PHYSICS 208
EXAM C a
November 9, 2010
NAME
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(10) 1. An alpha particle is moving at a speed of 5 x 1 mis in a direction perpendicular to a
uniform magnetic field of strength 4 x 1 02 T. The charge on an alpha particle is 3.2 x 1 ü’ C
and its mass is 6.6 x 1027 kg. The time it takes the alpha particle to complete one revolution
around its path is
a. 5.2 x 10
s; b. 1.0 x 106s; c. 3.2 x l0s; d. 6.5 x 10
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(10) 2. Two parallel wires carry currents in the same direction. The current in one wire doubles
while the current in the other also doubles and the distance between the wires doubles. The force
between the wires
a. remains the same; b. doubles; c. halves; d. quadruples;
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(10) 3. A ten ioop coil of area 0.23 m
2 is in a 0.047 T uniform magnetic field oriented so that the
maximum flux goes through the coil. The coil is then rotated so that the flux through it goes to
zero in 0.34 s. The average emf induced in the coil during the 0.34 s is
a. 0; b. 3.2 x 102 V; c. 3.2 x 10’ V; d. 1.0 V;
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(10) 4. At what rate would the current in a 100 mH inductor have to change to induce an emf of
1000 V?
a. lOAJs; b. 100A/s; c. 1000 AJs; d. 10,000A/s;
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(20) 5. Use Ampere’s Law to find the magnetic field for the region between r
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(20) 6. Using Lenz’s law, determine the direction of the current in resistor ab of the figure when:
a. switch S is opened after having been closed for several minutes.
b. coil B is brought closer to coil A with the switch closed.
c. the resistance of R is decreased while the switch remains closed.
EXPLAiN YOUR ANSWER by stating the direction of magnetic fields, flux, etc.
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(20) 7. A circuit is formed by a thin, 50 cm long metal bar (insignificant resistance) with mass
750 g held in place as shown by the magnetic force pushing up on it due to the current through
the wire and the 0.45 T magnetic field. The resistor in the circuit is 25 . What is the minimum
voltage battery that can be used without the bar falling?
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