# Physics questions

I'm review for a test in the morning and just making sure I have this right. The professor did not include a key for the review.

ere is a link to the image for both problems.

In the figure, two curved plastic rods form a circle of radius R in the standard x-y plane.
The charges of the rods are +q and -q on the rods. The charges are distributed uniformly.
What is the magnitude of the electric field at the center of the circle?

A k (4q / (pi R2))
B k (q2 / (2 pi R))
C k (2q / (pi R))
D k (q2 / (pi R2))
E 0

I believe it is 0 because the charge is distributed uniformly across the surface of the field.

In the figure, two curved plastic rods form a circle of radius R in the standard x-y plane.
The charges of the rods are +q and -q on the rods. The charges are distributed uniformly.
What is the direction of the electric field at the center of the circle?

A down (-y)
B up (+y)
C left (-x)
D right (+x)
E 0

Since the top half is the + rod and the bottom half is the - rod I believe it would travel down because the field travels from + to -.

The units of electric field are
A J / C
B J / (C m)
C J / m
D J C
E J m / C

I thought the units of electric field were N/C. Am I overlooking something?

Electric field lines enter charge B while electric field lines exit charge A. The number of
lines entering B equals the number of lines exiting A. The charges A and B are separated
from each other by a distance R. Therefore

A A is positive and B is negative
B A is negative and B is positive
C The magnitude of the electric field is the same everywhere
D The electric field is strongest midway between A and B
E A and B must have the same sign

I believe it is A. A is positive and B is negative because field lines point away from positive charges and toward negative charges.

Let me know if I'm on the right track.

Redbelly98
Staff Emeritus