# Solenoid Field Strength - Missing Raduis?

1. Apr 30, 2015

### Euler2718

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

A long solenoid has 35 turns/cm. With a current of 4.0A, what is the field strength at the core?

2. Relevant equations

$$\beta = \frac{\mu N I}{2r}$$

3. The attempt at a solution

I have no idea with the radius is; that's the entire problem statement above.

2. Apr 30, 2015

### TSny

The formula you gave is for a circular coil of short length. A different formula applies to the long solenoid. Check your notes or textbook.

3. Apr 30, 2015

### x86

Using Ampere's law, we can derive the magnetic field inside a solenoid. It is B = u_o i n, where n is turns/length