Electromagnetic Induction, Generators, and Transformers

A changing magnetic environment can drive charge through a circuit. That single idea sits behind generators, transformers, induction cooktops, and many sensors.

This lesson supports Chapter 19 of High School Physics.

Changing magnetic flux induces an emf

Magnetic flux through a flat loop can be written \(\Phi_B=BA\cos\theta\) for a uniform field, with the angle measured between the field and the loop’s area normal. Faraday’s law gives the average induced emf over a time interval as \[\mathrm{emf}_{\text{avg}}=-N\frac{\Delta\Phi_B}{\Delta t}.\] The average emf magnitude increases when the flux change per turn is larger, the same change occurs in less time, or more turns share that flux change.

Lenz’s law gives the direction

The minus sign in Faraday’s law represents Lenz’s law. The induced current creates a magnetic effect that opposes the change in flux. It opposes the change, not necessarily the original magnetic field. This distinction prevents common direction errors.

Generators and transformers move energy

A generator converts mechanical energy into electrical energy by changing flux through coils. A transformer uses changing current in one coil to create changing flux and an induced voltage in another. For an ideal transformer, \(\frac{V_s}{V_p}=\frac{N_s}{N_p}\), and input power equals output power.

Worked example

An ideal transformer has \(100\) primary turns and \(500\) secondary turns. With an alternating voltage of \(12\text{ V}\) rms across the primary, \(\frac{V_s}{V_p}=\frac{N_s}{N_p}=5\), so the secondary voltage is \(60\text{ V}\) rms.

Watch the idea in action

Khan Academy India – English gives a focused explanation of the chapter’s central idea. Pause before each calculation and predict the next step on paper.

Practice questions

1. What quantity must change to induce an emf?
2. Write Faraday’s law for average induced emf.
3. What does Lenz’s law determine?
4. What energy conversion occurs in a generator?
5. Write the ideal transformer voltage-turns relation.
6. Can a transformer operate from a perfectly steady direct current?

Answers

1. Magnetic flux.
2. \(\mathrm{emf}_{\text{avg}}=-N\frac{\Delta\Phi_B}{\Delta t}\).
3. The direction of induced current.
4. Mechanical energy to electrical energy.
5. \(\frac{V_s}{V_p}=\frac{N_s}{N_p}\).
6. No. It needs changing current and flux.

Keep studying

High School Physics includes 12 full-length practice tests: two in the book and ten online.

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