Page 341 - Schaum's Outline of Theory and Problems of Applied Physics

326 MAGNETISM [CHAP. 27

Magnet
Wire
loop Commutator

Brush

Current
Magnetic field of
magnet

Fig. 27-10. (From Konrad B. Krauskopf and Arthur Beiser, The Physical Universe, 10th Ed., c 2003, The McGraw-
Hill Companies. Reproduced with permission of The McGraw-Hill Companies.)

SOLVED PROBLEM 27.12
A stream of protons is moving parallel to a stream of electrons. Do the streams tend to come together or
to move apart?
The electric force between the streams is attractive, but the magnetic force is repulsive because the charges are
opposite. Which of the forces is stronger depends on how fast the particles are moving.

SOLVED PROBLEM 27.13
In a certain electric motor wires that carry a current of 5 A are perpendicular to a magnetic field of 0.8 T.
What is the force on each centimeter of these wires?

F = IL B = (5A)(0.01 m)(0.8T) = 0.04 N

SOLVED PROBLEM 27.14
The wires that supply current to a 120-V, 2-kW electric heater are 2 mm apart. What is the force per
meter between the wires?
Since P = IV , the current in the wires is

P 2000 W
I = I 1 = I 2 = = = 16.7A
V 120 V

Since s = 2mm = 2 × 10 −3 m, the force between the wires is

F µ 0 I 1 I 2 (4π × 10 −7 T·m/A)(16.7A) 2
= = = 0.028 N/m
L 2π s (2π)(2 × 10 −3 m)

The currents are in opposite directions, so the force is repulsive.