**Our Objective**

To demonstrate large emf is induced when direct current is switched off in an inductive circuit.

**The Theory **

**Electromagnetic induction **

Changing the magnetic field induces emf. It is electromagnetic induction. It was discovered by Michael Faraday. Faraday’s law of electromagnetic induction is the “Magnitude of induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit.”

Induced emf (ε)= - dΦ/dt, Φ-magnetic flux through the circuit

- The negative sign indicates induced current opposes the change in a magnetic field.
- Len’s law gives the direction of induced current. The direction of the induced current such that it opposes the change in the magnetic field.

For example:

A bar magnet brings towards the loop. The magnetic field increases across the loop. To oppose the increase in a magnetic field, a current induces in the loop. The magnetic field of the induced current opposes the approaching magnetic field. Loop’s north pole directly faces the north pole of the magnet to repel the approaching magnetic field. The direction of the induced current in the loop is counter clockwise direction.

**Self-induction **

Emf is induced in a single isolated coil due to changes in magnetic flux through the coil by varying currents through the same coil. This is self-induction.

- Induced emf (ε)=-L (di/dt), L=self-inductance, di/dt=change in current.
- Induced emf opposes any change in current in that coil.
- Induced emf does not depend on the current in the circuit. It depends on the rate of change of the current.
- Induced emf depends on self-inductance.

Self-inductance of solenoid, L/l =μ_{0}n^{2}A

**l**- length of solenoid

**n**- no of turns per unit length

**A**-Cross sectional area of solenoid

μ_{0}= 4 π x 10 ^{-7 }Tm/A

- When current increases emf induces in a coil so as to oppose an increase in current.

- When the current through coil decreases induced emf so as to oppose decrease in current.

**Learning Outcomes **

- Students understand the phenomena of electromagnetic induction and self-induction.
- Students are able to find the direction of induced emf.