Physics 202 MWF 10:20 Spring 1994 (Ford)
Exam I Chapts. 24-27 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 small object with charge q is released from rest at point a where the electric potential is +12.0 V. The particle moves because of the electric force on it and as it passes through point b where the potential is +4.0 V its kinetic energy is 3.0 × 10-6 J. The electric force on the object is the only force on it. What is q (sign and magnitude)?

Ans. ___+3.75 × 10-7 C___


(20 pts) 2. Point charges q1 = -5.0 × 10-6 C and q2 = -2.0 × 10-6 C are 0.10 m apart, as shown in the sketch.



a) What are the magnitude and direction of the resultant electric field at point P, which is midway between the two charges?

Ans. magnitude ___1.08 × 107 N/C___

direction (to the left or to the right?) ___to the left___

b) A negative charge q3 = -4.0 × 10-6 C is placed at point P. What are the magnitude and direction of the force exerted on q3 by the resultant electric field of the other two charges?

Ans. magnitude ___43.2 N___

direction (to the left or to the right?) ___to the right___


(22 pts) 3. Point charges q1 = -2.0 × 10-6 C and q2 = +3.0 × 10-6 C are held fixed at opposite corners of a rectangle as shown in the sketch.



a) Calculate the electric potential at point a and the electric potential at point b due to these two charges.

Ans. Va = ___-2.25 × 104 V___

Vb = ___+9.00 × 104 V___

b) Calculate the work done by the electric force on a third charge when it moves from point a to point b along a diagonal of the rectangle. (You must indicate whether this work is positive or negative.) The third charge has q3 = -8.0 × 10-6 C.

Ans. ___+0.90 J___


(16 pts) 4. Positive charge is uniformly distributed throughout a spherical volume of radius R. The charge per unit volume is . (The charge is distributed in an insulator, not in a conductor.) In terms of what is the magnitude of the electric field

a) outside the volume a distance r from the center of the sphere (r>R) (to get credit for this problem you must show clearly how you obtain your answer)

Ans. ___E = R3/ 30r2___

b) inside the volume a distance r from the center of the sphere (r<R) (to get credit for this problem you must show clearly how you obtain your answer)

Ans. ___E = r/ 30___


(24 pts) 5. In the circuit shown in the sketch C1 = 4.0 µF, C2 = 8.0 µF, C3 = 5.0 µF, and C4 = 3.0 µF. The potential difference applied across the network is 90 V. Calculate the charge on each capacitor and the potential difference across each capacitor.



Ans. Q1 ___144 µC___

Q2 ___288 µC___

Q3 ___270 µC___

Q4 ___162 µC___

V1 ___36 V___

V2 ___36 V___

V3 ___54 V___

V4 ___54 V___