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College Physics With an Integrated Approach to Forces and Kinematics

Alan Giambattista, Betty McCarthy Richardson , Robert C. Richardson

Chapter 20

Electromagnetic Induction - all with Video Answers

Educators


Chapter Questions

02:02

Problem 1

In Fig. 20.2, a metal rod of length $L$ moves to the right at speed $v$. (a) What is the current in the rod, in terms of $v, B, L$, and $R ?(\mathrm{~b})$ In what direction does the current flow? (c) What is the direction of the magnetic force on the rod? (d) What is the magnitude of the magnetic force on the rod (in terms of $v, B, L$, and $R)$ ?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:08

Problem 2

Suppose that the current were to flow in the direction opposite to that found in Problem 1. (a) In what direction would the magnetic force on the rod be? (b) In the absence of an external force, what would happen to the rod's kinetic energy? (c) Why is this not possible? Returning to the correct direction of the current, sketch a rough graph of the kinetic energy of the rod as a function of time.

Mayukh Banik
Mayukh Banik
Numerade Educator
02:30

Problem 3

To maintain a constant emf, the moving rod of Fig. $20.2$ must maintain a constant velocity. In order to maintain a constant velocity, some external force must pull it to the right. (a) What is the magnitude of the external force required, in terms of $v, B, L$, and $R ?$ (See Problem 1.) (b) At what rate does this force do work on the rod? (c) What is the power dissipated in the resistor?
(d) Overall, is energy conserved? Explain.

Mayukh Banik
Mayukh Banik
Numerade Educator
02:08

Problem 4

In Fig. 20.2, what would the magnitude (in terms of $v$, $L, R$, and $B$ ) and direction (CW or CCW) of the current be if the direction of the magnetic field were: (a) into the page; (b) to the right (in the plane of the page);
(c) up (in the plane of the page); (d) such that it has components both out of the page and to the right, with a $20.0^{\circ}$ angle between the field and the plane of the page?

Mayukh Banik
Mayukh Banik
Numerade Educator
06:52

Problem 5

A $15.0-g$ conducting rod of length $1.30 \mathrm{~m}$ is free to slide downward between two vertical rails without friction. The rails are connected to an $8.00-\Omega$ resistor, and the entire apparatus is placed in a $0.450-\mathrm{T}$ uniform magnetic field. Ignore the resistance of the rod and rails. (a) What is the terminal yelocity of the rod? (b) At this terminal velocity, compare the magnitude of the change in gravitational potential energy ner second with the power dissipated in the resistor.

Vishal Gupta
Vishal Gupta
Numerade Educator
02:34

Problem 6

When the armature of an ac generator rotates at $15.0 \mathrm{rad} / \mathrm{s}$, the amplitude of the induced emf is $27.0 \mathrm{~V}$. What is the amplitude of the induced emf when the armature rotates at $10.0 \mathrm{rad} / \mathrm{s} ?$

Vishal Gupta
Vishal Gupta
Numerade Educator
01:19

Problem 7

The armature of an ac generator is a circular coil with 50 turns and radius $3.0 \mathrm{~cm}$. When the armature rotates at $350 \mathrm{rpm}$, the amplitude of the emf in the coil is $17.0 \mathrm{~V}$. What is the strength of the magnetic field (assumed to be uniform)?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:09

Problem 8

The armature of an ac generator is a rectangular coil $2.0 \mathrm{~cm}$ by $6.0 \mathrm{~cm}$ with 80 turns. It is immersed in a uniform magnetic field of magnitude $0.45 \mathrm{~T}$. If the amplitude of the emf in the coil is $17.0 \mathrm{~V}$, at what angular speed is the armature rotating?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:53

Problem 9

In Fig. $20.6$, side 3 of the rectangular coil in the electric generator rotates about the axis at constant angular speed $\omega$. The figure with this problem shows side 3 by itself. (a) First consider the right half of side 3 . Although the speed of the wire differs depending on the distance from the axis, the direction is the same for the entire right half. Use the magnetic force law to find the direction of the force on electrons in the right half of the wire. (b) Does the magnetic force tend to push electrons along the wire, either toward or away from the axis? (c) Is there an induced emf along the length of this half of the wire? (d) Generalize your answers to the left side of wire 3 and the two sides of wire 1 . What is the net emf due to these two sides of the coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:29

Problem 10

A solid copper disk of radius $R$ rotates at angular velocity $\omega$ in a perpendicular magnetic field $B$. The figure shows the disk rotating clockwise and the magnetic field into the page. (a) Is the charge that accumulates on the edge of the disk positive or negative? Explain. (b) What is the potential difference between the center of the disk and the edge?

Mayukh Banik
Mayukh Banik
Numerade Educator
06:49

Problem 11

A square loop of wire of side $2.3 \mathrm{~cm}$ and electrical resistance $79 \Omega$ is near a long straight wire that carries a current of $6.8 \mathrm{~A}$ in the direction indicated. The long wire and loop both lie in the plane of the page. The left side of the loop is $9.0 \mathrm{~cm}$ from the wire. (a) If the loop is at rest, what is the induced emf in the loop? What are the magnitude and direction of the induced current in the loop? What are the magnitude and direction of the magnetic force on the loop? (b) Repeat if the loop is moving to the right at a constant speed of $45 \mathrm{~cm} / \mathrm{s}$. (c) In (b), find the electric power dissipated in the loop and show that it is equal to the rate at which an external force, pulling the loop to kecp its speed constant, docs work.

Mayukh Banik
Mayukh Banik
Numerade Educator
09:45

Problem 12

A solid metal cylinder of mass $m$ rolls down parallel metal rails spaced a distance $L$ apart with a constant acceleration of magnitude $a_{0}$ [part (a) of figure]. The rails are inclined at an angle $\theta$ to the horizontal. Now the rails are connected clectrically at the top and immersed in a magnetic field of magnitude $B$ that is perpendicular to the plane of the rails [part (b) of figure]. (a) As it rolls down the rails, in what direction does current flow in the cylinder? (b) What direction is the magnetic force on the cylinder? (c) Instead of rolling at constant acceleration, the cylinder now approaches a terminal speed $v_{t}$. What is $v_{1}$ in terms of $L, m, R, a_{0}, \theta$, and $B ? R$ is the total electrical resistance of the circuit consisting of the cylinder, rails, and wire; assume $R$ is constant (that is, the resistances of the rails themselves are negligible).

David Morabito
David Morabito
Numerade Educator
01:21

Problem 13

A horizontal desk surface measures $1.3 \mathrm{~m} \times 1.0 \mathrm{~m}$. If Earth's magnetic field has magnitude $0.44 \mathrm{~m} \mathrm{~T}$ and is directed $65^{\circ}$ below the horizontal, what is the magnetic flux through the desk surface?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:45

Problem 14

A square loop of wire, $0.75 \mathrm{~m}$ on each side, has one edge along the positive z-axis and is tilted toward the $y z$ -plane at an angle of $30.0^{\circ}$ with respect to the horizontal $(x z-$ plane). There is a uniform magnetic field of $0.32 \mathrm{~T}$ pointing in the positive $x$ -axis direction. (a) What is the flux through the loop? (b) If the angle increases to $60^{\circ}$, what is the new flux through the loop?
(c) While the angle is being increased,which direction will current flow through the top side of the loop?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:06

Problem 15

A long straight wire carrying a steady current is in the plane of a circular loop of wire. See the figure with Multiple-Choice Question 4 . (a) If the loop is moved closer to the wire, what direction does the induced current in the loop flow? (b) At one instant, the induced emf in the loop is $3.5 \mathrm{mV}$. What is the rate of change of the magnetic flux through the loop at that instant in webers per second?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:30

Problem 16

A long straight wire carrying a current $I$ is in the plane of a circular loop of wire. See the figure with MultipleChoice Question 4 . The current $I$ is decreasing. Both the loop and the wire are held in place by external forces. The loop has resistance $24 \Omega$. (a) In what direction does the induced current in the loop flow? (b) In what direction is the external force holding the loop in place? (c) At one instant, the induced current in the loop is $84 \mathrm{~mA}$. What is the rate of change of the magnetic flux through the loop at that instant in webers per second?

Mayukh Banik
Mayukh Banik
Numerade Educator
05:04

Problem 17

Two wire loops are side by side, as shown. The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise as viewed from the right.
While $I_{1}$ is increasing, does current flow in loop 2 ? If so, does it flow clockwise or counterclockwise as viewed from the right? Explain.

Vishal Gupta
Vishal Gupta
Numerade Educator
00:45

Problem 18

Two wire loops are side by side, as shown. The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise as viewed from the right.
While $I_{1}$ is increasing, what is the direction of the magnetic force exerted on loop 2, if any? Explain.

Mayukh Banik
Mayukh Banik
Numerade Educator
01:53

Problem 19

Two wire loops are side by side, as shown. The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise as viewed from the right.
While $I_{1}$ is constant, does current flow in loop $2 ?$ If so, does it flow clockwise or counterclockwise as viewed from the right? Explain.

Vishal Gupta
Vishal Gupta
Numerade Educator
01:11

Problem 20

Two wire loops are side by side, as shown. The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise as viewed from the right.
Refer to Fig. 20.2. The rod has length $L$ and its position is $x$ at some instant, as shown in the figure. Express your answers in terms of $x, L, v, B$ (the magnetic field strength), and $R$, as needed. (a) What is the area enclosed by the conducting loop at this instant? (b) What is the magnetic flux through the loop at this instant? (c) The rod moves to the right at speed $v$. At what rate is the flux changing? (d) According to Faraday's law, what is the induced emf in the loop? Compare your answer with Eq. (20-2a), (e) What is the induced current $I$ ?
(f) Explain why the induced current flows counterclockwise around the loop.

Mayukh Banik
Mayukh Banik
Numerade Educator
05:41

Problem 21

A circular conducting coil with radius $3.40 \mathrm{~cm}$ is placed in a uniform magnetic field of $0.880 \mathrm{~T}$ with the plane of the coil perpendicular to the magnetic field. The coil is rotated $180^{\circ}$ about the axis in $0.222$ s. (a) What is the average induced emf in the coil during this rotation? (b) If the coil is made of copper with a diameter of $0.900 \mathrm{~mm}$, what is the average current that flows through the coil during the rotation?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:59

Problem 22

Verify that, in SI units, $\Delta \Phi_{\mathrm{B}} / \Delta t$ can be measured in volts - in other words, that $1 \mathrm{~Wb} / \mathrm{s}=1 \mathrm{~V}$.

Mayukh Banik
Mayukh Banik
Numerade Educator
02:01

Problem 23

The component of the external magnetic field along the central axis of a 50 -turn coil of radius $5.0 \mathrm{~cm}$ increases from 0 to $1.8 \mathrm{~T}$ in $3.6 \mathrm{~s}$. (a) If the resistance of the coil is $2.8 \Omega$, what is the magnitude of the induced current in the coil? (b) What is the direction of the current if the axial component of the ficld points away from the viewer?

Mayukh Banik
Mayukh Banik
Numerade Educator
09:15

Problem 24

In the figure, switch $S$ is initially open. It is closed, and then opened again a few seconds later. (a) In what direction does current flow through the ammeter when switch $S$ is closed? (b) In what direction does current flow when switch $S$ is then opened? (c) Sketch a qualitative graph of the current through the ammeter as a function of time. Take the current to be positive to the

Vishal Gupta
Vishal Gupta
Numerade Educator
01:38

Problem 25

Another example of motional emf is a rod attached at one end and rotating in a plane perpendicular to a uniform magnetic field. We can analyze this motional emf using Faraday's law.
(a) Consider the area that the rod sweeps out in each revolution and find the magnitude of the emf in terms of the angular frequency $\omega$, the length of the rod $R$, and the strength of the uniform magnetic field $B .$ (b) Write the emf magnitude in terms of the speed $v$ of the tip of the rod and compare this with motional emf magnitude of a rod moving at constant velocity perpendicular to a uniform magnetic field.

Mayukh Banik
Mayukh Banik
Numerade Educator
09:24

Problem 26

(a) For a particle moving in simple harmonic motion, the position can be written $x(t)=x_{\mathrm{m}} \cos \omega t .$ What is the velocity $v_{x}(t)$ as a function of time for this particle?
(b) Using the small-angle approximation for the sine function, find the slope of the graph of $\Phi(t)=\Phi_{0} \sin \omega t$ at $t=0 .$ Does your result agree with the value of $\Delta \Phi / \Delta t=\omega \Phi_{0} \cos \omega t$ at $t=0 ?$

David Morabito
David Morabito
Numerade Educator
01:33

Problem 27

Two loops of wire are next to one another in the same plane. (a) If the switch $S$ is closed, does current flow in loop 2 ? If so, in what direction?
(b) Does the current in loop 2 flow for only a brief moment, or does it continue? (c) Is there a magnetic force on loop 2 ? If so, in what direction? (d) Is there a magnetic force on loop $1 ?$ If so, in what direction?

Mayukh Banik
Mayukh Banik
Numerade Educator
03:47

Problem 28

A dc motor has coils with a resistance of $16 \Omega$ and is connected to an emf of $120.0 \mathrm{~V}$. When the motor operates at full speed, the back emf is $72 \mathrm{~V}$. (a) What is the current in the motor when it first starts up? (b) What is the current when the motor is at full speed? (c) If the current is $4.0 \mathrm{~A}$ with the motor operating at less than full speed, what is the back emf at that time?

Vishal Gupta
Vishal Gupta
Numerade Educator
01:01

Problem 29

Tim is using a cordless electric weed trimmer with a dc motor to cut the long weeds in his back yard. The trimmer generates a back emf of $18.00 \mathrm{~V}$ when it is connected to an emf of $24.0 \mathrm{~V}$ dc. The total electrical resistance of the electric motor is $8.00 \Omega$. (a) How much current flows through the motor when it is running smoothly? (b) Suddenly the string of the trimmer gets wrapped around a pole in the ground and the motor quits spinning. What is the current through the motor when there is no back emf? What should Tim do?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:01

Problem 30

A dc motor is connected to a constant emf of $12.0 \mathrm{~V}$. The resistance of its windings is $2.0 \Omega$. At normal operating speed, the motor delivers $6.0 \mathrm{~W}$ of mechanical power. (a) What is the initial current drawn by the motor when it is first started up? (b) What current does it draw at normal operating speed? (c) What is the back emf induced in the windings at normal speed?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:41

Problem 31

A step-down transformer has 4000 turns on the primary and 200 turns on the secondary. If the primary voltage amplitude is $2.2 \mathrm{kV}$, what is the secondary voltage amplitude?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:36

Problem 32

A step-down transformer has a turns ratio of $1 / 100$. An ac voltage of amplitude $170 \mathrm{~V}$ is applied to the primary. If the primary current amplitude is $1.0 \mathrm{~mA}$, what is the secondary current amplitude?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:12

Problem 33

A doorbell uses a transformer to deliver an amplitude of $8.5 \mathrm{~V}$ when it is connected to a $170-\mathrm{V}$ amplitude line. If there are 50 turns on the secondary, (a) what is the turns ratio? (b) How many turns does the primary have?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:17

Problem 34

The primary coil of a transformer has 250 turns; the secondary coil has 1000 turns. An alternating current is sent through the primary coil. The emf in the primary is of amplitude $16 \mathrm{~V}$. What is the emf amplitude in the secondary?

Vishal Gupta
Vishal Gupta
Numerade Educator
00:17

Problem 35

When the emf for the primary of a transformer is of amplitude $5.00 \mathrm{~V}$, the secondary emf is $10.0 \mathrm{~V}$ in amplitude. What is the transformer turns ratio $\left(N_{2} / N_{1}\right)$ ?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:39

Problem 36

A transformer with a primary coil of 1000 turns is used to step up the standard $170-\mathrm{V}$ amplitude line voltage to a 220-V amplitude. How many turns are required in the secondary coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:24

Problem 37

A transformer with 1800 turns on the primary and 300 turns on the secondary is used in an electric slot car racing set to reduce the input voltage amplitude of $170 \mathrm{~V}$ from the wall output. The current in the secondary coil is of amplitude $3.2 \mathrm{~A}$. What is the voltage amplitude across the secondary coil and the current amplitude in the primary coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:11

Problem 38

A transformer for an answering machine takes an ac voltage of amplitude $170 \mathrm{~V}$ as its input and supplies $a$ $7.8-V$ amplitude to the answering machine. The primary has 300 turns, (a) How many turns does the secondary have? (b) When idle, the answering machine uses a maximum power of $5.0 \mathrm{~W}$. What is the amplitude of the current drawn from the $170-V$ line?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:20

Problem 39

A 2 -m-long copper pipe is held vertically. When a marble is dropped down the pipe, it falls through in about $0.7 \mathrm{~s}$. A magnet of similar size and shape takes much longer to fall through the pipe. (a) As the magnet is falling through the pipe with its north pole below its south pole, what direction do currents flow around the pipe above the magnet? Below the magnet (CW or CCW as viewed from the top)? (b) Sketch a graph of the speed of the magnet as a function of time. [Hint: What would the graph look like for a marble falling through honey?]

Mayukh Banik
Mayukh Banik
Numerade Educator
01:40

Problem 40

In Problem 39 , the pipe is suspended from a spring scale. The weight of the pipe is $12.0 \mathrm{~N} ;$ the weight of the marble and magnet are each $0.3 \mathrm{~N}$. Sketch graphs to show the reading of the spring scale as a function of time for the fall of the marble and again for the fall of the magnet. Label the vertical axis with numerical values.

Mayukh Banik
Mayukh Banik
Numerade Educator
01:03

Problem 41

Two wire loops are side by side, as shown in the figure with Problems $17-20 .$ The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise ss viewed from the right.
When the current in loop 1 is $I_{1}=0.75 \mathrm{~A}$, the magnetic flux through loop 2 is $2.35 \times 10^{-8} \mathrm{~T} \cdot \mathrm{m}^{2}$. What is the mutual inductance of the two loops?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:30

Problem 42

Two wire loops are side by side, as shown in the figure with Problems $17-20 .$ The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise ss viewed from the right.
When the current in loop 1 decreases at a steady rate of $28 \mathrm{~A} / \mathrm{s}$, the induced emf in loop 2 is $1.40 \mathrm{mV}$. (a) What is the direction of the current in loop 2 as viewed from the right? (b) What is the mutual inductance of the two loops?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:13

Problem 43

Two wire loops are side by side, as shown in the figure with Problems $17-20 .$ The current $I_{1}$ in loop 1 is supplied by an external source (not shown) and is clockwise ss viewed from the right.
When the current in loop 1 increases at a steady rate of $9.0 \mathrm{~A} / \mathrm{s}$, the current in loop 2 is $0.185 \mathrm{~mA}$. The resistance of loop 2 is $0.66 \Omega$. (a) What is the direction of the current in loop 2 as viewed from the right? (b) What is the mutual inductance of the two loops?

Mayukh Banik
Mayukh Banik
Numerade Educator
04:23

Problem 44

Two solenoids, of $N_{1}$ and $N_{2}$ turns respectively, are wound on the same form. They have the same length $\underline{L}$ and radius $r$. (a) What is the mutual inductance of these two solenoids? (b) If an ac current
$$I_{1}(t)=I_{\mathrm{m}} \sin \omega t$$
flows in solenoid $1\left(N_{1}\right.$ turns), write an expression for the total flux through solenoid 2. (c) What is the maximum induced emf in solenoid $2 ?$ [Hint: Refer to Eq. (20-7).]

Mayukh Banik
Mayukh Banik
Numerade Educator
00:58

Problem 45

A solenoid is made of $300.0$ turns of wire, wrapped around a hollow cylinder of radius $1.2 \mathrm{~cm}$ and length $6.0 \mathrm{~cm}$. What is the self-inductance of the solenoid?

Mayukh Banik
Mayukh Banik
Numerade Educator
03:58

Problem 46

A solenoid of length $2.8 \mathrm{~cm}$ and diamcter $0.75 \mathrm{~cm}$ is wound with 160 turns per $\mathrm{cm}$. When the current through the solenoid is $0.20 \mathrm{~A}$, what is the magnetic flux through one of the windings of the solenoid?

Mayukh Banik
Mayukh Banik
Numerade Educator
04:20

Problem 47

. If the current in the solenoid in Problem 46 is decreasing at a rate of $35.0 \mathrm{~A} / \mathrm{s}$, what is the induced emf (a) in one of the windings? (b) in the entire solenoid?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:48

Problem 48

An ideal solenoid has length $\ell$. If the windings are compressed so that the length of the solenoid is reduced to $0.50 \ell$, what happens to the inductance of the solenoid?

Mayukh Banik
Mayukh Banik
Numerade Educator
04:51

Problem 49

In this problem, you derive the expression for the selfinductance of a long solenoid [Eq. ( $20-15 \mathrm{a}$ )]. The solenoid has $n$ turns per unit length, length $\ell$, and radius $r$. Assume that the current flowing in the solenoid is $I$.
(a) Write an expression for the magnetic field inside the solenoid in terms of $n, \ell, r, l$, and universal constants.
(b) Assume that all of the field lines cut through each turn of the solenoid. In other words, assume the field is uniform right out to the ends of the solenoid-a good approximation if the solenoid is tightly wound and sufficiently long. Write an expression for the magnetic flux through one turn. (c) What is the total flux linkage through all turns of the solenoid? (d) Use the definition of self-inductance [Eq. (20-14)] to find the selfinductance of the solenoid.

Vishal Gupta
Vishal Gupta
Numerade Educator
01:48

Problem 50

Compare the electric energy that can be stored in a capacitor to the magnetic energy that can be stored in an inductor of the same size (that is, the same volume). For the capacitor, assume that air is between the plates; the maximum electric field is then the breakdown strength of air, about $3 \mathrm{MV} / \mathrm{m}$. The maximum magnetic field attainable in an ordinary solenoid with an air core is on the order of $10 \mathrm{~T}$.

Mayukh Banik
Mayukh Banik
Numerade Educator
01:22

Problem 51

The current in a $0.080-\mathrm{H}$ solenoid increases from $20.0 \mathrm{~mA}$ to $160.0 \mathrm{~mA}$ in $7.0 \mathrm{~s}$. Find the average $\mathrm{emf}$ in the solenoid during that time interval.

Penny Riley
Penny Riley
Numerade Educator
02:21

Problem 52

Calculate the equivalent inductance $L_{\mathrm{eq}}$ of two ideal inductors, $L_{1}$ and $L_{2}$, connected in series in a circuit. Assume that their mutual inductance is negligible. [Hint: Imagine replacing the two inductors with a single equivalent inductor $L_{\mathrm{seg}}$. How is the emf in the series equivalent related to the emfs in the two inductors? What about the currents?]

Mayukh Banik
Mayukh Banik
Numerade Educator
02:27

Problem 53

Calculate the equivalent inductance $L_{\mathrm{eq}}$ of two ideal inductors, $L_{1}$ and $L_{2}$, connected in parallel in a circuit. Assume that their mutual inductance is negligible.

Mayukh Banik
Mayukh Banik
Numerade Educator
03:15

Problem 54

In Section $20.9$, in order to find the energy stored in an inductor, we assumed that the current was increased from zero at a constant rate. In this problem, you will prove that the energy stored in an inductor is $U_{\mathrm{L}}=\frac{1}{2} L I^{2}-$ that is, it only depends on the current $I$ and not on the previous time dependence of the current. (a) If the current in the inductor increases from $i$ to $i+\Delta i$ in a very short time $\Delta t$, show that the energy added to the inductor is $\Delta U=$ Lidi.(b) Showthat, on a graph of $L i$ versus $i$, for any small current interval $\Delta i$, the energy added to the inductor can be interpreted as the area under the graph for that interval.
(c) Now show that the energy stored in the inductor when a current $I$ flows is $U=\frac{1}{2} L I^{2}$.

Mayukh Banik
Mayukh Banik
Numerade Educator
04:39

Problem 55

A $5.0-\mathrm{m} \mathrm{H}$ inductor and a $10.0-\Omega$ resistor are connected in series with a $6.0-\mathrm{V}$ de battery. (a) What is the voltage across the resistor immediately after the switch is closed? (b) What is the voltage across the resistor after the switch has been closed for a long time? (c) What is the current in the inductor after the switch has been closed for a long time? $\quad 500$

Vishal Gupta
Vishal Gupta
Numerade Educator
07:02

Problem 56

In a circuit, a parallel combination of a $10.0$ $\Omega$ resistor and a $7.0-\mathrm{mH}$ inductor
is connected in series with a $5.0-\Omega$ resistor, a $6.0-V$ de battery, and a switch. (a) What are the voltages across the $5.0-\Omega$ resistor and the $10.0-\Omega$ resistor, respectively, immediately after the switch is closed? (b) What are the voltages across the 5.0-\Omega resistor and the $10.0-\Omega$ resistor, respectively, after the switch has been closed for a long time? (c) What is the current in the $7.0-\mathrm{mH}$ inductor after the switch has been closed for a long time?

Vishal Gupta
Vishal Gupta
Numerade Educator
01:57

Problem 57

Refer to Problem 56. After the switch has been closed for a very long time, it is opened. What are the voltages across (a) the $5.0-\Omega$ resistor and (b) the $10.0-\Omega$ resistor immediately after the switch is opened?

Mayukh Banik
Mayukh Banik
Numerade Educator
05:21

Problem 58

No currents flow in the circuit before the switch is closed. Consider all circuit elements to be ideal. (a) At the instant the switch is closed, what are the values of the currents $I_{1}$ and $I_{2}$, the potential differences across the resistors, the power supplied by the battery, and the induced $\mathrm{emf}$ in the inductor? (b) After the switch has been closed for a long time, what are the values of the currents $I_{1}$ and $I_{2}$, the potential differences across the resistors, the power supplied by the battery, and the induced emf in the inductor?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:15

Problem 59

A coil of wire is connected to an ideal $6.00-\mathrm{V}$ battery at $t=0 .$ At $t=10.0 \mathrm{~ms}$, the current in the coil is $204 \mathrm{~mA}$. One minute later, the current is $273 \mathrm{~mA}$. Find the resistance and inductance of the coil. [Hint: Sketch $I(t) .]$

Mayukh Banik
Mayukh Banik
Numerade Educator
01:58

Problem 60

A $0.67$ -mH inductor and a $130-\Omega$ resistor are placed in series with a 24-V battery. (a) How long will it take for the current to reach $67 \%$ of its maximum value?
(b) What is the maximum energy stored in the inductor? (c) How long will it take for the energy stored in the inductor to reach $67 \%$ of its maximum value? Comment on how this compares to the answer in part (a).

Mayukh Banik
Mayukh Banik
Numerade Educator
01:55

Problem 61

The windings of an electromagnet have inductance $L=8.0 \mathrm{H}$ and resistance $R=2.0 \Omega .$ A $100.0-\mathrm{V} \quad \mathrm{dc}$
power supply is connected to the windings by closing switch $S_{2}$. (a) What is the current in the windings?
(b) The electromagnet is to be shut off. Before disconnecting the power supply by opening switch $S_{2}$, a shunt resistor with resistance $20.0 \Omega$ is connected in parallel across the windings. Why is the shunt resistor needed? Why must it be connected before the power supply is disconnected? (c) What is the maximum power dissipated in the shunt resistor? The shunt resistor must be chosen so that it can handle at least this much power without damage. (d) When the power supply is disconnected by opening switch $S_{2}$, how long does it take for the current in the windings to drop to $0.10 \mathrm{~A}$ ? (e) Would a larger shunt resistor dissipate the energy stored in the electromagnet faster? Explain.

Mayukh Banik
Mayukh Banik
Numerade Educator
03:05

Problem 62

A coil has an inductance of $0.15 \mathrm{H}$ and a resistance of $33 \Omega$. The coil is connected to a $6.0-\mathrm{V}$ ideal battery. When the current reaches half its maximum value:
(a) At what rate is magnetic energy being stored in the inductor? (b) At what rate is energy being dissipated?
(c) What is the total power that the battery supplies?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:21

Problem 63

The time constant $\tau$ for an $L R$ circuit must be some combination of $L, R$, and $\mathscr{8}$. (a) Write the units of each of these three quantities in terms of $\mathrm{V}, \mathrm{A}$, and $\mathrm{s}$.
(b) Show that the only combination that has units of scconds is $L / R$.

Mayukh Banik
Mayukh Banik
Numerade Educator
00:48

Problem 64

In the circuit, switch $S$ is opened at $t=0$ after having been closed for a long time. (a) How much energy is stored in the inductor at $t=0 ?$ (b) What is the instantaneous rate of change of the inductor's energy at $t=0$ ?
(c) What is the average rate of change of the inductor's energy between $t=0.0$ and $t=1.0 \mathrm{~s} ?$ (d) How long does it take for the current in the inductor to reach $0.0010$ times its initial value?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:21

Problem 65

In the circuit for Problem 56 , after the switch has been closed for a long time, it is opened. How long docs it take for the energy stored in the inductor to decrease to $0.10$ times its initial value?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:40

Problem 66

A $0.30-\mathrm{H}$ inductor and a $200.0-\Omega$ resistor are connected in series to a $9.0-\mathrm{V}$ battery. (a) What is the maximum current that flows in the circuit? (b) How long after connecting the battery does the current reach half its maximum value? (c) When the current is half its maximum value, find the energy stored in the inductor, the rate at which energy is being stored in the inductor, and the rate at which energy is dissipated in the resistor. (d) Redo parts (a) and (b) if, instead of being negligibly small, the internal resistances of the inductor and battery are $75 \Omega$ and $20.0 \Omega$, respectively.

Mayukh Banik
Mayukh Banik
Numerade Educator
01:42

Problem 67

A coil has an inductance of $0.15 \mathrm{H}$ and a resistance $33 \Omega$. The coil is connected to a $6.0-\mathrm{V}$ battery. After long time elapses, the current in the coil is no longe changing. (a) What is the current in the coil? (b) What the energy stored in the coil? (c) What is the rate energy dissipation in the coil? (d) What is the induce emf in the coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:43

Problem 68

Switch $S_{2}$ has been closed for a long time. (a) If switch $S_{1}$ is closed, will a current flow in the left-hand coil? If so, what direction will it flow across the ammeter? (b) After some time, switch $S_{1}$ is opened again while switch $S_{2}$ remains closed. Will a current flow in the left coil? If so, what direction will it flow across the ammeter?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:05

Problem 69

In the ac generator of Fig. $20.6$, the emf produced $j$ $\mathscr{E}(t)=\omega B A \sin \omega t .$ If the generator is connected to load of resistance $R$, then the current that flows is
$$I(t)=\frac{\omega B A}{R} \sin \omega t$$
(a) Find the magnetic forces on sides 2 and 4 at the instant shown in Fig. 20.7. (Remember that $\theta=\omega t$.
(b) Why do the magnetic forces on sides 1 and 3 no cause a torque about the axis of rotation? (c) From the magnetic forces found in (a), calculate the torque on the loop about its axis of rotation at the instant shown in Fig. 20.7. (d) In the absence of other torques, would the magnetic torque make the loop increase or decrease it angular velocity? Explain.

Mayukh Banik
Mayukh Banik
Numerade Educator
01:50

Problem 70

A circular metal ring is suspended above a solenoid. The magnetic field due to the solenoid is shown. The current in the solenoid is increasing. (a) What is the direction of the current in the ring?
(b) The flux through the ring is proportional to the current in the solenoid. When the current in the solenoid is $12.0 \mathrm{~A}$, the magnetic flux through the ring is $0.40 \mathrm{~Wb}$. When the current increases at a rate of $240 \mathrm{~A} / \mathrm{s}$, what is the induced emf in the ring? (c) Is there a net magnetic force on the ring? If so, in what direction? (d) If the ring is cooled by immersing it in liquid nitrogen, what happens to its electrical resistance, the induced current, and the magnetic force? The change in size of the ring is negligible. (With a sufficiently strong magnetic field, the ring can be made to shoot up high into the air.)

Mayukh Banik
Mayukh Banik
Numerade Educator
00:24

Problem 71

The strings of an electric guitar are made of ferromagnetic metal. The pickup consists of two components. A magnet causes the part of the string near it to be magnetized. The vibrations of the string near the pickup coil produce an induced emf in the coil. The electrical signal in the coil is then amplified and used to drive the speakers. In the figure, the string is moving away from the coil. What is
the direction of the induced current in the coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:30

Problem 72

A toroid has a square cross section of side $a$. The toroid has $N$ turns and radius $R$. The toroid is narrow $(a<<R)$ so that the magnetic field inside the toroid can be considered to be uniform in magnitude. What is the self-inductance of the toroid?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:16

Problem 73

An ideal toroid has $N$ turns and self-inductance L. A single turn of wire is wrapped around the toroid [see part (a) of the figure]. (a) What is the mutual inductance between the toroid and the single turn of wire? (b) What would the mutual inductance be if the turn of wire had an area twice as large as the crosssectional area of the toroid [see part (b) of the figure]?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:26

Problem 74

Suppose you wanted to use Earth's magnetic field to make an ac generator at a location where the magnitude of the field is $0.50 \mathrm{mT}$. Your coil has $1000.0$ turns and a radius of $5.0 \mathrm{~cm} .$ At what angular velocity would you have to rotate it in order to generate an $\mathrm{emf}$ of amplitude $1.0 \mathrm{~V} ?$

Mayukh Banik
Mayukh Banik
Numerade Educator
01:18

Problem 75

A uniform magnetic field of magnitude $0.29 \mathrm{~T}$ makes an angle of $13^{\circ}$ with the plane of a circular loop of wire. The loop has radius $1.85 \mathrm{~cm}$. What is the magnetic flux through the loop?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:29

Problem 76

A solenoid is $8.5 \mathrm{~cm}$ long, $1.6 \mathrm{~cm}$ in diameter, and has 350 turns. When the current through the solenoid is $65 \mathrm{~mA}$, what is the magnetic flux through one turn of the solenoid?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:11

Problem 77

How much energy due to Earth's magnetic field is present in $1.0 \mathrm{~m}^{3}$ of space near Earth's surface, at a place where the field has magnitude $0.45 \mathrm{mT}$ ?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:59

Problem 78

The largest constant magnetic field achieved in the laboratory is about $40 \mathrm{~T}$. (a) What is the magnetic energy density due to this field? (b) What magnitude electric field would have an equal energy density?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:31

Problem 79

A TV tube requires a $20.0$ -kV-amplitude power supply.
(a) What is the turns ratio of the transformer that raises the $170-V$ -amplitude household voltage to $20.0 \mathrm{kV}$ ?
(b) If the tube draws $82 \mathrm{~W}$ of power, find the currents in the primary and secondary windings. Assume an ideal transformer.

Mayukh Banik
Mayukh Banik
Numerade Educator
00:52

Problem 80

The alternator in an automobile generates an emf of amplitude $12.6 \mathrm{~V}$ when the engine idles at $1200 \mathrm{rpm}$. What is the amplitude of the emf when the car is being driven on the highway with the engine at $2800 \mathrm{rpm} ?$

Mayukh Banik
Mayukh Banik
Numerade Educator
02:00

Problem 81

The outside of an ideal solenoid $\left(N_{1}\right.$ turns, length $L$, radius $r$ ) is wound with a coil of wire with $N_{2}$ turns.
(a) What is the mutual inductance?
(b) If the current in the solenoid is changing at a rate $\Delta I_{1} / \Delta t$, what is the magnitude of the induced emf in the coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
00:59

Problem 82

A standard ammeter must be inserted in series into the circuit (Section 18.9). An induction ammeter has the great advantage of being able to measure currents without making any electrical connection to the circuit. An iron ring is hinged so that it can be snapped around a wire. A coil is wrapped around the iron ring; the ammeter uses the induced emf in the coil to determine the current flowing in the wire. (a) Does the induetion ammeter work equally well for ac and de currents? Explain. (b) Can the induction ammeter be placed around both wires connected to an appliance to measure the current drawn by the appliance? Explain.-

Mayukh Banik
Mayukh Banik
Numerade Educator
02:12

Problem 83

A flip coil is a device used to measure a magnetic field. A coil of radius $r, N$ turns, and electrical resistance $R$ is initially perpendicular to a magnetic field of magnitude $B$. The coil is connected to a special kind of galvanometer that measures the total charge $Q$ that flows through it. To measure the field, the flip coil is rapidly flipped upside down. (a) What is the change in magnetic flux through the coil in one flip? (b) If the time interval during which the coil is flipped is $\Delta t$, what is the average induced emf in the coil? (c) What is the average current that flows through the galvanometer? (d) What is the total charge $Q$ in terms of $r, N, R$, and $B ?$

Mayukh Banik
Mayukh Banik
Numerade Educator
01:22

Problem 84

A 100 -turn coil with a radius of $10.0 \mathrm{~cm}$ is mounted so the coil's axis can be oriented in any horizontal direction. Initially the axis is oriented so the magnetic flux from Earth's field is maximized. If the coil's axis is rotated through $90.0^{\circ}$ in $0.080 \mathrm{~s}$, an emf of $0.687 \mathrm{mV}$ is induced in the coil. (a) What is the magnitude of the horizontal component of Earth's magnetic field at this location? (b) If the coil is moved to another place on Earth and the measurement is repeated, will the result be the same?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:11

Problem 85

A bar magnet is initially far from a circular loop of wire. The magnet is moved at constant speed along the axis of the loop. It moves toward the loop, proceeds to pass through it, and then continues until it is far away on the right side of the loop. Sketch a qualitative graph of the current in the loop as a function of the position of the bar magnet. Take the current to be positive when it is counterclockwise as viewed from the left.

Mayukh Banik
Mayukh Banik
Numerade Educator
03:06

Problem 86

A bar magnet approaches a coil [part (a) of figure].
(a) In which direction does current flow through the galvanometer as the magnet approaches? (b) In part (b) of the figure, the magnet is initially at rest inside the coil. It is then pulled out from the left side. In which direction docs current flow through the galvanometer as the magnet is pulled away? (c) In both situations, how does the magnitude of the current depend on the number of turns in the coil? (The resistance of the coil is negligible compared to the resistance of the galvanometer.) (d) How does the current depend on the speed of the magnet?
(c) How does the magnitude of the current change if two such magnets were used, held together with the north poles together and the south poles together? (f) How does the magnitude of the current change if two such magnets were used, held together with the opposite poles together? (g) Would the experiment give similar results if the magnet remains stationary and the coil moves instead? Explain.

Mayukh Banik
Mayukh Banik
Numerade Educator
02:27

Problem 87

An ideal inductor of inductance $L$ is connected to an ac power supply, which provides an emf $\mathscr{E}(t)=8_{\mathrm{m}} \sin \omega t .$
(a) Write an expression for the current in the inductor as a function of time. [Hint: See Eq. (20-7).] (b) What is the ratio of the maximum emf to the maximum current? This ratio is called the reactance. (c) Do the maximum emf and maximum current occur at the same time? If not, how much time separates them?

Mayukh Banik
Mayukh Banik
Numerade Educator
03:44

Problem 88

An airplane is flying due north at $180 \mathrm{~m} / \mathrm{s}$. Earth's magnetic field has a northward component of $0.30 \mathrm{mT}$ and an upward component of $0.38 \mathrm{mT}$. (a) If the wingspan (distance between the wingtips) is $46 \mathrm{~m}$, what is the motional emf between the wingtips? (b) Which wingtip is positively charged?

Mayukh Banik
Mayukh Banik
Numerade Educator
01:21

Problem 89

Repeat Problem 88 if the plane flies $30.0^{\circ}$ west of south at $180 \mathrm{~m} / \mathrm{s}$ instead.

Mayukh Banik
Mayukh Banik
Numerade Educator
02:13

Problem 90

The magnetic field between the poles of an electromagnet is $2.6 \mathrm{~T}$. A coil of wire is placed in this region so that the field is parallel to the axis of the coil. The coil has electrical resistance $25 \Omega$, radius $1.8 \mathrm{~cm}$, and length $12.0 \mathrm{~cm}$. When the current supply to the electromagnet is shut off, the total charge that flows through the coil is $9.0 \mathrm{mC}$. How many turns are there in the coil?

Mayukh Banik
Mayukh Banik
Numerade Educator
02:10

Problem 91

An ideal solenoid $\left(N_{1}\right.$ turns, length $L_{1}$, radius $\left.r_{1}\right)$ is placed inside another ideal solenoid $\left(N_{2}\right.$ turns, length $L_{2}>L_{1},$, radius $r_{2}>r_{1}$ ) such that the axes of the two coincide.
(a) What is the mutual inductance?
(b) If the current in the outer solenoid is changing at a rate $\Delta l_{2} / \Delta t$, what is the magnitude of the induced emf in the inner solenoid?

Mayukh Banik
Mayukh Banik
Numerade Educator