30.19: Magnetic Damping

Eddy currents can produce significant drag on motion, called magnetic damping. For instance, when a metallic pendulum bob swings between the poles of a strong magnet, significant drag acts on the bob as it enters and leaves the field, quickly damping the motion.

If, however, the bob is a slotted metal plate, the magnet produces a much smaller effect. When a slotted metal plate enters the field, an emf is induced by the change in flux; however, it is less effective because the slots limit the size of the current loops. Moreover, adjacent loops have currents in opposite directions, and their effects cancel. When an insulating material is used, the eddy current is extremely small, so magnetic damping on insulators is negligible. If eddy currents are to be avoided in conductors, they must be slotted or constructed of thin layers of conducting material separated by insulating sheets.

Magnetic damping is used in sensitive laboratory balances. To have maximum sensitivity and accuracy, the balance must be as friction-free as possible. If it is friction-free, it oscillates for a very long time. Magnetic damping is a simple and ideal solution. With magnetic damping, the drag is proportional to the speed and becomes zero at zero velocity. Thus, the oscillations are quickly damped, after which the damping force disappears, allowing the balance to be very sensitive. In most balances, magnetic damping is accomplished with a conducting disc that rotates in a fixed field.

Consider a metallic plate fixed to one end swinging freely about its mean position, placed between two electromagnets.

When there is no magnetic field, the plate keeps oscillating. Once the electromagnets are turned on, the plate slows down and stops quickly.

This reduction in motion without any physical contact is called magnetic damping, caused by the opposing nature of eddy currents.

Now swing the plate again to understand this mechanism.

When the plate enters the magnetic field, the magnetic flux on the plate changes.

As per Faraday's law, eddy currents are induced in the plate, and according to Lenz's law, their direction is anticlockwise. So, the plate experiences a magnetic force that opposes its motion.

Similarly, when the plate goes out of the magnetic field, magnetic flux decreases, and eddy currents are induced in the clockwise direction.

As a result, the plate again experiences a magnetic force that opposes its exit from the magnetic field.

So, as a combined effect, the oscillations of the metallic plate are quickly damped.