Fall ’12 PHY 122 Homework Solutions #7

HW7: Ch. 26 P 34, 36 Ch.27 Q 2, 4, 8,

18 P 2, 8, 17, 19, 37

Chapter 26 Problem 34 Determine the magnitudes and directions of the currents in each

resistor shown in Fig. 26–52. The batteries have emfs of E1 = 9.0V

and E2 = 12.0V and the resistors have values of R1=25Ω, R2 = 48Ω

and R3=35Ω. (a) Ignore internal resistance of the batteries. (b)

Assume each battery has internal resistance

r=1.0Ω.

Chapter 26 Problem 36 (a)Determine the currents I1,I2 and I3 in Fig. 26–53. Assume the

internal resistance of each battery is r=1.0Ω (b) What is the terminal

voltage of the 6.0-V battery?

Chapter 27 Question 2 Draw the magnetic field lines around a straight section of wire

carrying a current horizontally to the left.

Solution

The magnetic field lines are concentric circles around the wire.

With the current running to the left, the field is directed

counterclockwise when looking from the left end. So, the field

goes into the page above the wire and comes out of the page

below the wire.

Chapter 27 Question 4

In the relation F=I l x B which pairs of the vectors (F, l,B) are always

at 90°? Which can be at other angles?

Solution

F is always perpendicular to both B and l. B and l can be at any

angle with respect to each other.

Chapter 27 Question 8 A positively charged particle in a nonuniform magnetic field follows

the trajectory shown in Fig. 27–35. Indicate the direction of the

magnetic field at points near the path, assuming the path is always in

the plane of the page, and indicate the relative magnitudes of the

field in each region.

Solution

In the areas where the particle’s path is curving up towards the top

of the page, the magnetic field is directed into the page. Where the

particle’s path curves downward towards the bottom of the page,

the magnetic field is directed out of the page. Where the particle is

moving in a straight line, the magnetic field direction is parallel or

anti-parallel to the particle’s velocity. The strength of the magnetic

field is greatest where the radius of curvature of the path is the

smallest.

Chapter 27 Question 18 How could you tell whether moving electrons in a certain region of

space are being deflected by an electric field or by a magnetic field

(or by both)?

Solution

If the direction of the velocity of the electrons is changing but their

speed is not, then they are being deflected by a magnetic field only,

and their path will be circular or helical. If the speed of the electrons

is changing but the direction is not, then they are being accelerated

by an electric field only. If both speed and direction are changing,

the particles are possibly being deflected by both magnetic and

electric fields, or they are being deflected by an electric field that is

not parallel to the initial velocity of the particles. In the latter case,

the component of the electron velocity antiparallel to the field

direction will continue to increase, and the component of the

electron velocity perpendicular to the field direction will remain

constant. Therefore, the electron will asymptotically approach a

straight path in the direction opposite the field direction. If the

particles continue with a circular component to their path, there

must be a magnetic field present.

Chapter 27 Problem 2

Calculate the magnitude of the magnetic force on a 240-m length of

wire stretched between two towers and carrying a 150-A current.

The Earth’s magnetic field of 5.0x10-5

T makes an angle of 68° with

the wire.

Chapter 27 Problem 8 A long wire stretches along the x axis and carries a 3.0-A current to

the right (+x). The wire is in a uniform magnetic field B = (0.20i –

0.36j + 0.25k)T. Determine the components of the force on the wire

per cm of length.

Chapter 27 Problem 17 Determine the direction of B for each case in Fig. 27–43, where F

represents the maximum magnetic force on a positively charged

particle moving with velocity v.

Chapter 27 Problem 19 A doubly charged helium atom whose mass is 6.63 x 10-27 kg is accelerated by a voltage of 2700 V. (a) What will be its radius of curvature if it moves in a plane perpendicular to a uniform 0.340-T field? (b) What is its period of revolution?

Chapter 27 Problem 37 A circular coil 18.0 cm in diameter and containing twelve loops lies

flat on the ground. The Earth’s magnetic field at this location has

magnitude 5.50 x 10-5

T and points into the Earth at an angle of

66.0° below a line pointing due north. If a 7.10-A clockwise current

passes through the coil, determine (a) the torque on the coil, and (b)

which edge of the coil rises up, north, east, south, or west.