30.2 : Faraday's Law

Faraday's law state that the induced emf is the negative change in the magnetic flux per unit of time. Any change in the magnetic field or change in the orientation of the area of the coil with respect to the magnetic field induces a voltage (emf). The magnetic flux measures the number of magnetic field lines through a given surface area. Magnetic flux is estimated from the integral of the dot product of the magnetic field vector and the area vector. The negative sign describes the direction in which the induced emf drives the current around a circuit. However, that direction is most easily determined with a rule known as Lenz's law.

In many practical applications, the circuit of interest consists of a number (N) of tightly wound turns. Each turn experiences the same magnetic flux. Therefore, the net magnetic flux through the circuit is N times the flux through one turn, and Faraday's law is written as the negative of N multiplied by the rate of change of magnetic flux through them.

Users of electrical appliances use a ground fault interrupter (GFI) device, which works as a circuit breaker, to avoid getting an electrical shock. This device works on the principle of Faraday's law. Another application of Faraday's law is seen in the electric guitar. In the electric guitar, a pick coil is placed near a vibrating string, and the coil is made of metal that can be magnetized. A permanent magnet inside the coil magnetizes the portion of the string while it is vibrating at a particular frequency, which results in a change in the magnetic flux. This change in flux produces an emf that is fed to the amplifier to produce a sound that listeners can hear.