Physics 42 Chapter 31 HW
Objective Questions 1-11; Problems: 9, 13, 14, 17, 20, 23, 25, 28, 30, 36, 55
Problems: 9, 13, 14, 17, 20, 23, 25, 28, 30, 36, 55
9.
An aluminum ring of radius 5.00 cm and resistance 3.00 × 10–4 Ω is placed on top of a long air-core solenoid
with 1 000 turns per meter and radius 3.00 cm, as shown in Figure P31.7. Over the area of
the end of the solenoid, assume that the axial component of the field produced by the
solenoid is half as strong as at the center of the solenoid. Assume the solenoid produces
negligible field outside its cross-sectional area. The current in the solenoid is increasing at a
rate of 270 A/s. (a) What is the induced current in the ring? At the center of the ring, what
are (b) the magnitude and (c) the direction of the magnetic field produced by the induced
current in the ring?
14. A long solenoid has 400 turns per meter and carries a current given by I = (30.0 A)(1 – e – 1.60 t ). Inside the
solenoid and coaxial with it is a coil that has a radius of 6.00 cm and consists of a total of 250 turns of fine
wire (Fig. P31.13). What emf is induced in the coil by the changing current?
17.
A toroid having a rectangular cross section (a = 2.00 cm by b = 3.00 cm) and inner radius R = 4.00 cm
consists of 500 turns of wire that carries a sinusoidal current I = Imax sin ωt, with Imax = 50.0 A and a
frequency f = ω/2π = 60.0 Hz. A coil that consists of 20 turns of wire links with the toroid, as in Figure
P31.17. Determine the emf induced in the coil as a function of time.
P31.17 In a toroid, all the flux is confined to the inside of the toroid.
B
0N I 5000I

2 r
2 r
B   BdA 
B 
5000Im ax
 b R 
asin  tln 
 R 
2
N


5000Im ax
adr
sin  t
2
r
FIG. P31.17
dB
 5000Im ax 
 b R 
 20
 aln 
cos t

 R 


dt
2
  3.00  4.00 cm 
104
4  107 N A 2  50.0 A   377 rad s 0.020 0 m  ln 
 cos t
2
4.00 cm


 0.422 V  cos t

20.
Use Lenz’s law to answer the following questions concerning the direction of induced currents. (a)
What is the direction of the induced current in resistor R in Figure P31.28a when the bar magnet is moved
to the left? (b) What is the direction of the current induced in the resistor R immediately after the switch S
in Figure P31.28b is closed? (c) What is the direction of the induced current in R when the current I in
Figure P31.28c decreases rapidly to zero? (d) A copper bar is moved to the right while its axis is maintained
in a direction perpendicular to a magnetic field, as shown in Figure P31.28d. If the top of the bar becomes
positive relative to the bottom, what is the direction of the magnetic field?
ˆ and B decreases; therefore, the
B ext  Bexti
ext
ˆ
induced field is B 0  B0i (to the right) and the current in the resistor
P31.20
(a)
is directed to the right .
(b)
 
ˆ increases; therefore, the induced field
B ext  Bext i
 
B 0  B0  ˆ
i is to the right, and the current in the resistor is directed
to the right .
(c)
 
B ext  Bext kˆ into the paper and Bext decreases; therefore,
 
the induced field is B 0  B0  kˆ into the paper, and the current in
the resistor is directed to the right .
(d)
By the magnetic force law, FB  q v  B  . Therefore, a positive charge will move to the top of the bar if B is
into the paper .
23.
Figure P31.20 shows a top view of a bar that can slide without friction. The resistor is 6.00 Ω and a
2.50-T magnetic field is directed perpendicularly downward, into the paper. Let ℓ = 1.20 m. (a) Calculate
the applied force required to move the bar to the right at a constant speed of 2.00 m/s. (b) At what rate is
energy delivered to the resistor?
P31.23
(a)
FB  I  B  I B
When
and   B v
we get
I

R
B v
B2 2v  2.50  1.20  2.00

 3.00 N .
 B 
R
R
6.00
2
FB 
The applied force is 3.00 N to the right .
(b)
P  I2R 
B2 2v2
 6.00 W or P  Fv  6.00 W
R
2
30. A rectangular coil with resistance R has N turns,
each of length ℓ and width w as shown in Figure
P31.29. The coil moves into a uniform magnetic field B
with constant velocity v. What are the magnitude and
direction of the total magnetic force on the coil (a) as it
enters the magnetic field, (b) as it moves within the
field, and (c) as it leaves the field?
55.
A conducting rod of length ℓ = 35.0 cm is free to
slide on two parallel conducting bars as shown in
Figure P31.50. Two resistors R1 = 2.00 Ω and R2 = 5.00 Ω
are connected across the ends of the bars to form a
loop. A constant magnetic field B = 2.50 T is directed perpendicularly into the page. An external agent pulls
the rod to the left with a constant speed of v = 8.00 m/s. Find (a) the currents in both resistors, (b) the total
power delivered to the resistance of the circuit, and (c) the magnitude of the applied force that is needed to
move the rod with this constant velocity.
P31.55 The emf induced between the ends of the moving bar is   B v   2.50 T 0.350 m   8.00 m s  7.00 V .
The left-hand loop contains decreasing flux away from you, so the induced current in it will be
clockwise, to produce its own field directed away from you. Let I1 represent the current flowing upward through
the 2.00- resistor. The right-hand loop will carry counterclockwise current. Let I3 be the upward current in the
5.00- resistor.
(a)Kirchhoff’s loop rule then gives:
and
(b)
7.00 V  I1  2.00   0 I1  3.50 A
7.00 V  I3  5.00   0
I3  1.40 A .
The total power dissipated in the resistors of the circuit is
P   I1   I3    I1  I3    7.00 V  3.50 A  1.40 A   34.3 W
(c)
.
Method 1: The current in the sliding conductor is downward with value I2  3.50 A  1.40 A  4.90 A . The
magnetic field exerts a force of Fm  I B   4.90 A  0.350 m  2.50 T  4.29 N directed
conductor. An outside agent must then exert a force of 4.29 N
toward the right on this
to the left to keep the bar moving.
Method 2: The agent moving the bar must supply the power according to P  F  v  Fvcos0 . The force
required is then:
P
34.3 W
F 
 4.29 N .
v 8.00 m s