Electromagnetic induction is the process through which an emf is induced in a conductor due to a changing magnetic field.
In Faraday's experiment, moving a magnet into or out of a coil induces an electric current, which is detected by a galvanometer. The faster the magnet moves, the greater the deflection of the needle. The direction of needle deflection depends on whether the magnet is moving toward or away from the coil—reversing as the direction of motion changes. When the magnet is stationary, the needle returns to the center, indicating no induced current.
Based on the observations in Faraday's experiment, we can demonstrate the two laws of electromagnetic induction:
Magnetic flux, $\Phi$, is the magnetic field in a given area. It depends on the strength of the magnetic field ($B$), the area of the loop, and the angle between the field and the area.
Magnetic flux linkage is the total magnetic flux linked with a coil, taking into account all the turns in the coil. If $N$ is the number of turns in a coil, then flux linkage is $N \times \Phi$,
Lenz’s Law is a direct consequence of the law of conservation of energy. When a magnet is pushed into a coil, an electric current is induced in the coil. This current generates a magnetic field that opposes the motion of the magnet, in accordance with Lenz’s Law. As a result, you must exert a force to overcome this resistance, doing mechanical work in the process. This mechanical work is not lost; it is transformed into electrical energy within the coil. In this way, energy is conserved — no energy is created out of nothing, but simply converted from one form/store to another.
The following are examples in which the effects of Lenz's law are demonstrated.
| Example | Description |
|---|---|
| Dropping a magnet through a copper tube | Induced eddy currents in the tube oppose the motion of the magnet, causing it to fall slowly. |
| Magnetic braking in roller coasters or trains | Moving conductors induce currents that produce opposing magnetic forces, slowing motion without contact. |
| Jumping ring on an electromagnet | Switching on the electromagnet induces a current in the ring that creates a repelling force, causing it to jump. |
| Induction cooktops | Rapidly changing magnetic fields induce currents in metal cookware, heating it through resistance. |
Here is an interesting video on how the changing magnetic fields create induced current opposing the changes and hence, levitating an aluminum plate.
| Letter | Explanation |
|---|---|
| C | Cutting of magnetic field lines occurs when a magnet or coil moves relative to one another. Change in magnetic flux linkage occurs when any of the following changes: strength of the magnetic field, the area of the loop, and the angle between the field and the area. |
| F | According to Faraday’s Law, this change induces an electromotive force (emf) in the coil. The greater the rate of change, the larger the emf induced. |
| I | If the circuit is closed, the induced emf causes an electric current to flow—this is the induced current. |
| L | Lenz’s Law states that the direction of the induced current is such that it produces a magnetic effect that opposes the original change that caused it. |
| E | The effect of the induced current could be a deflection in a galvanometer, deceleration of a magnet near a conducting plate, or heating due to resistance. |