Loops and Solenoids

At the centre of a flat coil of N turns:

$$B = \frac{\mu_0 N I}{2R}$$

A counterclockwise current: the field at the centre comes out of the page.

Fingers along the current, thumb along the field.

Inside a solenoid the field is uniform:

$$B = \mu_0 n I,\quad n = \frac{N}{L}$$

Inside a solenoid the field is uniform.