Couple sur une boucle de courant dans un champ magnétique

A magnetic dipole moment is a vector quantity describing a current-carrying closed loop's magnetic properties. Its magnitude equals the product of the loop's area and current flowing through it, while its direction is perpendicular to the loop's plane, following the right-hand rule. This concept allows torque on current loops to be expressed in terms of the magnetic dipole moment.

Torque on a current loop is calculated using the equation where I is current, A is loop area, B is magnetic field strength, and θ is the angle between current and field. Maximum torque occurs when θ = 90°, making sin θ = 1. Minimum torque occurs when θ = 0°, indicating the loop is aligned with the magnetic field.

In a uniform magnetic field, magnetic forces acting along the loop's plane lie on the same rotation axis, causing their torques to cancel. Similarly, torques from forces perpendicular to the plane balance each other. This symmetry results in zero net force, though net torque can still exist depending on loop orientation.

Electric motors use torque from magnetic forces on current-carrying wire loops to produce rotational motion. When current flows through loops placed between magnets, the magnetic field applies torque to the shaft, converting electrical energy into mechanical energy. This principle is fundamental to motor design and operation.

Torque magnitude depends on sin θ, where θ is the angle between current direction and magnetic field. When θ = 90°, sin θ = 1 and torque is maximum. When θ = 0°, sin θ = 0 and torque is minimum. This angular dependence allows control of torque by adjusting loop orientation relative to the field.

For a multi-turn loop, multiply the single-turn torque equation by the number of turns N. For example, a 200-turn square loop with area 0.04 m² carrying 15 A in a 1-T field produces maximum torque of 120 N·m when perpendicular to the field. The torque equation becomes τ = NIAB sin θ.

The torque direction is perpendicular to both the magnetic dipole moment vector and the magnetic field, following the right-hand rule. The magnetic dipole moment points perpendicular to the loop's plane in the direction determined by curling fingers with current flow. This vector relationship defines the rotational axis and direction of the loop's motion.