Physics 202 MWF 10:20 Spring 2001 (Ford)

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

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

(3 pts) 1. The electric field is always in the direction of
a) increasing electric potential
b) decreasing electric potential
c) constant electric potential


(3 pts) 2. If the potential at the surface of a solid conducting sphere of radius R = 0.2 m is 500 V, what is the potential at the center of the sphere?
a) zero
b) infinity
c) 250 V
d) 500 V
e) 2500 V
f) 12,500 V


(3 pts) 3. An electron (charge -e) moves from point A where V_A = +90 V to point B where V_B = +130 V. If the only force on the electron is the electric force, how is the kinetic energy KE of the electron at A related to its kinetic energy at B?
a) KE_A = KE_B
b) KE_A > KE_B
c) KE_A < KE_B


(3 pts) 4. Point charges are arranged at the corners of a square, with identical charges at opposite corners. The net force due to the other charges on the charge q that is in the upper left corner is zero. What is the sign of q ?


a) positive
b) negative


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


(20 pts) 5. A charge q₁ = - 6.0 × 10^-6 C is placed on the x-axis at x = 0.4 m and a charge q₂ = +5.0 × 10^-6 C is placed at the point (x = 0.4 m, y = 0.3 m). What is the magnitude of the resultant electric field at the origin due to these two charges?


Ans.______________


(18 pts) 6. A small spherical insulator of mass 0.06 kg and charge q is suspended from the end of a thin wire of negligible mass. There is a uniform electric field of 5000 N/C directed to the left. The sphere hangs at rest when the wire makes an angle of 37° with the vertical, as shown in the sketch. Find the sign (positive or negative) and magnitude of the charge q.


Ans. sign ______________

magnitude _______________


(18 pts) 7. A small, charged metal sphere that carries a net charge of q₁ = -5.0 × 10^-6 C is held stationary by insulating supports. A second small metal sphere, with a net charge of q₂ = +3.0 × 10^-6 C is projected toward q₁. When the two spheres are 0.800 m apart, q₂ is moving toward q₁and has kinetic energy 0.8 J. Assume that the two spheres can be treated as point charges and neglect the effects of gravity. What is the kinetic energy of q₂ when it is 0.400 m from q₁?


Ans.______________


(16 pts) 8. A hollow spherical conductor has a net charge of 5.0 × 10^-6 C. A point charge of -7.0 × 10^-6 C is placed at the center of the sphere, in the hollow cavity. The hollow conductor has inner radius r₁ = 0.20 m and outer radius r₂ = 0.30 m.


a) What is the charge (sign and magnitude) on the outer surface of the spherical conductor?

Ans. __________________

b) What is the magnitude of the resultant electric field at a distance of r = 0.10 m from the center of the hollow sphere? (This point is within the hollow cavity.)

Ans._________________


(16 pts) 9. A parallel-plate capacitor has charge Q = 9.0 × 10^-6 C and capacitance C₀ = 3.0 × 10^-6 F when there is air between the plates. The separation between the plates is 0.002 m.

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

Ans._________________

b) With the charge on the plates of the capacitor kept constant (Q = 9.0 × 10^-6 C) a dielectric with = 5 is inserted between the plates, completely filling the volume between the plates. After the dielectric has been inserted, what is the potential difference between the plates of the capacitor?

Ans._______________