# Homework Help: Does the electric field reach a value equal to half its maximum value?

1. Jun 30, 2013

### pr_angeleyes

A non-conducting sphere of radius R = 7 cm carries a charge Q = 4 mC distributed uniformly throughout its volume. At what distance, measured from the center of the sphere does the electric field reach a value equal to half its maximum value?

My attempt:

Emax= q(4mC)/7
r= 7/2 cm = 3.5

is this correct? because one of my friend said 9.9... so idk if im right or are both values

2. Jun 30, 2013

### zephyr5050

I'm not sure how exactly you determined the maximum of the electric field. It appears that all you did was divide the charge by the radius. This is not a correct formula and doesn't even give you the units of an electric field. My suggestion would be to use Gauss' Law to find the electric field as a function of radius. This will allow you to determine at what radius the maximum occurs (although with a bit of experience it should be fairly obvious). From there you can set the equation for the field equal to half that value and solve for r.

Just a hint, there should actually be two radii at which the field is equal to half its maximum.

Last edited: Jun 30, 2013
3. Jun 30, 2013

### howie8594

I agree with your friend.