Question

The magnetic field contribution dB from a current element is exactly zero when:

Options

• the distance r is very small

• point P is perpendicular to the current element (θ = 90°), so sinθ = 1

• point P lies exactly along the direction of the current element (θ = 0° or 180°), so sinθ = 0

• the current I is very large

Answer 1

When P lies exactly along the direction of the current element (θ = 0° or 180°), sinθ = 0, so dB = 0. No magnetic field is produced directly ahead of or behind a current element — the field only builds up in the surrounding perpendicular directions.