Electromagnetic Induction & Transformers – Key Concepts

Ten question-and-answer flashcards covering Faraday’s law, Lenz’s law, motional emf, self and mutual inductance, transformers, losses, and energy storage in inductors.

What is electromagnetic induction?

The phenomenon in which an electromotive force (emf) is produced in a circuit whenever the magnetic flux linked with the circuit changes.

State Faraday’s first law of electromagnetic induction.

The magnitude of the induced emf in a circuit is directly proportional to the rate of change of magnetic flux through the circuit.

Give the mathematical form of Faraday’s law for a coil of N turns.

ε = –N (dΦ/dt), where ε is the induced emf and Φ is the magnetic flux through one turn.

What does Lenz’s law say about the direction of induced current?

The induced current flows in such a direction that its own magnetic field opposes the change in magnetic flux that produced it (ε = –N dΦ/dt, negative sign shows opposition).

Define motional emf and provide its basic formula.

Motional emf is the emf induced by the physical motion of a conductor in a magnetic field; for a straight conductor of length l moving with velocity v perpendicular to a uniform field B, ε = B l v.

What is self-inductance and its SI unit?

Self-inductance (L) is the property of a coil by which it opposes any change in current through itself by inducing an emf; its SI unit is the henry (H).

Define mutual inductance and name the electrical device that operates on this principle.

Mutual inductance (M) is the property where a changing current in one coil induces an emf in a nearby coil; a transformer operates on the principle of mutual induction.

For an ideal transformer, what is the relation between the voltages and the turns in the primary and secondary coils?

E₁/E₂ = N₁/N₂, where E is voltage (emf) and N is the number of turns; hence a step-up transformer has N₂ > N₁ and a step-down transformer has N₂ < N₁.

Name two major losses in a transformer and one way to reduce each.

1) Copper loss – minimized by using low-resistance (thick) copper windings; 2) Flux leakage – minimized by using a shell-type core to confine magnetic flux.

Write the expression for the energy stored in an inductor.

The magnetic energy stored in an inductor is W = ½ L I², where L is inductance and I is the current.