Spring 2008 Physics 102 Hour Exam 2(28 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 121. The exam period was 90 minutes; the mean score was 97.4; the median was 98. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

• 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 next four questions pertain to the the following situation.

A conducting rod is sliding at a velocity v = 0.12 m/s on conducting rails, such that a rectangular loop is formed. At time t = 0 the rod is at position l = 0.5 m (see picture). The entire device is placed in a uniform magnetic field B = 0.45 T directed into the page. The width w = 0.4 m. The conducting rails are long.

What is the magnetic flux Φ through the conducting loop at time t = 0 ?

(a)   Φ = 0 T-m2
(b)   Φ = 0.09 T-m2
(c)   Φ = 0.198 T-m2
(d)   Φ = 0.45 T-m2
(e)   Φ = 12 T-m2

QUESTION 2*

What is the magnetic flux Φ through the conducting loop at time t = 5 sec?

(a)   Φ = 0 T-m2
(b)   Φ = 0.09 T-m2
(c)   Φ = 0.198 T-m2
(d)   Φ = 0.45 T-m2
(e)   Φ = 12 T-m2

QUESTION 3*

What is the induced EMF ε in the loop at time t = 5 sec?

(a)   ε = 0 V
(b)   ε = 22 mV
(c)   ε = 40 mV
(d)   ε = 0.45 V
(e)   ε = 12 V

QUESTION 4**

What is the direction of the induced current I in the loop at time t = 5 sec?

(a)   clockwise
(b)   counter-clockwise
(c)   No current is induced.

QUESTION 5**

All of the conductors in the problem are made of a wire with resistance λ = 5 Ω/m (Ohm per meter). What is the induced current I in the loop at time t = 5 sec?

(a)   I = 0 A
(b)   I = 0.04 mA
(c)   I = 0.43 mA
(d)   I = 1.4 mA
(e)   I = 0.2 A

QUESTION 6*

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

A circular conducting loop is placed next to a very long straight wire carrying current I (see picture). The loop and the wire lie in a plane.

What is the direction of the induced current in the loop in each of the following cases?

The loop is held in its place:

(a)   No EMF is induced, hence no loop current.
(b)   clockwise
(c)   counter-clockwise

QUESTION 7**

The loop is moved left (towards the wire) at a velocity v:

(a)   No EMF is induced, hence no loop current.
(b)   clockwise
(c)   counter-clockwise

QUESTION 8**

The loop is moved up (towards the top of the page) at a velocity v:

(a)   No EMF is induced, hence no loop current.
(b)   clockwise
(c)   counter-clockwise

QUESTION 9**

The current in the wire is gradually increased from I to 2I:

(a)   No EMF is induced, hence no loop current.
(b)   clockwise
(c)   counter-clockwise

QUESTION 10**

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

Given are a uniform electric field, E = 30,000 V/m inside a capacitor (see picture), and a uniform magnetic filed, B = 10 T, perpendicular to the plane of the page, present in all of the area.

Identical charged particles are moving in a trajectory marked by the dashed line, under the influence of the two fields: The trajectory inside the capacitor is a straight line, and the trajectory outside the capacitor is circular, with radius r = 1 cm.

What is the direction of the magnetic field B?

(a)   into the page
(b)   out of the page
(c)   Cannot be determined from the question.

QUESTION 11*

What is the velocity v of the charged particles?

(a)   v = 0.01 m/s
(b)   v = 195 m/s
(c)   v = 3000 m/s
(d)   v = 105 m/s
(e)   v = 3 × 108 m/s

QUESTION 12*

The charge of each particle is |q| = 3 × 10- 19 C. What is the momentum p = mv?

(a)   p = 5.2 × 10-31 kg-m/s
(b)   p = 2.2 × 10-27 kg-m/s
(c)   p = 3.0 × 10-20 kg-m/s
(d)   p = 1.6 × 10-3 kg-m/s
(e)   p = 0.1 kg-m/s

QUESTION 13*

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

A particle of charge +q moves at a velocity v in an area with uniform magnetic field B, directed at the +y direction. What is the direction of the particle’s acceleration a in each of the following cases:

What is the direction of the particle’s acceleration a in each of the following cases?

v is in the +x direction:

(a)   a = 0
(b)   a is in the +x direction.
(c)   a is in the +y direction.
(d)   a is directed into the page.
(e)   a is directed out of the page.

QUESTION 14*

v is in the -y direction:

(a)   a = 0
(b)   a is in the +x direction.
(c)   a is in the -y direction.
(d)   a is directed into the page.
(e)   a is directed out of the page.

QUESTION 15*

v is directed out of the page:

(a)   a = 0
(b)   a is in the -x direction.
(c)   a is in the +x direction.
(d)   a is in the -y direction.
(e)   a is directed out of the page.

QUESTION 16*

A rectangular coil of 12 turns has sides a = 0.2 m and b = 0.1 m. The coil rotates about the y axis with angular velocity ω = 20 rad/s in a uniform magnetic field Bx = 0.05 T (By = Bz = 0). A resistor is connected in series with the coil as shown in the diagram.

What is the maximum voltage Vmax produced across the resistor by the coil?

(a)   Vmax = 10 mV
(b)   Vmax = 50 mV
(c)   Vmax = 240 mV
(d)   Vmax = 300 mV
(e)   Vmax = 320 mV

QUESTION 17*

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

An ideal transformer with 5 turns on the primary and 10 turns on the secondary is shown in the figure. A sinusoidal voltage source having V(t) = 10 sin(ωt) [in volts] is connected across the primary and a 1 Ω resistor is connected across the secondary. ω = 100 rad/s.

What is the maximum value of the voltage Vmax in the secondary circuit?

(a)   Vmax = 5 volts
(b)   Vmax = 10 volts
(c)   Vmax = 20 volts

QUESTION 18**

What is the maximum value of the current Imax in the primary circuit (not the secondary)?

(a)   Imax = 2.0 amp
(b)   Imax = 5.0 amp
(c)   Imax = 10.0 amp
(d)   Imax = 20.0 amp
(e)   Imax = 40.0 amp

QUESTION 19*

Let PP be the power provided in the primary circuit and PS be the power dissipated in the secondary circuit. Which one of the following statements is true?

(a)   PP > PS
(b)   PP = PS
(c)   PP < PS

QUESTION 20*

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

A generator is built by winding 30 turns around in a circle and then attaching it to a horizontal motor-driven shaft as shown in the figure below. The loop rotates in a uniform vertical magnetic field B.

When the orientation of the loop is as shown in the figure, the flux of magnetic field through the loop is at its maximum value.

(T)   True
(F)   False

QUESTION 21*

When the orientation of the loop is as shown in the figure, the induced current in the loop is at its maximum value.

(T)   True
(F)   False

QUESTION 22**

When the orientation of the loop is as shown in the figure, the torque on the loop due to the magnetic field is at its maximum value.

(T)   True
(F)   False

QUESTION 23*

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

A circular conducting loop with resistance of 5 Ω and diameter d = 0.4 m surrounds a long solenoid magnet. The solenoid consists of a coil with 2 × 104 turns/meter wrapped around a hollow cylinder of radius R = 0.15 m. The current in the solenoid is flowing in the counterclockwise direction producing a uniform magnetic field inside the solenoid of B = 0.75 T.

From the end view, the magnetic field at the center of the solenoid is pointing

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

QUESTION 24*

Calculate the magnitude of the current Is flowing in the solenoid.

(a)   Is = 9.5 A
(b)   Is = 16.4 A
(c)   Is = 29.8A
(d)   Is = 58.7 A
(e)   Is = 68.2 A

QUESTION 25***

Now the current in the solenoid is steadily decreased to zero over a period of 5 seconds.

Calculate IL the magnitude of the average induced current in the outside loop while the current in the solenoid is being decreased. (Be careful all of the answers can be obtained using numbers given in the problem.)

(a)   IL = 2.12 × 10-3 A
(b)   IL = 2.46 × 10-3 A
(c)   IL = 3.18 × 10-3 A
(d)   IL = 3.76 × 10-3 A
(e)   IL = 4.30 × 10-3 A

QUESTION 26**

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

Three long parallel wires carry currents, equal in magnitude (I1 = I2 = I3 = 3 A) in the directions as shown. The wires all lie in a plane.

What is the magnitude of the magnetic field B at point P (0.5 meter below wire #1)?

(a)   B = 9.2 × 10-6 T
(b)   B = 8.5 × 10-6 T
(c)   B = 5.3 × 10-6 T
(d)   B = 3.7 × 10-6 T
(e)   B = 1.5 × 10-6 T

QUESTION 27*

What is the direction of the magnetic field at this point?

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

QUESTION 28**

What is the direction of the total force on wire #3 due to wire #1 and wire #2?

(a)   up
(b)   The force is zero.
(c)   down

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.