Electromagnetic Induction | Class 12 Physics Notes

6.4 FARADAY'S LAW OF INDUCTION

Faraday's law of electromagnetic induction states that an electromotive force (emf) is induced in a coil whenever the magnetic flux through the coil changes with time. The magnitude of the induced emf is equal to the rate of change of magnetic flux through the coil, and its direction is such that it opposes the change in flux (Lenz's law).

Mathematically, the induced emf ε is given by ε = - dΦ_B/dt, where Φ_B is the magnetic flux through one turn of the coil. For a coil of N turns, the total emf induced is ε = - N dΦ_B/dt. The negative sign indicates the direction of the induced emf, which is explained by Lenz's law.

The magnetic flux through a coil can be changed by varying the magnetic field strength B, changing the area A of the coil, or changing the angle θ between the magnetic field and the coil's area vector. Examples include moving a magnet towards or away from a coil, moving a current-carrying coil near another coil, changing the coil's shape, or rotating the coil in a magnetic field.

Faraday's law explains the observations from the experiments of Faraday and Henry, where relative motion or changing current induced emf and current in coils. The law is fundamental to the operation of electrical devices like transformers, generators, and inductors.

Example 6.1 discusses how to increase galvanometer deflection by using a soft iron core, a powerful battery, or rapid motion, and how to demonstrate induced current using a small bulb instead of a galvanometer. Example 6.2 and 6.3 provide numerical problems calculating induced emf and current in loops and coils due to changing magnetic fields.

📊 Diagram: See figure_7: Michael Faraday [1791-1867] Faraday made numerous contributions to science, viz., the discovery of electromagnetic induction, the laws of electrolysis, benzene, and the fact that the plane of polarisation is rotated in an electric field. He is also credited with the invention of the electric motor, the electric generator and the transformer. He is widely regarded as the greatest experimental scientist of the nineteenth century.

🧪 Activity: Observing induced emf by changing magnetic flux through coils via motion or current variation; replacing galvanometer with bulb to demonstrate induced current.

🔗 Connection: This section leads to Lenz's law, which explains the direction of the induced emf and current, and the principle of conservation of energy.