# Homework Help: Faraday's Law and a solenoid

1. Nov 24, 2009

### llauren84

1. The problem statement, all variables and given/known data
A long solenoid with 1000 turns per meter and a radius 2.00 cm carries an oscillating current given by I = (5.00A) sin (100 pi t). What is the electric field induced at a radius r = 1.00 cm.from the axis of the solenoid? What is the direction of the electric field when the current is increasing counterclockwise in the coil?

2. Relevant equations
Eq. 1: $$\epsilon=\frac{-d\phi}{dt}$$

Eq. 2: $$\phi=BA$$

Eq. 3: $$B=\mu_0 n \frac{-dI}{dt}$$

Eq. 4: $$E=\frac{\epsilon}{2 \pi r}$$

3. The attempt at a solution

I think that you just sub Eq.3 into Eq.2 and then Eq.2 into Eq.3 and then into Eq 4 and take the derivative of I:

$$E = \frac{R^{2} \mu_0 n}{2 r} 500 \pi sin (100\pit)$$

My confusion is where to put the r=1cm and R=2cm.

Also, how do you know which direction the E field is going?

Last edited: Nov 24, 2009
2. Nov 24, 2009

### Redbelly98

Staff Emeritus
Agreed.

Agreed. Question for you: what is A here? More specifically, what size loop are you using to get Φ?

Eq. 3 is wrong. Look it up again, what is the B-field inside a solenoid?

Agreed. Question for you: what is r here? More specifically, what size circle are you using to relate ε and E?

What happened to Eq. 1? I think you better show what steps you took to get this equation.

See my comments after Eq's 2 and 4.

Lenz's Law is useful here.