Download Magnetism In-Class Practice Problems

Magnetism In-Class Practice Problems (“Magnetic Forces and Fields”)
1.
2.
A long, straight wire carries a current of 14 A. Next to the wire is a square loop with sides 1.0 m in length, as
shown in Figure 1. The loop carries a current of 2.5 A in the direction indicated. (a) What is the direction of the
net force exerted on the loop? Explain. (b) Calculate the magnitude of the net force acting on the loop.
Suppose the 14-A current in the straight wire in Figure 1 is reversed in direction, but the current in the loop is
unchanged. (a) Calculate the magnitude and direction of the net force acting on the loop. (b) If the loop is
extended in the horizontal direction, so that it is 1.0 m high and 2.0 m wide, does the net force exerted on the
loop increase or decrease? By what factor? Explain. (c) If, instead, the loop is extended in the vertical direction,
so it is 2.0 m high and 1.0 m wide, does the net force exerted on the loop increase or decrease? Explain.
Figure 1
3.
4.
5.
A beam of protons with various speeds is directed in the positive x direction. The beam enters a region with a
uniform magnetic field of magnitude 0.52 T pointing in the negative z direction, as indicated in Figure 2. It is
desired to use a uniform electric field (in addition to the magnetic field) to select from this beam only those
protons with a speed of 142
.  105 m / s —that is, only these protons should be undeflected by the two fields. (a)
Determine the magnitude and direction of the electric field that yields the desired result. (b) Suppose the
electric field is to be produced by a parallel-plate capacitor with a plate separation of 2.5 cm. What potential
difference is required between the plates? (c) Which plate in Figure 2 (top or bottom) should be positively
charged? Explain.
Two parallel wires, each carrying a current of 2.2 A in the same direction, are shown in Figure 3. Find the
direction and magnitude of the net magnetic field at points A, B, and C.
A powerful bolt of lightning can carry a current of 225 kA. (a) Treating a lightning bolt as a long, thin wire,
calculate the magnitude of the magnetic field produced by such a bolt of lightning at a distance of 75 m.
(b) If two such bolts strike simultaneously at a distance of 25 m from each other, what is the magnetic force per
meter exerted by one bolt on the other?
Figure 3
6.
Figure 2
Figure 4
Consider the two current-carrying wires shown in Figure 4. The current in wire 1 is 3.7 A; the current in wire 2
is adjusted to make the net magnetic field at point A equal to zero. (a) Is the magnitude of the current in wire 2
greater than, less than, or the same as that in wire 1? Explain. (b) Find the magnitude and direction of the
current in wire 2.
Answers
1. (a) Fnet points towards the wire…explain with FBD. (b) Fnet = 3 x 105 N
2. (a) Fnet = 3 x 105 N away from the wire. (b) Fnet increases by a factor of two…explain.
(c) Fnet increases…explain.
3. (a) E = 74 kV/m in the negative y-direction (b) V = 1.8 kV
(c) The top plate should be positively charged…explain.
4. BA = 7.8 T out of the page
BB = 0
BC = 7.8 T into the page
5. (a) B = 600 T
(b) F/L = 410 N/m
6. (a) I2  I1 because…explain.
(b) I2 = 1.2 A to the lleftt