# Electric Field of a Solenoid

1. Jul 31, 2013

### freddy13

1. The problem statement, all variables and given/known data

A long solenoid of radius R, length L (which
is much longer than R), number of windings
N, has a current I that is changing in time at
a rate (dI/dt). What is the magnitude of the
electric eld a distance R=2 from the axis of
the solenoid?

2. Relevant equations

μ0(N/L)I

3. The attempt at a solution

I thought I had this completely right until I get to the R/2 Part. if you just throw the R/2 part into the equation, then you get the B field I believe, but the answer is: μ0(dI/dt)(N/L)R/4.

I am not seeing where the 4 in the denominator comes from unless we had some kind of divisible action of R going on somewhere, or something. Not really sure how to proceed!

2. Aug 1, 2013

### rude man

Start by stating the problem correctly. What do you mean by "R=2"? Did you mean R/2? 2R?