Physics 202 MWF 10:20 Spring 1997 (Ford)
Exam III Chapts. 31-34 in SZY College Physics 7e
Show all your work. Partial credit will be given if earned. Write your
answers in the blanks provided.
(15 pts) 1. A long, straight coil has 2000 turns and negligible resistance.
When the current in the coil is 4.0 A, the average magnetic flux through
each turn
of the coil is 0.050 Wb. If the current in the coil is changing
at the uniform rate of 0.030 A/s, what is the magnitude of
the potential difference between the two ends of the coil?
Ans. ________________
(18 pts) 2. Two long straight wires carry currents with the magnitudes
and directions shown in the sketch. The distance between the wires is
0.20 m. What are the magnitude and direction of the resultant magnetic field
produced by these two currents at a point P that is in the plane of the
paper and 0.10 m above the top wire?
Ans. B = _________________
direction _______________
(18 pts) 3. A long coaxial cable consists of two concentric cylindrical
conductors. The central solid cylinder has radius a, and the outer
hollow cylinder has inner radius b and outer radius c.
There are equal
and opposite currents i in the two conductors. Let r
be the radial distance from the center of the cable. Calculate the
magnitude B of the magnetic field at points where
a) a < r < b (between the conductors)
Ans. ______________________
b) r > c (outside the whole cable) Ans. __________________
c) r < a (within the central solid conductor)
Ans. _________________
(15 pts) 4. Consider the circuit shown in the sketch. The conducting loop is in
a region of uniform magnetic field. The wires from the conducting loop go to
a resistor; note that the two ends of the resistor are labeled a
and b.
Calculate the direction of the induced current
through the resistor R,
from
a to b or from b to a, in each of the
following situations:
a) The magnetic field is directed into the plane of the paper
and is increasing.
Ans. __________________
b) The magnetic field is directed out of plane of the paper
and is decreasing.
Ans. ________________
c) The magnetic field is directed into the plane of the paper
and the loop is quickly rotated 90°, so its plane becomes parallel to the
field.
Ans. __________________
(16 pts) 5. Consider the circuit sketched below:
a) Switch S1 is closed with S2 kept open.
Just after S1 is closed what is
i) the potential vac across the resistor?
Ans. _________________
ii) the rate
i/t
at which the current through the resistor is
increasing?
Ans. ________________
b) After switch S1 has been closed for a long time, S2
is closed and S1
is opened. Just after S2 is closed what is
i) the potential vac across the resistor?
Ans. _________________
ii) the magnitude
|i/t|
of the rate at which the current
through the resistor is decreasing?
Ans. ___________________
(18 pts) 6. Consider the ac circuit sketched below. Note that there is no
inductor in the circuit. The source has voltage amplitude 180 V and
angular frequency 50 rad/s. The resistor has resistance
600 
and the capacitor has capacitance
C = 5.0 × 10-5 F.
a) Draw the phasor diagram for this circuit.
b) Does the source voltage lag or lead the current?
Ans. _______________
c) Calculate the current amplitude.
Ans. _________________
d) What is the voltage amplitude across the capacitor?
Ans. ___________________
e) Does the voltage amplitude across the capacitor lag or lead the current?
Ans. _________________
f) An inductor with inductance L is to be added in series to this circuit.
For what value of L will the resulting current amplitude be a maximum?
Ans. _______________