Download Circular Motion HW-1

AP Physics 2
Induction-In-Class Assignment (“Induced emf, Work, & Energy)
Conceptual Questions
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A conducting rod slides on two wires in a region with a magnetic field. The two wires are not connected. Is a
force required to keep the rod moving with constant speed? Explain.
Many equal-arm balances have a small metal plate attached to one of the two arms. The plate passes between the
poles of a magnet mounted in the base of the balance. Explain the purpose of this arrangement.
A penny is placed on edge in the powerful magnetic field of an MRI solenoid. If the penny is tipped over, it
takes several seconds for it to land on one of its faces. Explain.
Recently, NASA tested a power generation system that involves connecting a small satellite to the space shuttle
with a conducting wire several miles long. Explain how such a system can generate electrical power.
A metal rod of resistance R can slide without friction on two zero-resistance rails, as shown in Figure 1. Describe
the rod’s motion when the switch is closed. Your discussion should include the effects of motional emf.
Figure 1
Problems
Mechanical Work and Electrical Energy
1. A Boeing KC-135A airplane has a wingspan of 39.9 m and flies at constant altitude in a northerly direction
with a speed of 850 km/h. If the vertical component of the Earth’s magnetic field is 5.0  10 6 T, what is the
induced emf between the wing tips?
2. Figure 2 shows a zero-resistance rod sliding to the right on two zero-resistance rails separated by the distance
L  0.45 m. The rails are connected by a 12.5 -  resistor, and the entire system is in a uniform magnetic field
with a magnitude of 0.750 T. (a) Find the speed at which the bar must be moved to produce a current of 0.125
A in the resistor. (b) Would your answer to part (a) change if the bar was moving to the left instead of to the
right? Explain.
Figure 2
3.
Referring to part (a) of Problem 2, (a) find the force that must be exerted on the rod to maintain a constant
current of 0.125 A in the resistor. (b) What is the rate of energy dissipation in the resistor? (c) What is the
mechanical power delivered to the rod?
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The light bulb in the circuit shown below has a resistance of 12  and consumes 5.0 W of power; the rod is
1.25 m long and moves to the left with a constant speed of 3.1 m/s. (a) What is the strength of the magnetic
field? (b) What external force is required to maintain the rod’s constant speed? (c) Find the current that flows
in the circuit. (d) Suppose we wish to make the bulb shines brighter than 5.0 W; what speed must the rod have
if the current in the circuit is to be 0.85 A?
Suppose the mechanical power delivered to the rod above is 7.5 W. Find (a) the current in the circuit and (b)
the speed of the rod. (Assume the externally applied magnetic field, the resistance of the light bulb, and the
length of the rod are all the same as in #4.)
Answers
1.  = 47 mV
2. (a) v = 4.6 m/s
3. (a) F = 42 mN
4. (a) B = 2.0 T
5. (a) I = 0.79 A
(b) The equation for v is independent of the direction of motion of the bar, so the answer to
part (a) would not change.
(b) P = 195 mW
(c) P = 195 mW
(b) Fexternal = 1.6 N
(c) I = 0.65 A
(d) v = 4.1 m/s
(b) v = 3.8 m/s