# 7: Force on a Current in a Magnetic Field

- Page ID
- 5458

- 7.1: Introduction
- In Chapter 6 we showed that when an electric current is situated in an external magnetic field it experiences a force at right angles to both the current and the field. Indeed we used this to define both the magnitude and direction of the magnetic field. The magnetic field is defined in magnitude and direction.

- 7.4: Magnetic Moment
- If a compass needle, or indeed any bar magnet, is placed in an external magnetic field, it experiences a torque – the one exception being if the needle is placed exactly along the direction of the field. The torque is greatest when the needle is oriented at right angles to the field.

- 7.9: Magnetogyric Ratio
- The magnetic moment and the angular momentum are both important properties of subatomic particles. Each of them, however, depends on the angular speed of rotation of the particle. The ratio of magnetic moment to angular momentum, on the other hand, is independent of the speed of rotation, and tells us something about how the mass and charge are distributed within the particle. This ratio is called the magnetogyric ratio.