Physics 202 MWF 10:20 Spring 1994 (Ford)
Exam II Chapts. 28-31 in SZY College Physics 7e



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


(18 pts) 1. A 4.00 × 10^-6 F capacitor that is initially uncharged is connected in series with a resistor R and an emf source with = 400 V and negligible internal resistance.

a) Just after the connection is completed the current through the resistor is 5.0 A. What is the resistance R of the resistor?

Ans. __80 __

b) After time t the current through the resistor has dropped to 2.0 A. At this time what is the voltage across the capacitor?

Ans. __240 V__

c) After a long time what is the charge q on the capacitor?

Ans. __0.0016 C__


(21 pts) 2. In the circuit sketched below the current through the 4 resistor is 5 A in the direction shown.

a) What is the current through the 3 resistor? Is this current in the direction from a to b or b to a?

Ans. I = __5 A__

direction __a to b__

b) What is the emf ?

Ans. __40 V__

c) What is the potential difference V_ab? Which point is at higher potential, point a or point b?

Ans. V_ab __20 V__

the point that is at higher potential is point __a__


(20 pts) 3. You have a galvanometer that has resistance 50 and that requires a current of 0.4 A for full-scale deflection.

a) What is the shunt resistance required to convert the galvanometer to an ammeter reading 2.00 A full-scale?

Ans. __12.5 __

b) What is the current through the resistance R in the circuit below?

Ans. __2 A__

c) The ammeter from part (a) is placed into the circuit as shown. What then is the current through the resistance R?

Ans. __1.5 A__


(20 pts) 4. A long, straight wire carries a current of 6.0 A in the direction shown. An electron (charge q = -1.60 × 10^-19 C and mass m = 9.11 × 10^-31 kg) is moving near the wire with a speed of v = 4.0 × 10⁵ m/s.

a) At the location of the electron, 0.020 m above the wire, what are the magnitude and direction (up, down, right, left, into paper, out of paper) of the magnetic field produced by the current in the wire?

Ans. B = __6.0 × 10^-5 T__

direction __into paper__

b) At the instant that the electron is moving away from the wire, what are the magnitude and direction (up, down, right, left, into paper, out of paper) of the force exerted on the electron by the magnetic field of the wire?

Ans. F = __3.84 × 10^-18 N___________

direction __right__


(21 pts) 5. A uniform rectangular loop of wire has mass 0.020 kg and dimensions 2 cm by 6 cm. The loop is pivoted about side ab as a frictionless axis. There is a uniform magnetic field of 0.60 T in the +y-direction. The loop is in equilibrium when its plane makes an angle of 37° with the yz plane.

a) What is the magnitude of the torque about the axis along ab due to the gravity force on the loop?

Ans. __3.54 × 10^-3 N__

b) What is the current in the loop? Is the direction of this current from a to b or from b to a?

Ans. I = __6.16 A__

direction __a to b__