Consider a metal disk that has a part of it influenced by an external magnetic field. As the disk is made of metal, the movement of the metal through the region of magnetic field causes eddy currents to flow. Using the right-hand push rule, it can be shown that the eddy current within the magnetic field will be upwards. The current follows a downward return path through the metal outside the region of magnetic influence.
The magnetic field exerts a force on the induced eddy current. This can be shown to oppose the motion of the disk in the example on the previous page by applying the right-hand push rule. In this way eddy currents can be utilised in smooth braking devices in trams and trains. An electromagnet is switched on so that an external magnetic field affects part of a metal wheel or the steel rail below the vehicle. Eddy currents are established in the part of the metal that is influenced by the magnetic field. These currents inside the magnetic field experience a force that acts in the opposite direction to the relative motion of the train or tram, as explained below. In the case of the wheel, the wheel is slowed down. In the case of the rail, the force acts in a forward direction on the rail and there is an equal and opposite force that acts on the train or tram. Note: The right-hand push rule is used twice.
The first time we use it, we show that an eddy current is produced. The thumb points in the direction of movement of the metal disk through the field because we imagine that the metal contains many positive charges moving through the field. We push in the direction of the force on these charges. This push gives us the direction of the eddy current.
The second time we use the right-hand push rule, we show that there is a force opposing the motion of the metal. Our thumb is put in the direction of the current in the field (the eddy current), then we push in the direction of the force on the moving charges (which are part of the metal disk). We then see that the force is always in the opposite direction to the movement of the metal.
Eddy currents are used for electromagnetic braking in many free-fall amusement park rides.
A copper plate attached to the ride capsule passes between fixed strong magnets near the bottom of the ride, inducing eddy currents and associated magnetic poles in the copper plate. Each fixed magnet in turn induces a like pole as the plate approaches and an opposite pole as the plate leaves. The combined effect of interaction between the permanent and the induced fields slows the ride down smoothly because the strength of the eddy currents in the plate is directly proportional to the speed of the plate moving between the poles. As the ride slows the braking force is reduced.
Some trains use electromagnets close to the metal rails to induce eddy currents in the rails. These eddy currents produce magnetic fields in the rails, a like pole ahead of each electromagnet and an opposite pole behind it. The interaction between the magnetic fields opposes the forward motion of the electromagnets and the train to which they are attached. Because the strength of the induced eddy currents is proportional to the speed of the train, the braking force is reduced as the train slows, resulting in a smooth stop.
Triple beam balances commonly used in school laboratories have an aluminium plate fixed to the end of the beam. As the beam swings, the plate passes through the field of a permanent horseshoe magnet. Eddy currents are induced in the plate, setting up magnetic fields and damping the motion of the balance.