Loops and Solenoids

At the centre of a flat coil of N turns:

B = μ₀ 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 = μ₀ n I, n = N / L

Inside a solenoid the field is uniform.