Physics 202 MWF 10:20 Spring 1996 (Ford)
Exam III Chapts. 31-34 in SZY College Physics 7e


Show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(16 pts) 1. Two long, straight, parallel wires are 0.60 m apart. The lower wire (wire #1) has a current I₁ = 4.0 A that is in the direction from left to right, as shown in the sketch. Point P is 0.40 m above wire #1.

a) What are the magnitude and direction of the magnetic field B₁ at point P due to wire #1? (Show the direction of B₁ in the sketch.)

Ans. B₁ = __2 × 10^-6 T__

direction __out of paper__

b) What must be the magnitude and direction of the current I₂ in wire #2 for the resultant field at point P due to both wires to be zero?

Ans. I₂ = __2.0 A__

direction (also show on sketch) __left to right__


(18 pts) 2. Consider the circuit sketched below:

a) Switch S₁ is closed with S₂ kept open. Just after S₁ is closed what is

i) the potential v_ac across the resistor?

Ans. __0__

ii) the rate i/t at which the current through the resistor is increasing?

Ans. __250 A/s__

b) After switch S₁ has been closed for a long time, S₂ is closed and S₁ is opened. Just after S₂ is closed what is

i) the potential v_ac across the resistor?

Ans.__100 V__

ii) the magnitude |i/t| of the rate at which the current through the resistor is decreasing?

Ans. __250 A/s__


(16 pts) 3. An L-C circuit consists of an inductor with L =0.800 H and a capacitor with C = 6.00 × 10^-4 F. The initial charge on the capacitor is 5.00 µC and the initial current in the inductor is zero.

a) During the current oscillations, what is the maximum energy stored in the inductor?

Ans. __2.08 × 10^-8 J__

b) When the charge on the capacitor is 3.00 µC, what is the energy stored in the inductor?

Ans. __1.33 × 10^-8 J__


(14 pts) 4. Determine the direction of the current induced in circuit II in each of the following situations. (State whether the induced current passes through the resistor R₂ in the direction a to b or the direction b to a.)

a) Switch S is closed.

Ans. __a to b__

b) With switch S remaining closed, coil II is moved farther away from coil I.

Ans. __b to a__


(18 pts) 5. In the sketch below the rod ab makes contact with metal rails that are 0.50 m apart. The resistance of the rail plus rod circuit is 5.0 and may be considered to be constant. There is a uniform magnetic field with magnitude B = 2.0 T and direction out of the plane of the paper. The rod is pulled to the left at a constant speed of v = 0.40 m/s.

a) Is the induced current in the circuit clockwise or counterclockwise? (Show the direction of the induced current on the sketch.)

Ans. __clockwise__

b) What is the magnitude of the induced current?

Ans. __0.08 A__

c) What external force F_ext is required to move the rod at constant speed?

Ans. __0.08 N__


(18 pts) 6. In an L-R-C series circuit R = 600 , L = 0.200 H, and C = 8.00 × 10^-8 F. The voltage amplitude of the source is V = 120 V. The source is operated at the resonant angular frequency of the circuit.

a) What is the current amplitude?

Ans. __0.200 A__

b) What is the voltage amplitude V_R of the voltage across the resistor?

Ans. __120 V__

c) What is the voltage amplitude V_L of the voltage across the inductor?

Ans. __316 V__

d) What is the phase angle ø of the source voltage with respect to the current?

Ans. __0°__