current-carrying wire

Meaning: Moving charges create a magnetic field, so a wire carrying current is surrounded by circular field lines centered on the wire. Right-hand rule: thumb along the conventional current, fingers curl in the direction of B. The field is stronger closer to the wire. A wire in an external field also feels a force, because its moving charges do.

Equation: B = μ0I/(2πr), where B is the field (T), μ0 = 4π × 10⁻⁷ T·m/A, I is the current (A), and r is the perpendicular distance from the wire (m).

On the MCAT: Find the field direction or size at a point: doubling I doubles B, and doubling r halves it. For the force on a wire in an external field, F = ILB sinθ (F in N, L is the length of wire in the field in m, θ is the angle between the current and B). Parallel wires with currents in the same direction attract; opposite directions repel. Fields from several wires add as vectors.

Common confusion: The field circles the wire; it does not point along the wire or radially away from it. The force on a wire in an external field is perpendicular to both I and B, and it is zero when the wire is parallel to B (θ = 0).