Solenoid Field Strength - Missing Raduis?

[Euler2718]

Homework Statement

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

Homework Equations

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

The Attempt at a Solution

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

 

[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.

 

[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