Physics Questions: Electric Field Magnitude/Direction/Units

[aaronmilk3]

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]

It may be too late to respond, but here are my thoughts.

I don't see the figure you referred to. Do questions 1 & 2 refer to the same figure? If so, how can the electric field point in some direction, but be zero? Or do the questions refer to different figures?

For the units question, you are correct that the units are N/C. But there are other equivalent ways to express that. Use the equivalent units for J in the answers provided, and you should find something equivalent to N/C.

I agree with your answer for the last question, field lines point away from positive charges and towards negative charges.