What Determines the Electric Field Magnitude at Half the Radius of a Solenoid?

[freddy13]

Homework Statement

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?

Homework Equations

μ₀(N/L)I

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!

 

[rude man]

Homework Statement

What is the magnitude of the
electric eld a distance R=2 from the axis of
the solenoid?

!

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