# Homework Help: Magnetic Field of a Solenoid

1. May 2, 2012

### GermanMC

1. The problem statement, all variables and given/known data
I've been given a solenoid and I have N/l = 100 turns/meter, dI/dt = 0.05A/s and a radius of 0.005m. I am only concerned with the dI/dt.

2. Relevant equations

B = μ*N/l*I
$\Phi$ = $\int\int$ B$\bullet$n dl
$\epsilon$ = d$\Phi$/dt

3. The attempt at a solution

For B I get the field but how do I deal with my amperage in amps/s instead of just amps. Once I get that I'll take the derivative of the flux with respect to time and I'll be fine. Any clues will be much appreciated.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. May 2, 2012

### marcusl

When you take d(phi)/dt of your 2nd expression, you can interchange the order of differentiation and integration because both are linear operations. d/dt comes inside and operates on B. You find dB/dt from your first expression, which looks wrong BTW. (Shouldn't I be in the denominator?)