Chapter 23

Question 1

Suppose a point charge is located at the center of a sphericalsurface. The electric field at the surface of the sphere and thetotal flux through the sphere are determined. Now the radiusof the sphere is halved. What happens to the flux through thesphere and the magnitude of the electric field at the surfaceof the sphere?
(a) The flux and field both increase.
(b) The flux and field both decrease.
(c) The flux increases, and the field decreases.
(d) The flux decreases, and the field increases.
(e) The flux remains the same, and the field increases.
(f) The flux decreases, and the field remains the same.

Answer 1

e becaused the same amount of field lines pass through the sphere, and the distance from the charge to the surface is less.

Question 2

If the net flux through a gaussian surface is zero, the following four statements could be true. Which of the statements must be true?
(a) There are no charges inside the surface.
(b) The net charge inside the surface is zero.
(c) The electric field is zero everywhere on the surface.
(d) The number of electric field lines entering thesurface equals the number leaving the surface

Answer 2

b, d

Question 3

A spherical gaussian surface surrounds a point charge q. Describe what happens to the total flux through the surface if
(A) the charge is tripled,
(B) the radius of the sphere is doubled,
(C) the surface is changed to a cube, and
(D) the charge is moved to another location inside the surface.

Answer 3

A: flux is tripled
B: Flux is the same
C: Flux is the same
D: Flux is the same

Question 4

When can’t Gauss’s law be used?

Answer 4

dipole, charged disk, triangle with point at each corner

Question 5

Assume a uniformly charged ring of radius R and charge Q produces an electric field E-ring at a point P on its axis, at distance x away from the center of the ring as in figure (a).Now imagine the same charge Q is spread uniformly over the circular area the ring encloses, forming a flat disk of charge with the same radius. How does the field E-disk produced by the disk at P compare with the field produced by the ring at the same point?
1. Edisk < Ering
2. Edisk = Ering
3. Edisk > Ering
4. impossible to determine

Answer 5

3

Question 6

A cubical gaussian surface surrounds a long, straight, charged filament that passes perpendicularly through two opposite faces. No other charges are nearby. Over how many of the cube’s faces is the electric field zero?
1. 0
2. 2
3. 4
4. 6

Answer 6

1

Question 7

A cubical gaussian surface surrounds a long, straight,charged filament that passes perpendicularly throughtwo opposite faces. No other charges are nearby.Through how many of the cube’s faces is the electricflux zero?
1. 0
2. 2
3. 4
4. 6

Answer 7

2

Question 8

In which of the following contexts can Gauss’s law not be readily applied to find the electric field?
1. near a long, uniformly charged wire
2. above a large, uniformly charged plane
3. inside a uniformly charged ball
4. outside a uniformly charged sphere
5. Gauss’s law can be readily applied to find theelectric field in all contexts.

Answer 8

5

Question 9

ank the electric fluxes through each gaussian surfaceshown in the figure from largest to smallest. Display anycases of equality in your ranking

a sphere q
b cube q
c cylinder 3Q
d sphere with charge outside 4Q

1. a > b > c > d
2. b > a > d > c
3. c > a = b > d
4. d > b = c > a

Answer 9

3