6.30 MHz? 52 Calculate the inductance of an LC circuit that oscillates at 120 Hz when the capacitance is 8.00 F 53 An LC circuit like the one shown in Figure 32.14 contains an 82.0-mH inductor and a 17.0-F capacitor that initially carries a 180-C charge The switch is thrown closed at t ϭ (a) Find the frequency (in hertz) of the resulting oscillations At t ϭ 1.00 ms, ﬁnd (b) the charge on the capacitor and (c) the current in the circuit 1039 Problems 54 The switch in Figure P32.54 is connected to point a for a long time After the switch is thrown to point b, what are (a) the frequency of oscillation of the LC circuit, (b) the maximum charge that appears on the capacitor, (c) the maximum current in the inductor, and (d) the total energy the circuit possesses at t ϭ 3.00 s? 10.0 Ω a b 0.100 H S 1.00 µ µF 12.0 V Figure P32.54 WEB 55 An LC circuit like that illustrated in Figure 32.14 consists of a 3.30-H inductor and an 840-pF capacitor, initially carrying a 105-C charge At t ϭ the switch is thrown closed Compute the following quantities at t ϭ 2.00 ms: (a) the energy stored in the capacitor; (b) the energy stored in the inductor; (c) the total energy in the circuit (Optional) Section 32.6 The RLC Circuit 56 In Figure 32.19, let R ϭ 7.60 ⍀, L ϭ 2.20 mH, and C ϭ 1.80 F (a) Calculate the frequency of the damped oscillation of the circuit (b) What is the critical resistance? 57 Consider an LC circuit in which L ϭ 500 mH and C ϭ 0.100 F (a) What is the resonant frequency ? (b) If a resistance of 1.00 k⍀ is introduced into this circuit, what is the frequency of the (damped) oscillations? (c) What is the percent difference between the two frequencies? 58 Show that Equation 32.29 in the text is Kirchhoff’s loop rule as applied to Figure 32.19 59 Electrical oscillations are initiated in a series circuit containing a capacitance C, inductance L, and resistance R (a) If R V !4L/C (weak damping), how much time elapses before the amplitude of the current oscillation falls off to 50.0% of its initial value? (b) How long does it take the energy to decrease to 50.0% of its initial value? ADDITIONAL PROBLEMS 60 Initially, the capacitor in a series LC circuit is charged A switch is closed, allowing the capacitor to discharge, and after time t the energy stored in the capacitor is onefourth its initial value Determine L if C is known 61 A 1.00-mH inductor and a 1.00-F capacitor are connected in series The current in the circuit is described by I ϭ 20.0t, where t is in seconds and I is in amperes The capacitor initially has no charge Determine (a) the voltage across the inductor as a function of time, (b) the voltage across the capacitor as a function of time, and (c) the time when the energy stored in the capacitor ﬁrst exceeds that in the inductor 62 An inductor having inductance L and a capacitor having capacitance C are connected in series The current in the circuit increases linearly in time as described by I ϭ Kt The capacitor is initially uncharged Determine (a) the voltage across the inductor as a function of time, (b) the voltage across the capacitor as a function of time, and (c) the time when the energy stored in the capacitor ﬁrst exceeds that in the inductor 63 A capacitor in a series LC circuit has an initial charge Q and is being discharged Find, in terms of L and C, the ﬂux through each of the N turns in the coil, when the charge on the capacitor is Q /2 64 The toroid in Figure P32.64 consists of N turns and has a rectangular cross-section Its inner and outer radii are a and b, respectively (a) Show that Lϭ 0N 2h b ln 2 a (b) Using this result, compute the self-inductance of a 500-turn toroid for which a ϭ 10.0 cm, b ϭ 12.0 cm, and h ϭ 1.00 cm (c) In Problem 14, an approximate formula for the inductance of a toroid with R W r was derived To get a feel for the accuracy of that result, use the expression in Problem 14 to compute the approximate inductance of the toroid described in part (b) Compare the result with the answer to part (b) h a b Figure P32.64 65 (a) A ﬂat circular coil does not really produce a uniform magnetic ﬁeld in the area it encloses, but estimate the self-inductance of a ﬂat circular coil, with radius R and N turns, by supposing that the ﬁeld at its center is uniform over its area (b) A circuit on a laboratory table consists of a 1.5-V battery, a 270-⍀ resistor, a switch, and three 30cm-long cords connecting them Suppose that the circuit is arranged to be circular Think of it as a ﬂat coil with one turn Compute the order of magnitude of its selfinductance and (c) of the time constant describing how fast the current increases when you close the switch 66 A soft iron rod ( m ϭ 800 ) is used as the core of a solenoid The rod has a diameter of 24.0 mm and is 1040 CHAPTER 32 Inductance 10.0 cm long A 10.0-m piece of 22-gauge copper wire (diameter ϭ 0.644 mm) is wrapped around the rod in a single uniform layer, except for a 10.0-cm length at each end, which is to be used for connections (a) How many turns of this wire can wrap around the rod? (Hint: The diameter of the wire adds to the diameter of the rod in determining the circumference of each turn Also, the wire spirals diagonally along the surface of the rod.) (b) What is the resistance of this inductor? (c) What is its inductance? 67 A wire of nonmagnetic material with radius R carries current uniformly distributed over its cross-section If the total current carried by the wire is I, show that the magnetic energy per unit length inside the wire is 0I 2/16 68 An 820-turn wire coil of resistance 24.0 ⍀ is placed around a 12 500-turn solenoid, 7.00 cm long, as shown in Figure P32.68 Both coil and solenoid have crosssectional areas of 1.00 ϫ 10Ϫ4 m2 (a) How long does it take the solenoid current to reach 63.2 percent of its maximum value? Determine (b) the average back emf caused by the self-inductance of the solenoid during this interval, (c) the average rate of change in magnetic ﬂux through the coil during this interval, and (d) the magnitude of the average induced current in the coil R1 S ε L R2 Figure P32.69 Problems 69 and 70 closed for a long time, the current in the inductor drops to 0.250 A in 0.150 s What is the inductance of the inductor? 71 In Figure P32.71, the switch is closed for t Ͻ 0, and steady-state conditions are established The switch is thrown open at t ϭ (a) Find the initial voltage across L just after t ϭ Which end of the coil is at the higher potential: a or b? (b) Make freehand graphs of the currents in R and in R as a function of time, treating the steady-state directions as positive Show values before and after t ϭ (c) How long after t ϭ does the current in R have the value 2.00 mA? 2.00 kΩ R1 14.0 Ω S 6.00 kΩ R2 12 500 turns 60.0 V S Figure P32.68 I(t ) ϭ R1 72 The switch in Figure P32.72 is thrown closed at t ϭ Before the switch is closed, the capacitor is uncharged, and all currents are zero Determine the currents in L, C, and R and the potential differences across L, C, and R (a) the instant after the switch is closed and (b) long after it is closed L R [1 Ϫ e Ϫ(RЈ/L)t ] where RЈ ϭ R 1R 2/(R ϩ R ) 70 In Figure P32.69, take ϭ 6.00 V, R ϭ 5.00 ⍀, and R ϭ 1.00 ⍀ The inductor has negligible resistance When the switch is thrown open after having been 0.400 H Figure P32.71 69 At t ϭ 0, the switch in Figure P32.69 is thrown closed Using Kirchhoff’s laws for the instantaneous currents and voltages in this two-loop circuit, show that the current in the inductor is L 18.0 V b 24.0 Ω 820 turns + – ε a C S Figure P32.72 ε0 1041 Problems 73 To prevent damage from arcing in an electric motor, a discharge resistor is sometimes placed in parallel with the armature If the motor is suddenly unplugged while running, this resistor limits the voltage that appears across the armature coils Consider a 12.0-V dc motor with an armature that has a resistance of 7.50 ⍀ and an inductance of 450 mH Assume that the back emf in the armature coils is 10.0 V when the motor is running at normal speed (The equivalent circuit for the armature is shown in Fig P32.73.) Calculate the maximum resistance R that limits the voltage across the armature to 80.0 V when the motor is unplugged Armature 7.50 Ω R 450 mH 12.0 V 10.0 V Figure P32.73 74 An air-core solenoid 0.500 m in length contains 000 turns and has a cross-sectional area of 1.00 cm2 (a) If end effects are neglected, what is the self-inductance? (b) A secondary winding wrapped around the center of the solenoid has 100 turns What is the mutual inductance? (c) The secondary winding carries a constant current of 1.00 A, and the solenoid is connected to a load of 1.00 k⍀ The constant current is suddenly stopped How much charge ﬂows through the load resistor? 75 The lead-in wires from a television antenna are often constructed in the form of two parallel wires (Fig P32.75) (a) Why does this conﬁguration of conductors have an inductance? (b) What constitutes the ﬂux loop for this conﬁguration? (c) Neglecting any magnetic ﬂux inside the wires, show that the inductance of a length x of this type of lead-in is Lϭ 0x wϪa ln a where a is the radius of the wires and w is their centerto-center separation Note: Problems 76 through 79 require the application of ideas from this chapter and earlier chapters to some properties of superconductors, which were introduced in Section 27.5 76 Review Problem The resistance of a superconductor In an experiment carried out by S C Collins between 1955 and 1958, a current was maintained in a superconducting lead ring for 2.50 yr with no observed loss If the inductance of the ring was 3.14 ϫ 10Ϫ8 H and the sensitivity of the experiment was part in 109, what was the maximum resistance of the ring? (Hint: Treat this as a decaying current in an RL circuit, and recall that e Ϫx Х Ϫ x for small x.) 77 Review Problem A novel method of storing electrical energy has been proposed A huge underground superconducting coil, 1.00 km in diameter, would be fabricated It would carry a maximum current of 50.0 kA through each winding of a 150-turn Nb3Sn solenoid (a) If the inductance of this huge coil were 50.0 H, what would be the total energy stored? (b) What would be the compressive force per meter length acting between two adjacent windings 0.250 m apart? 78 Review Problem Superconducting Power Transmission The use of superconductors has been proposed for the manufacture of power transmission lines A single coaxial cable (Fig P32.78) could carry 1.00 ϫ 103 MW (the output of a large power plant) at 200 kV, dc, over a distance of 000 km without loss An inner wire with a radius of 2.00 cm, made from the superconductor Nb3Sn, carries the current I in one direction A surrounding superconducting cylinder, of radius 5.00 cm, would carry the return current I In such a system, what is the magnetic ﬁeld (a) at the surface of the inner conductor and (b) at the inner surface of the outer conductor? (c) How much energy would be stored in the space between the conductors in a 000-km superconducting line? (d) What is the pressure exerted on the outer conductor? I a = 2.00 cm TV set I TV antenna I b = 5.00 cm a b I Figure P32.75 Figure P32.78 1042 CHAPTER 32 Inductance 79 Review Problem The Meissner Effect Compare this problem with Problem 63 in Chapter 26 on the force attracting a perfect dielectric into a strong electric ﬁeld A fundamental property of a Type I superconducting material is perfect diamagnetism, or demonstration of the Meissner effect, illustrated in the photograph on page 855 and again in Figure 30.34, and described as follows: The superconducting material has B ϭ everywhere inside it If a sample of the material is placed into an externally produced magnetic ﬁeld, or if it is cooled to become superconducting while it is in a magnetic ﬁeld, electric currents appear on the surface of the sample The currents have precisely the strength and orientation required to make the total magnetic ﬁeld zero throughout the interior of the sample The following problem will help you to understand the magnetic force that can then act on the superconducting sample Consider a vertical solenoid with a length of 120 cm and a diameter of 2.50 cm consisting of 400 turns of copper wire carrying a counterclockwise current of 2.00 A, as shown in Figure P32.79a (a) Find the magnetic ﬁeld in the vacuum inside the solenoid (b) Find the energy density of the magnetic ﬁeld, and note that the units J/m3 of energy density are the same as the units N/m2(ϭPa) of pressure (c) A superconducting bar 2.20 cm in diameter is inserted partway into the solenoid Its upper end is far outside the solenoid, where the magnetic ﬁeld is small The lower end of the bar is deep inside the solenoid Identify the direction required for the current on the curved surface of the bar so that the total magnetic ﬁeld is zero within the bar The ﬁeld created by the supercurrents is sketched in Figure P32.79b, and the total ﬁeld is sketched in Figure Btot B0 I (a) (b) (c) Figure P32.79 P32.79c (d) The ﬁeld of the solenoid exerts a force on the current in the superconductor Identify the direction of the force on the bar (e) Calculate the magnitude of the force by multiplying the energy density of the solenoid ﬁeld by the area of the bottom end of the superconducting bar ANSWERS TO QUICK QUIZZES 32.1 When it is being opened When the switch is initially open, there is no current in the circuit; when the switch is then closed, the inductor tends to maintain the nocurrent condition, and as a result there is very little chance of sparking When the switch is initially closed, there is current in the circuit; when the switch is then opened, the current decreases An induced emf is set up across the inductor, and this emf tends to maintain the original current Sparking can occur as the current bridges the air gap between the poles of the switch 32.2 (b) Figure 32.8 shows that circuit B has the greater time constant because in this circuit it takes longer for the current to reach its maximum value and then longer for this current to decrease to zero after switch S is closed Equation 32.8 indicates that, for equal resistances R A and R B , the condition B Ͼ A means that LA Ͻ LB 32.3 (a) M12 increases because the magnetic ﬂux through coil increases (b) M12 decreases because rotation of coil decreases its ﬂux through coil 32.4 (a) No Mutual inductance requires a system of coils, and each coil has self-inductance (b) Yes A single coil has self-inductance but no mutual inductance because it does not interact with any other coils 32.5 From Equation 32.25, I max ϭ Q max Thus, the amplitude of the I - t graph is times the amplitude of the Q - t graph 32.6 Equation 32.31 without the cosine factor The dashed lines represent the positive and negative amplitudes (maximum values) for each oscillation period, and it is the Q ϭ Q maxe ϪRt /2L part of Equation 32.31 that gives the value of the ever-decreasing amplitude ... be observed Thus, physics students learn Faraday’s law of induction rather than “Colladon’s law of induction. ” 981 31.1 Faraday’s Law of Induction N S N S N S Galvanometer (a) Galvanometer (b)... to their ultimate source, namely, electric charges 31.1 12.6 & 12.7 A demonstration of electromagnetic induction A changing potential difference is applied to the lower coil An emf is induced... of the electric motor, electric generator, and transformer, as well as the discovery of electromagnetic induction and the laws of electrolysis Greatly inﬂuenced by religion, he refused to work
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