Summer 2009 Physics 102 Hour Exam 2
(22 questions)

The grading button and a description of the scoring criteria are at the bottom of this page.Basic questions are marked by a single star *. More difficult questions are marked by two stars **. The most challenging questions are marked by three stars ***.

True-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 82. The exam period was 75 minutes; the mean score was 62.8; the median was 62. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

Some helpful information:
• A reminder about prefixes: p (pico) = 10-12; n (nano) = 10-9; μ (micro) = 10-6; m (milli) = 10-3; k (kilo) = 10+3; M or Meg (mega) = 10+6; G or Gig (giga) = 10+9.


This question and the next two pertain to the the following situation.

An electron (mass 9.11 × 10-31 kg, charge 1.6 × 10-19 C) moves rightward at speed 105 m/s as it passes from a field-free region into a region where there is a magnetic field of strength B = 0.03 T. Shortly thereafter, it exits, moving leftward back into the region where B = 0.

Find L, the distance between where the electron entered the field and where it exited the field.

(a)   22 μm
(b)   26 μm
(c)   32 μm
(d)   38 μm
(e)   44 μm


How much time did the electron spend in the region where the magnetic field B = 0.03 T ?

(a)   0.38 ns
(b)   0.6 ns
(c)   1.2 ns


In which direction does this magnetic field point?

(a)   into the page
(b)   out of the page


This question and the next two pertain to the the following situation.

Consider two parallel wires, one carrying current I1 = 20 A to the right, and at a distance L = 8 cm below it, another carrying I2 = 12 A to the left.

What is the magnetic field a distance 5 cm below the top wire (3 cm above the bottom one)?

(a)   1.6 × 10-4 T, out of page
(b)   8.0 × 10-5 T, out of page
(c)   0
(d)   8.0 × 10-5 T, into page
(e)   1.6 × 10-4 T, into page


What is the net force exerted by the bottom wire on a 6 cm segment of the top wire?

(a)   3.6 × 10-5 N
(b)   4.8 × 10-5 N
(c)   6 × 10-5 N


Do the two wires attract each other?

(a)   yes, they attract each other
(b)   no, they do not attract each other


This question and the next three pertain to the the following situation.

The phasor diagram shown applies to a certain RLC circuit at some particular frequency. Note that the lengths of three vectors are labeled in volts.

At the instant shown, the voltage across the capacitor is increasing.

(T)   True
(F)   False


In this circuit, the current "leads" (peaks just before) the voltage across the function generator.

(T)   True
(F)   False


Suppose a peak current of exactly 1 amp flows in this circuit. What is its total impedance Z?

(a)   Z = 4 Ω
(b)   Z = 5 Ω
(c)   Z = 7.8 Ω
(d)   Z = 9 Ω
(e)   Z = 9.2 Ω


Assume the same phasor diagram describes another circuit, this one oscillating at a frequency f = 400 Hz. If the peak current in the circuit is 12 mA, what is the inductance?

(a)   0.13 H
(b)   0.2 H
(c)   0.3 H


A source of alternating current is attached to the primary of a transformer. A 270 Ω resistor is attached to the secondary. There are 400 turns of wire in the primary and 540 turns of wire in the secondary (the diagram may not accurately depict this). The resistor converts electrical energy to heat at an average rate of 1.9 W. What is the peak voltage in the primary?

(a)   16.8 V
(b)   22.6 V
(c)   23.7 V
(d)   30.5 V
(e)   32 V


This question and the next one pertain to the the following situation.

The solenoid shown has a radius of 3 cm and 200 turns of wire along an 18 cm axis.

What constant current would be required to produce a magnetic field B = 0.01 T in the interior of this solenoid?

(a)   7.2 A
(b)   43 A
(c)   240 A


Suppose a varying current flows, rising steadily from 0 to 300 mA over a time of 7 seconds. What is the electromotive force experienced by a single turn of the wire composing this solenoid?

(a)   4.2 × 10-8 V
(b)   8.5 × 10-8 V
(c)   1.7 × 10-7 V


This question and the next two pertain to the the following situation.

A bar is free to slide along a pair of metal tracks separated by a distance of L = 28 cm. (You may ignore friction between the bar and the tracks.) At one end, the tracks are connected electrically by a resistor R = 3.9 Ω. You should assume the metal bar completes an electric circuit. A magnetic field B = 0.15 T points into the page.

If the bar is moving to the left at 8 m/s, what current flows through the resistor?

(a)   86 mA
(b)   150 mA
(c)   336 mA


How fast would the bar have to move in order for a magnetic force of 0.33 N to act on it?

(a)   0.8 m/s
(b)   2 m/s
(c)   15 m/s
(d)   140 m/s
(e)   730 m/s


In what direction does the induced current flow through the bar?

(a)   upward
(b)   downward


This question and the next two pertain to the the following situation.

In this RLC circuit, R = 180 Ω, L = 22 mH, and C = 4.7 μF. A function generator whose operating frequency can be adjusted applies an alternating voltage.

Suppose the maximum generator voltage Vmax = 20 V when f = 290 Hz. What rms current flows through this circuit?

(a)   0.07 A
(b)   0.1 A
(c)   0.15 A


Call VC,rms the rms voltage across the capacitor, and Irms the rms current. What is the ratio


(again assuming f = 290 Hz)?

(a)   8.6 × 10-3 Ω
(b)   120 Ω
(c)   860 &Omega


Imagine the resistor were a light bulb. You operate the function generator at a frequency f = 290 Hz, and stare blankly at the bulb. Is this bulb glowing as brightly as possible? Or would adjusting the frequency make it glow brighter? (Assume that you can vary the frequency without affecting the voltage.)

(a)   It glows most brightly at f = 290 Hz.
(b)   Must reduce frequency to make it glow brighter.
(c)   Must increase frequency to make it glow brighter.


This and the next two questions pertain to the the following situation.

Three turns of wire are wound in a square of side 41 cm. The wire itself has negligible resistance, but completes a circuit across an external resistance of 12 Ω. By connecting the loop to a regulated motor, you can get it to spin at a constant angular frequency in a magnetic field of strength B = 0.9 T. (The magnetic field points out of the page, as shown). At the instant shown, the loop lies in a plane perpendicular to the magnetic field, with the right side of the wire loop moving out of the page. The maximum current is observed to be 100 mA, though not necessarily at the instant shown.

What torque does the magnetic field exert on the loop when the current I = 100 mA?

(a)   1.0 × 10-3 Nm
(b)   2.1 × 10-3 Nm
(c)   1.8 × 10-2 Nm
(d)   4.5 × 10-2 Nm
(e)   2.1 × 10-1 Nm


Find ω, the (constant) angular velocity of the spinning loop

(a)   2.6 rad/sec
(b)   3.3 rad/sec
(c)   7.6 rad/sec
(d)   5.1 rad/sec
(e)   7.9 rad/sec


After one quarter cycle, the right side of the loop will be closest to us (and the left side will have moved behind it, deeper into the page). At that instant, in which direction does current flow in that side closest to us?

(a)   At that instant, current will flow down along that side.
(b)   At that instant, current will flow upward along that side.
(c)   At that instant, no current flows in the loop.n