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.


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


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


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


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?



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.


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


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

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


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


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


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


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

(T)   True
(F)   False


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


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


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


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°


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


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


In what direction is the wave propagating?

(a)   +x
(b)   -x
(c)   +y


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


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


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


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

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


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


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


The person is driving

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