General Physics II
Recitation Quiz 5 - Magnetic Field and Forces
Feb. 17, 2010
Name:
For full credit, make your work clear. Show the formulas you use, all the essential steps, and results with
correct units and correct number of significant figures. For multiple-choice questions, choose the best answer.
Permeability of free space µ0 = 4π × 10−7 T · m/A.
1(3). A uniform magnetic field points into the page as shown. The diagram shows the instantaneous velocity
of an electron e. The magnetic field will
a.
b.
c.
d.
Cause the electron to accelerate along its original direction
Cause the electron to decelerate
Change the direction of the electron motion and will cause it to circle clock-wise
Change the direction of the electron motion and will cause it to circle counter clock-wise
2(3). A rectangular loop of wire and a straight wire are located in the same plane. The net force acting
between the straight wire and the loop is
a. Attractive
b. Repulsive
c. Zero
d. Produces a torque.
Forces F3 and F4 cancel. F1 is larger than F2 .
Opposite currents repel. Currents in the same direction attract.
3(4). Four very long straight parallel wires, located at the corners of a square of side l = 5 cm (points A, B, C
and D) carry equal currents I = 5 mA, perpendicular to the page as shown in Figure. Determine the direction
~ at the center O of the square.
and the magnitude of magnetic field B
~ A | = |B
~ B | = |B
~ C | = |B
~ D| =
B = |B
µ0 I√
2π `/ 2
~ total = B
~A + B
~B + B
~C + B
~ D ; Btotal = 2B
B
Btotal =
µ0 I√
π `/ 2
=
4π×10−7 T·m/
π
= 5.65 × 10−8 T
A×
5×10−3 √
A
0.05 m/ 2