Physics 202 MWF 10:20 Spring 2000 (Ford)

Exam I Chapts. 18--19 in Cutnell and Johnson 4e

Multiple choice conceptual questions. Circle the correct answer. You do not need to show any work and no partial credit will be given.

(4 pts) 1. A hollow spherical conductor has a net charge of -5q. What is the total charge on the exterior surface of the sphere when a point charge of +2q is placed in the hollow space at the center of the sphere?

a) -2q
b) -3q <--------- answer
c) -7q
d) +2q
e) +3q
f) +7q


(3 pts) 2. An electron (charge -e) is in a uniform electric field. The electric field is directed due east. What is the direction of the force that the field exerts on the electron?

a) north
b) south
c) east
d) west <--------- answer


(4 pts) 3. The electric field lines in a region of space are parallel and directed to the right, as shown in the sketch. Points A and B are shown. Which of the following statements about the electric potential is true:

a) V_A > V_B <--------- answer
b) V_B > V_A
c) V_A = V_B


(3pts) 4. Two conducting spheres are connected by a conducting wire. The radius of sphere A is twice that of sphere B, r_A = 2r_B. If the potential at the surface of sphere A is +200 V, what is the potential at the surface of sphere B?

a) +25 V
b) +50 V
c) +100 V
d) +200 V <--------- answer
e) +400 V
f) +800 V
g) +1600 V
h) +3200 V


On the following 5 problems show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(14 pts) 5. A small particle has charge -4.0 × 10^-3 C. It moves from point A, where the electric potential is V_A = +150 V, to point B, where the electric potential is V_B = -50 V. The electric force is the only force acting on the particle. If the particle has kinetic energy 3.0 J at point A what is its kinetic energy at point B?

Ans. ____2.2 J_____


(20 pts) 6. A charge q₁ = +5.0 × 10^-6 C is placed on the x-axis at x = 0.30 m and a charge q₂ = -3.0 × 10^-6 C is at the origin.

a) What are the x and y components of the net electric field produced by these two charges at a point P on the y-axis at y = 0.40 m? You must clearly indicate whether each component is positive or negative.

Ans. x-component ____-1.08 × 10⁵ N/C_____

y-component _____-2.5 × 10⁴ N/C____

b) A third charge q₃ = -2.0 × 10^-6 C is placed at point P. What are the x and y components of the net force on q₃ exerted by the other two charges?

Ans. x-component _____+0.216 N______

y-component _____+0.05 N______


(16 pts) 7. A hollow conducting spherical shell has radius r₁ = 0.050 m and net charge q₁ = +3.0 × 10^-6 C. It is concentric with a larger hollow conducting spherical shell has radius r₂ = 0.150 m and net charge q₂ = -5.0 × 10^-6 C. What are the magnitude and direction (radially outward or radially inward) of the net electric field produced by these two shells at the following distances from their common center:

a) 0.100 m

Ans. E = ____2.7 × 10⁶ N/C____________

direction ____outward_____

b) 0.200 m

Ans. E = ____4.5 × 10⁵ N/C______________

direction ____inward_____


(18 pts) 8. A parallel-plate capacitor with capacitance C₀ = 3.0 × 10^-6 F has air between its plates. The separation between its plates is 0.005 m. The capacitor is connected to a 20.0 V battery and charged up.

a) With the capacitor still connected to the battery, a dielectric with = 2.0 is inserted between the plates, completely filling the space between the plates. After the dielectric has been inserted, what is

(i) the magnitude of the charge on each plate?

Ans. ___1.2 × 10^-4 C________________

(ii) the magnitude of the electric field at a point midway between the plates of the capacitor?

Ans. ___4.0 × 10³ N/C________________

b) The capacitor without the dielectric is charged up as before, by connecting it to a 20.0 V batery. Now the battery is disconnected from the capacitor and then the dielectric with = 2 is inserted between the plates. After the dielectric has been inserted, what is

(i) the magnitude of the charge on each plate?

Ans. ____6.0 × 10^-5 C_____________

(ii) the magnitude of the electric field at a point midway between the plates of the capacitor?

Ans. ___2.0 × 10³ N/C_____________


(18 pts) 9. A point charge of q = +5.0 × 10^-6 C is held fixed in place. From a horizontal distance of 0.060 m a small sphere of mass 4.0 × 10^-3 kg and charge +2.0 × 10^-6 C is fired with an initial speed of 40 m/s toward the fixed charge. Gravity can be neglected. How far is the small sphere from the fixed charge when its speed is 25 m/s?



Ans. ____0.0261 m_____