Physics 202 MWF 10:20 Spring 2003 (Ford)

Exam I Chapts. 18 and 19 in Cutnell and Johnson 5th ed.

Multiple choice questions. Circle the correct answer. No work need be shown and no partial credit will be given.

A hollow insulating spherical shell has radius R = 0.08 m and has charge +4.0 × 10^-9 C distributed uniformly over its surface.

(4 pts) 1. The electric potential at the center of the sphere is

(a) infinite
(b) zero
(c) 450 V<-----------
(d) 5625 V

(3 pts) 2. A point charge q₀ = -5.0 × 10^-9 C is placed 0.04 m above the center of the shell. The electrical force on q₀ is



(a) infinite
(b) zero<---------
(c) toward the center of the shell
(d) away from the center of the shell

Identical point charges q = +5 × 10^-6 C are placed at opposite corners of a square. The length of each side of the square is 0.2 m. The other corners of the square are points A and B, as shown in the sketch.



(3 pts) 3. A point charge q₀ = -2 × 10^-6 C is placed at A. The net electrical force on q₀ is

(a) toward B<-----------
(b) away from B
(c) zero

(3 pts) 4. The work done on q₀ by the electrical force when q₀ moves from A to B is

(a) zero<------------
(b) negative
(c) positive

A hollow spherical conductor has inner radius a and outer radius b. A point charge q₁ = +2 × 10^-6 C is placed in the air space at the center of the sphere. The hollow sphere carries a net charge of -3 × 10^-6 C



(3 pts) 5. The charge on the outer surface of the hollow sphere is

(a) zero
(b) -3 × 10^-6 C
(c) +3 × 10^-6 C
(d) -5 × 10^-6 C
(e) +5 × 10^-6 C
(f) -1 × 10^-6 C<-------------
(g) +1 × 10^-6 C

(4 pts) 6. The electric field at a point in the air space inside the sphere (at r less than a) is

(a) zero
(b) k(2 × 10^-6 C)/r², outward<---------------
(c) k(2 × 10^-6 C)/r², inward
(d) k(1 × 10^-6 C)/r², outward
(e) k(1 × 10^-6 C)/r², inward
(f) k(3 × 10^-6 C)/r², outward
(g) k(3 × 10^-6 C)/r², inward

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

(22 pts) 7. A point charge q₁ = -5.0 × 10^-6 C is located on the -x-axis at x = -0.20 m. A point charge q₂ is located at the origin. The resultant electric field produced by these two point charges is zero at point P, which is on the +x-axis at x = +0.30 m.



a) What is the magnitude of q₂ and is it positive or negative?

Ans. magnitude ___1.8 × 10^-6 C____

positive or negative? ___positive___

b) What is the resultant electric potential at point P produced by charges q₁ and q₂? Take V = 0 at large distances from both charges.

Ans. ____-3.6 × 10⁴ V___


(19 pts) 8. A small particle has charge -5.0 × 10^-6 C and mass 2 × 10^-4 kg. It moves from point A, where the electric potential is V_A = +200 V, to point B, where the electric potential is V_B = +900 V. The electric force is the only force acting on the particle. The particle has speed 5 m/s at point A. What is its speed at point B?

Ans. ____7.75 m/s____


(18 pts) 9. Points A and B are in a region of space where the electric field is uniform. Point B is 0.20 m due east of point A. The electric potential at point A is +135 V. The electric field in this region has direction due west and has magnitude 1500 N/C.

a) What is the electric potential at point B?

Ans. ____+435 V__

b) What is the potential of point C, which is 0.20 m due south of point A?

Ans. _____+135 V___


(21 pts) 10. A parallel plate capacitor has a capacitance C = 5.0 × 10^-6 F when the volume between the plates is filled with air. It is connected to a battery and a charge of magnitude 3.0 × 10^-6 C goes onto each plate. The plates are separated by 4.0 mm.

a) What is the battery voltage?

Ans. ____0.60 V____

b) What is the magnitude of the electric field at a point midway between the plates?

Ans. ___150 V/m____

c) With the capacitor still connected to the battery, a slab of dielectric is inserted between the plates, completely filling the space between the plates. After then dielectric is inserted, the charge on each plate has magnitude 9.0 × 10^-6 C. What is the dielectric constant of the dielectric?

Ans. ____3.0____