Whenever the number of magnetic lines of forces in a circuit changes, an emf is produced and known as induced emf. The phenomenon is electromagnetic induction. An induced current is the current that flows when the circuit is closed. This induced emf and current last only for a time while the magnetic flux changes.
Experiment: Induction from a magnet moving through a coil
Faraday used a cardboard tube with insulated wire wrapped around it to form a coil. A voltmeter was connected across the coil, and the induced EMF read as a magnet was passed through the coil. He observed the following
- Magnet at rest in or near the coil: No voltage observed.
- The magnet moving toward the coil: Some voltage measured, rising to a peak as the magnet nears the coil’s centre.
- Magnet passes through the middle of the coil: Measured voltage rapidly changes sign.
- Magnet passes out and away from the coil: Voltage is measured in the opposite direction to the magnet’s earlier case moving into the coil.
The stationary magnet might produce a large magnetic field; no EMF can be induced because the flux through the coil is not changing. When the magnet moves closer to the coil the flux rapidly increases until the magnet is inside the coil. As it passes through the coil, the magnetic flux through the coil begins to decrease. The induced EMF is reversed gradually.