Electric Fields and Conducting Shells in Physics

1. Relative to the effort it would take to thoroughly read the chapter once, how much effort did you put into doing the reading assignment? a) 0-20% b) 20-50% c) 50-90% d) 90-110% e) 110-150% f) 150-200% g) >200%

2. Consider a thin spherical shell of radius 9.0 cm made of a perfectly conducting material centered on the origin of a Cartesian coordinate system. Two charged particles lie outside the ball on the x-axis of the same coordinate system: a particle with -5.0 microcoulombs of charge at x = -11 cm and a particle with charge +5.5 microcoulombs of charge at x = +11 cm. What is the direction of the electric field at the origin. a) The electric field is in the +x direction at the origin. b) The electric field is in the -x direction at the origin. c) The electric field is 0 at the origin. It has no direction.

3. 1.00 Coulomb of charge is placed on a solid sphere made of a perfectly conducting material. Find the magnitude of the electric field at a distance of 1.00 meters from the center of the sphere. Assume the radius of the sphere to be less than 1.00 meters a) 8.99 x 109 N/C b) 1.00 N/C c) 0.00 N/C d) None of the above.

4. Why is the interior of a car considered to be a good place to be during a thunder storm? a) A typical car is a good approximation to a conducting shell. Inside a conducting shell, the electric field is zero. b) A typical car is a good approximation to a conducting shell. Inside a conducting shell, the electric field is uniform (but not zero).

5. Consider an empty spherical gold shell having an inner radius 25.0 cm and thickness 1.5 mm (and hence an outer radius of 26.5 cm) and a charge of +2.40 mC. What is the direction of the electric field at a point on the interior surface of the sphere? a) Radially inward (toward the center of the sphere). b) Radially outward. c) There is no direction because the electric field is zero.

6. Consider an empty spherical gold shell having an inner radius 25.0cm and thickness 1.5mm (and hence an outer radius of 26.5cm) and a charge of +2.40mC. What is the direction of the electric field at a point 1nm inside the outer surface of the sphere? a) Radially inward (toward the center of the sphere). b) Radially outward. c) There is no direction because the electric field is zero.

7. Consider an empty spherical gold shell having an inner radius 25.0cm and thickness 1.5mm (and hence an outer radius of 26.5cm) and a charge of +2.40mC. What is the direction of the electric field at a point 1nm outside the outer surface of the sphere? a) Radially inward (toward the center of the sphere). b) Radially outward. c) There is no direction because the electric field is zero.

8. Suppose you calculate a force using F=qE and then use that force in Newton’s 2nd Law to calculate an acceleration. For the latter step, what kind of diagram is required? a) A before and after picture. b) A free body diagram. c) No diagram at all.

9. What is wrong with the following expression? a) The quantity on the left is a vector whereas the quantity on the right is a scalar. A non-zero vector cannot be equal to a scalar. b) Nothing.

10. What is wrong with the following expression? a) The quantity on the left is a vector whereas the quantity on the right is a scalar. b) The quantity on the left is a scalar whereas the quantity on the right is a vector. c) Nothing.

11. What is wrong with the following expression? a) The quantity on the left is a vector whereas the quantity on the right is a scalar. A non-zero vector cannot be equal to a scalar. b) Nothing.

12. What is wrong with the following expression? a) The quantity on the left is a vector whereas the quantity on the right is a scalar. b) The quantity on the left is a scalar whereas the quantity on the right is a vector. c) Nothing.

13. Suppose, during a test, you need the mass or charge of an electron or proton. Where do you find it? a) One doesn’t “find” any of these values. They have to be memorized. b) One finds the masses and the value of e, in coulombs, on the formula sheet. One has to know that the charge of a proton is +1e and the charge of an electron is -1e.