Physics 202 MWF 10:20 Spring 1998 (Ford)

Exam II Chapts. 28-30 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 capacitor with capacitance C = 4.0 × 10^-6 F and that is initially uncharged is connected in series with a resistor R = 120 and an emf source with = 400 V and negligible internal resistance. The switch is closed at time t = 0, completing the circuit. (Note: For a capacitor, C = Q/V.)

a) What is the current i through the resistor just after the switch is closed?

Ans. ___3.33 A___

b) At some time t after the circuit is completed the current through the resistor is 2.5 A. At this instant, what is the charge on the capacitor?

Ans. __4.0 × 10^-4 C___

c) A long time after the switch is closed, what is v_ab, the voltage across the resistor?

Ans. ___0___


(20 pts) 2. In sketch (a) the current in the circuit is I = 11 A in the direction shown. In sketch (b) the terminals of the 35 V battery have been reversed and the current is I = 3 A in the direction shown, opposite to the current direction in (a). Calculate the emf and internal resistance r₁ of battery #1.
sketch a


sketch b


Ans. r₁ = ___20.0 V___

r₁ = ___0.5 ____


(22 pts) 3. Consider the circuit sketched below. The two batteries have negligible internal resistance. The current I₁ through the resistor R₁ is 3.0 A, in the direction shown.

a) Calculate the current through each of the other two resistors. (To receive any partial credit you must clearly show the circuit loop to which you are applying Kirchhoff's loop rule, and the direction you are going around the loop.)

Ans. current through R₂ ____3.5 A____

through R₃ ___6.5 A____

b) Calculate the emf .

Ans. ____62.5 V____

c) Calculate the potential difference V_ab between points a and b.

Ans. ___52 V___

d) Which point, a or b, is at higher potential?

Ans. ___a____


(18 pts) 4. A particle with charge q and mass m = 3.00 × 10^-6 kg has velocity v = 20.0 m/s in the direction shown as it enters a region of uniform magnetic field of magnitude B = 4.0 T and direction into the plane of the paper. (The magnetic field is uniform inside the rectangular region shown in the sketch, and zero outside of this region.) The particle is found to travel in the semicircular path shown in the sketch. This path has radius R = 0.15 m.

a) Calculate the magnitude of the charge q.

Ans. q = ___1.0 × 10^-4 C____

b) Is the charge q positive or negative?

Ans. __negative___

(22 pts) 5. A uniform rectangular loop of wire has width 0.060 m and length 0.090 m, has a mass of 5.0 × 10^-3 kg, and is pivoted along one side about an axis that coincides with the z-axis of the coordinate system shown in the sketch. The current in the loop is 8.0 A, in the direction shown. A uniform magnetic field is parallel to the x-axis.

a) Find the magnitude B of the magnetic field that will cause the loop to swing up into an equilibrium position in which its plane makes an angle of 30.0° with the yz-plane.

Ans. ____0.0509 T___

b) To cause the loop to be in equilibrium in the position shown in the sketch, must the magnetic field be in the +x-direction or in the -x-direction?

Ans. __-x-direction______________