Chapter 6 - Electromagnetic Induction

Electric current produces a magnetic field. Can a magnetic field produce electric current?

A constant magnetic field alone does not necessarily produce current. A changing magnetic flux can.

Electromagnetic Induction Class 12 explains how changing magnetic conditions produce an induced electromotive force and current. This principle is central to electrical generators, transformers and induction-based technologies.

Magnetic flux

Magnetic flux measures how much magnetic field passes through a surface.

It depends on:

• Magnetic-field strength

• Surface area

• Angle between the magnetic field and area vector

Magnetic flux can change if any of these quantities changes.

Faraday’s experiments

Faraday observed that current was induced when:

• A magnet moved relative to a coil

• A coil moved through a magnetic field

• Current in a nearby coil changed

• Magnetic flux linked with a circuit changed

No continuous induced current appears merely because a magnet is held stationary relative to a coil.

Faraday’s laws

Faraday’s laws state that:

1. An induced EMF is produced when magnetic flux linked with a circuit changes.

2. Its magnitude depends on the rate of change of magnetic flux.

A faster change in flux produces a larger induced EMF.

For a coil with several turns, the flux linkage includes the number of turns.

Lenz’s law

Lenz’s law determines the direction of induced current.

The induced current produces an effect that opposes the change in magnetic flux responsible for it.

It opposes the change—not necessarily the original magnetic field.

For example, if magnetic flux into a loop is increasing, the induced field acts outward to oppose that increase.

Conservation of energy

Lenz’s law is a consequence of energy conservation.

If induced current supported the change that produced it, the process could continue without external work and generate unlimited energy.

Because the induced effect opposes the change, external work is required. That work is converted into electrical or thermal energy.

Induced EMF and relative motion

Magnetic flux may change through relative motion between a conductor and a magnetic field.

Students analyse situations involving:

• Magnets approaching or leaving coils

• Loops entering or leaving magnetic regions

• Changing area or orientation

• Changing magnetic-field strength

The first step is always to determine how the magnetic flux changes.

Self-induction

When current in a coil changes, the magnetic flux produced by that same coil also changes.

This changing flux induces an EMF in the coil itself. The effect is called self-induction.

The induced EMF opposes the change in current.

Self-inductance depends on the coil’s geometry, number of turns and magnetic medium.

Mutual induction

When changing current in one coil produces changing flux through a nearby coil, an EMF is induced in the second coil.

This is mutual induction.

The principle is used in transformers and other coupled-coil devices.

Common student difficulties

Students often:

• Think a stationary magnet always induces current

• Use magnetic field instead of magnetic flux

• Ignore surface orientation

• Reverse the direction predicted by Lenz’s law

• Say the induced field opposes the original field in every case

• Forget the number of turns

• Confuse self-induction and mutual induction

• Treat the negative sign in Faraday’s law as an ordinary numerical negative without physical meaning

How should you solve induction questions?

Use this sequence:

1. Select the loop or coil

2. Mark its area direction

3. Determine the original flux direction

4. Decide whether flux is increasing or decreasing

5. Use Lenz’s law to determine the induced field

6. Use the right-hand rule to determine current direction

7. Calculate induced EMF using the rate of flux change

Teachoo resources

Teachoo provides:

• Flux diagrams

• Faraday-law derivations

• Lenz-law direction questions

• Self and mutual-inductance explanations

• NCERT solutions

• Numerical and conceptual questions

• MCQs and case-based problems

Frequently Asked Questions

What causes electromagnetic induction?

A change in magnetic flux linked with a circuit produces induced EMF.

Does a constant magnetic field produce induced EMF?

Not by itself. Induction requires changing flux, which can occur through changes in field, area or orientation.

What does Lenz’s law oppose?

The induced effect opposes the change in magnetic flux that produces it.

What is self-induction?

It is the production of induced EMF in a circuit due to a change in its own current.

What is mutual induction?

It is the induction of EMF in one coil due to changing current in another nearby coil.

Why is the negative sign present in Faraday’s law?

It represents Lenz’s law and the opposition of the induced effect to the flux change.