### Spring 2007 Physics 102 Hour Exam 2(27 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 109. The exam period was 90 minutes; the mean score was 73.6; the median was 75. Click here to see page1 page2 of the formula sheet that came with the exam.

• A physics 102 light bulb acts just like a resistor: its resistance is constant, independent of the current flowing through the light bulb. The bulb's brightness increases with increasing current.
• 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.

### QUESTION 1*

This and the following two questions pertain to the diagram below showing two current-carrying wires.

Two currents are flowing in the same direction (out of the page) as shown.

What is the magnitude of magnetic field at wire #1 due to the current in wire #2?

(a)   8 × 10-8 T
(b)   4 × 10-8 T
(c)   2.5 × 10-7 T

### QUESTION 2***

What is the direction of magnetic force acting on wire #1?

(a)   up
(b)   down
(c)   out of the page
(d)   left
(e)   right

### QUESTION 3***

If the current flowing through both wires doubles to 0.2 A each, how would the magnitude of magnetic force acting on wire #1 change?

(a)   It will double.
(b)   It will increase by a factor of four.

### QUESTION 4*

This and the next question pertain to the diagram below.

A charged particle of mass m = 0.001 kg enters a region of constant magnetic field (10 T, out of the page) and exits after following a trajectory as shown. Its location in the horizontal direction is displaced by d = 0.2 meter when it exits compared to its location upon entry. The speed of the particle before it entered the region of constant magnetic field is 100 m/s.

What is the value of the charge?

(a)   +0.1 C
(b)   +0.02 C
(c)   -0.003 C
(d)   -0.02 C
(e)   -0.1 C

### QUESTION 5*

What is the speed of the particle after it exits the region of constant magnetic field?

(a)   100 m/s
(b)   200 m/s
(c)   50 m/s

### QUESTION 6**

This and the next question pertain to the diagram below.

A square loop (L = 0.1 m for each side) is placed in a region of constant magnetic field (B = 0.1 T, to the right). The plane of the loop makes an angle of 30° with respect to horizontal as shown. A current (I = 0.1 A) is flowing in the loop in the direction illustrated.

What is the magnitude of torque applied to this loop?

(a)   8.7 × 10-5 N-m
(b)   5.0 × 10-5 N-m
(c)   2.9 × 10-5 N-m

### QUESTION 7**

Which direction would the loop be rotated by the torque?

(a)   clockwise
(b)   counterclockwise

### QUESTION 8***

A long wire carries current (I = 0.5 A as shown). An electron is moving at the speed of 2000 m/s at a distance 10 cm from the current carrying wire, making an angle of 45° relative to the wire. The magnitude of the electron's charge is |q|=1.6 × 10-19 C and its mass is 9.1 × 10-31 kg.

What is the magnitude of the magnetic force on the electron?

(a)   2.3 × 10-22 N
(b)   3.2 × 10-22 N
(c)   5.1 × 10-22 N
(d)   6.5 × 10-22 N
(e)   8.4 × 10-22 N

### QUESTION 9**

This and the next question refer to the figure below.

An "American Idol" contestant ends her performance by singing a perfect C# into a microphone. Her microphone works as shown in the figure below: a magnet attached to a diaphragm moves back and forth in response to the sound waves, moving in and out of a coil.

The magnet oscillates back and forth in response to her voice such that the magnetic flux though the coil changes in time as shown in the plot at right.

Which one of the following graphs most accurately describes the emf in the coil?

(a)
(b)
(c)

### QUESTION 10*

If the number of turns in the coil were doubled, what would happen to the emf?

(a)   It would double.
(b)   It would remain the same.
(c)   It would be halved.

### QUESTION 11*

This and the next two questions refer to the figure below.

Two ideal transformers are connected as shown below. The first transformer (on the left) has 5 turns on the primary and 10 turns on the secondary, and the second transformer (on the right) also has 5 turns on the primary and 10 turns on the secondary. A 100 Ω resistor is placed across the secondary of the second transformer. A voltage source supplies a 60 Hz sinusoidal voltage with a maximum of 10 V. Note that the voltage at the secondary of the first transformer is the same as the voltage at the primary of the second transformer.

Calculate the maximum voltage across points a and b on the diagram.

(a)   5 V
(b)   10 V
(c)   20 V

### QUESTION 12*

What is the maximum current passing through the 100 Ω resistor on the diagram?

(a)   0.1 A
(b)   0.2 A
(c)   0.4 A
(d)   0.8 A
(e)   1.6 A

### QUESTION 13**

Compare the power supplied by the voltage source to the power dissipated in the resistor. Which one of the following statements is correct?

(a)   Psource  >  Presistor
(b)   Psource  =  Presistor
(c)   Psource  <  Presistor

### QUESTION 14***

This and the next question refer to the figure below.

A generator consists of a coil with 10 turns and an area of 0.01 m2 spinning in a uniform magnetic field of magnitude 0.1 T. The generator drives a light bulb with a resistance of 200 Ω.

The current in the light bulb cannot exceed 0.5 A; otherwise it burns out. What is the maximum rate at which the coil can spin so that the current does not exceed this limit?

(a)   1 × 104 revolutions/second
(b)   1 × 105 revolutions/second
(c)   1.6 × 103 revolutions/second
(d)   1.6 × 104 revolutions/second
(e)   3.2 × 104 revolutions/second

### QUESTION 15***

Which diagram correctly shows the direction of current flow in the coil?

(a)
(b)

### QUESTION 16***

This and the next question refer to the figure below.

A solenoid is connected to a battery with a switch. The ends of a second solenoid are connected to an ammeter.

The switch in the first solenoid is closed. In what direction does the current flow in the ammeter connected to the second solenoid immediately after the switch is closed?

(a)   to the right
(b)   to the left

### QUESTION 17**

What is the magnitude of the current in the second solenoid a long time after the switch is closed?

(a)   the same as in the previous question
(b)   zero

### QUESTION 18*

This and the next question pertain to a machine that fabricates inductors.

A machine that manufactures inductors is adjusted to make cylindrical coils of diameter 5 mm and length 20 mm. Each inductor is wound with 300 turns of wire.

What is the inductance of an inductor built with these properties?

(a)   0.016 μH
(b)   111.0 μH
(c)   325.0 μH

### QUESTION 19*

The machine is now reprogrammed so that it manufactures inductors of the same length and diameter, but with 600 turns of wire instead of 300. How do the inductances L300 and L600 of the old and new inductors compare?

(a)   L300 / L600 = 0.25
(b)   L300 / L600 = 0.50
(c)   L300 / L600 = 1.00
(d)   L300 / L600 = 2.00
(e)   L300 / L600 = 4.00

### QUESTION 20*

A sophisticated power supply ramps the current flowing through an inductor linearly so that the current is I(t) = I0t for t > 0. (No current flows when t < 0.) The current I(t) is shown in the graph at right.

Which one of the following graphs best represents the energy U(t) stored in the inductor?

(a)
(b)
(c)
(d)
(e)

### QUESTION 21*

An alternating current generator drives a sinusoidal signal through the circuit element hidden inside a box as shown in the following figure.

The generator is programmed to keep the amplitude of the current constant, but to double the frequency at a preset time. An oscilloscope is used to observe the voltage across the mystery circuit element.

The current I(t) provided by the AC generator, before and after the frequency is doubled (but the amplitude is held constant), is shown in the following graphs.

The voltage V(t) across the mystery object (as shown by the oscilloscope) is indicated in the following diagrams. Note that the voltage amplitude doubles.

The mystery object hidden inside the box is probably

(a)   a resistor.
(b)   an inductor.
(c)   a capacitor.

### QUESTION 22**

An oscilloscope shows the voltage drops as functions of time across two of the elements in an LRC circuit as shown in the figure. The upper oscilloscope trace displays the voltage drop across the resistor: VR = V1 - V2.

The second oscilloscope trace labeled Vx - Vy displays

(a)   the voltage drop across the capacitor: VC = V2 - V3.
(b)   the voltage drop across the inductor: VL = V3 - V4.
(c)   the power dissipated by the capacitor.
(d)   the total reactance (impedance) of the circuit.
(e)   the energy stored in the inductor.

### QUESTION 23*

This and the following two questions pertain to the following circuit.

An AC generator in the LRC circuit shown in the following figure produces a voltage V(t) = 1.414sin(ωt)=1.414sin(1000t). The values of inductance, capacitance, and resistance are shown on the diagram. Recall that ω = 2πf.

The rms voltage supplied by the AC generator is

(a)   0.50 V
(b)   0.707 V
(c)   1.00 V

### QUESTION 24**

The maximum current in the circuit is

(a)   141 mA
(b)   12 mA
(c)   2.71 mA
(d)   377 mA
(e)   105 mA

### QUESTION 25*

The phase difference Φ between the AC generator voltage and the current flowing in the circuit is

(a)   -42.0°
(b)   -100°
(c)   +76.2°
(d)   -12.1°
(e)   +90°

### QUESTION 26**

This and the next two questions refer to the following circuit.

An LRC circuit and a voltage phasor diagram for this circuit are shown in the following figure. The period T = 2π/ω of the voltage produced by the AC generator is 10 μs.

At the instant in time represented by the phasor, across which circuit element is the largest voltage difference to be found?

(a)   inductor
(b)   resistor
(c)   capacitor

### QUESTION 27**

Which one of the following phasor diagrams best represents the state of circuit 5 μs after the time corresponding to the above phasor diagram?

(a)
(b)
(c)
(d)
(e)

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.