Physics 202 MWF 10:20 Spring 1999 (Ford)

Exam I Chapts. 18--20 in Cutnell and Johnson 4e

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

(18 pts) 1. Point A is at a potential of +250 V. A small particle with charge q = -3.0 × 10-3 C has kinetic energy 0.50 J at point A and 0.90 J of kinetic energy at point B. The electric force is the only force on the particle. What is the potential of point B?

Ans. __383 V___


(22 pts) 2. Point charges q1 = -4.0 × 10-9 C and q2 = +6.0 × 10-9 C are held fixed on the x-axis as shown in the sketch. Point P is at the origin.

a) What are the magnitude and direction of the resultant electric field at point P produced by these two charges?

Ans. E = ____300 N/C____

direction __to right (+x)__________

b) A third point charge q3 = -8.0 × 10-9 C is placed at point P. What are the magnitude and direction of the resultant force on q3 exerted by the electric field of the other two charges?

Ans. F = ___2.4 × 10-6 N____

direction ___to left (-x)_____


(18 pts) 3. Two spherical shells have a common center. A charge q1 = 2 × 10-6 C is spread uniformly over the inner shell and charge q2 = - 5 × 10-6 C is spread uniformly over the outer shell. The inner shell has radius r1 = 0.10 m and the outer shell has radius r2 = 0.20 m.

Find the magnitude of the electric field at a distance from the common center of

a) 0.05 m (inside both shells)

Ans. ____0____

b) 0.15 m (between the shells)

Ans. ___8.0 × 105 N/C______

c) 0.25 m (outside both shells)

Ans. ___4.32 × 105 N/C____


(22 pts) 4. Three capacitors are connected to a battery as shown in the sketch. Sufficient time has passed for all the capacitors to be fully chrged. Capacitor C1 has charge q1 = 9.0 × 10-6 C. What is

a) the charge q2 on C2?

Ans. ____9.0 × 10-6____

b) the charge q3 on C3?

Ans. ___22.5 × 10-6 C_____

c) the voltage V of the battery?

Ans. ___4,5 V____


(20 pts) 5. In the circuit shown in the sketch what is the resistance R and the emf ? Note that two currents are given.

Ans. R = ___5.0 ___________

= ___ 42 V___