### Spring 2009 Physics 212 Hour Exam 3(25 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 99. The exam period was 90 minutes; the average score was 85.2; the median score was 88. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1*

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

The switch is initially open and the capacitor initially carries a charge Q0 = +10 μC on the top plate and -Q0 on the bottom plate. At t = 0 the switch is closed.

What is the maximum current Imax in this LC circuit? (Note: 1 mA = 10-3 A)

(a)   Imax = 1 mA
(b)   Imax = 5 mA
(c)   Imax = 8 mA
(d)   Imax = 10 mA
(e)   Imax = 12 mA

### QUESTION 2**

At what time t1 is the maximum current reached for the first time? (1 ms = 10-3 s)

(a)   t1 = 6.15 ms
(b)   t1 = 12.57 ms
(c)   t1 = 9.36 ms
(d)   t1 = 3.14 ms
(e)   t1 = 1.48 ms

### QUESTION 3*

Calculate the rate at which the current is changing when the charge on the capacitor is 5 μC.

(a)   dI / dt = 1.3 A/s
(b)   dI / dt = 2.0 A/s
(c)   dI / dt = 5.0 A/s

### QUESTION 4*

This and the next question refer to this circuit:

The current as a function of time is shown in the right- hand side figure.

Which one of the following graphs best describes the voltage across the inductor VL(t) = VB - VA as a function of time?

(a)
(b)
(c)

### QUESTION 5**

If the frequency of the generator increases (while holding εmax constant), the maximum voltage across the resistor

(a)   increases.
(b)   remains the same.
(c)   decreases.

### QUESTION 6*

This question and the next three refer to this situation:

At what frequency f = ω / 2π is Irms maximum?

(a)   0 Hz
(b)   3.55 × 10-5 Hz
(c)   712 Hz
(d)   4.47 × 103 Hz
(e)   3.18 × 106 Hz

### QUESTION 7*

Fill in the blank: the current through the capacitor ______ the voltage across the resistor.

(a)   leads.
(b)   lags.
(c)   is in phase with.

### QUESTION 8*

The above circuit, with a variable capacitor, is used in your radio. Station 1 and station 2 emit at frequencies separated by 0.2 MHz. You have trouble receiving one station without getting pickup from the other. One way to get better discrimination would be to decrease the resistance R in the circuit.

(T)   True
(F)   False

### QUESTION 9*

If the phasor diagram for this circuit looks like the figure at right, is ω greater than, less than, or equal to the resonance frequency ωo?

(a)   ω > ωo
(b)   ω < ωo
(c)   ω = ωo

### QUESTION 10*

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

What is the rms power provided to the circuit?

(a)   <P(t)> = 330 W
(b)   <P(t)> = 450 W
(c)   <P(t)> = 570 W
(d)   <P(t)> = 621 W
(e)   <P(t)> = 660 W

### QUESTION 11*

By changing the resistance R to an appropriate value, it is possible to make Φ positive in the above circuit.

(T)   True
(F)   False

### QUESTION 12*

A sinusoidal plane-polarized radio wave transports energy through air at a (mean) rate of I = 1 W/m2. Find the amplitudes E0 and B0 of the electric and magnetic fields respectively.

(a)   E0 = 27.5 V/m and B0 = 9.2 × 10-8 T
(b)   E0 = 1 V/m and B0 = 3 × 10-9 T
(c)   E0 = 1 V/m and B0 = 1 T
(d)   E0 = 13.7 V/m and B0 = 17.6 T
(e)   E0 = 34.3 V/m and B0 = 1.1 × 10-7 T

### QUESTION 13*

This and the next two questions refer to this situation:

The primary coil of an ideal transformer is connected to an AC voltage source as shown in the figure below. There are 100 turns in the primary coil and 20 turns in the secondary coil.

If the voltage drop measured across the resistor is V2 = 50 V, what is the voltage provided by the AC generator?

(a)   V1 = 10 V
(b)   V1 = 50 V
(c)   V1 = 250 V

### QUESTION 14*

If the load resistor R is reduced by a factor of 2, the current I1 in the primary will increase by a factor of

(a)   1.2
(b)   2
(c)   10

### QUESTION 15*

This and the next question refer to this situation:

A beam of light, initially polarized along the y-axis, passes through a linear polarizer and then through a quarter wave plate, aligned as shown in the diagram.

What is the polarization state of the light that emerges from the quarter wave plate?

(a)   circularly polarized
(b)   linearly polarized at 45°
(c)   linearly polarized at -45°

### QUESTION 16*

If the intensity of the incoming beam before it hits the polarizer is I, what is the intensity I2 of the outgoing beam after it leaves the quarter wave plate?

(a)   I2 = 0
(b)   I2 = I
(c)   I2 = I / 2
(d)   I2 = I / 4
(e)   I2 = I / √2

### QUESTION 17*

This and the next three questions concern the following situation:

A plane electromagnetic wave propagates in vacuum. The figure below represents a snapshot at time t = 0 of the electric field associated with this wave, which is described by E = E0 sin(ωt-kx) y with E0 = 1000 V/m. The frequency is f = 300 kHz. Points P and Q are along the x-axis.

Which one of the following options correctly relates BP, the magnitude of the magnetic field at point P at time t = 0, to BQ, the magnitude of the magnetic field at point Q at time t = 0?

(a)   BP = BQ = 0
(b)   BP > BQ = 0
(c)   BP = BQ > 0

### QUESTION 18*

In what direction is the wave propagating?

(a)   +x
(b)   -x
(c)   +y

### QUESTION 19*

If the amplitude of the magnetic field associated with this electromagnetic wave is doubled, the intensity I of this electromagnetic wave changes by a factor of:

(a)   2
(b)   1/2
(c)   4

### QUESTION 20*

The wavelength λ associated with this electromagnetic wave is:

(a)   λ = 0.1 m
(b)   λ = 1.0 m
(c)   λ = 100 m
(d)   λ = 1000 m
(e)   λ = 10000 m

### QUESTION 21*

This and the next question concern the following situation:

A beam of unpolarized light of intensity I0 travels in the +z-direction and goes through an arrangement of three linear polarizers as shown in the figure below. The planes of all polarizers are parallel to the xy plane.

Find the ratio between the final intensity If and initial intensity I0.

(a)   If / I0 = 0.16
(b)   If / I0 = 0.28
(c)   If / I0 = 0.38
(d)   If / I0 = 0.42
(e)   If / I0 = 0.75

### QUESTION 22*

If the second and third polarizers are interchanged, the final intensity will

(a)   increase.
(b)   remain the same.
(c)   decrease.

### QUESTION 23*

An inductor initially has 3 A of current passing through it at time t = 0. The current through the inductor increases at a constant rate until 5 A of current is flowing through it 1s later at t = 1 s. The voltage across the inductor is 4 μV while the current is increasing. What is the increase in the energy stored in the inductor between t = 0 and t = 1 s?

(a)   9 μJ
(b)   16 μJ
(c)   18 μJ
(d)   25 μJ
(e)   32 μJ

### QUESTION 24*

This and the next question concern the following situation:

A person is driving in such a way that a red (600 nm) light appears to be green (500 nm).

How fast is the person driving?

(a)   2.7 × 107 m/s
(b)   5.4 × 107 m/s
(c)   1.6 × 109 m/s

### QUESTION 25*

The person is driving

(a)   toward the light.
(b)   away from the light.

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.