Electromagnetic induction (A-level only) Flashcards – AQA A-level Physics (7408)

What is electromagnetic induction?

The production of an emf when a circuit’s magnetic flux linkage changes.

Under what condition does an induced current flow in a circuit?

The circuit must be closed.

What are two ways to induce an emf in a coil?

• Move a magnet relative to the coil.
• Change the magnetic field through a stationary coil.

What is the magnetic flux through a flat area in a uniform magnetic field?

$\Phi = BA\cos\theta$

Here $\theta$ is the angle between the field and the normal to the area.

What is the flux linkage of a coil with $N$ turns and the same flux $\Phi$ through each turn?

$N\Phi$

What does Faraday’s law state, and what is its equation for a uniform or average change in flux?

The magnitude of the induced emf equals the rate of change of flux linkage.

$\varepsilon = N\frac{\Delta\Phi}{\Delta t}$

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

Its effect opposes the change in magnetic flux linkage that produces it.

In a closed circuit, how does the induced current’s magnetic field relate to the change in flux linkage?

It opposes the change in flux linkage, not necessarily the original magnetic field.

What happens to the induced emf and, in a closed circuit, the induced current when the change in flux linkage is reversed?

Both reverse direction.

Which conservation principle is consistent with the opposition described by Lenz’s law?

Conservation of energy.

Why can motion through a magnetic field induce an emf between the ends of a straight conductor?

Charges moving with the conductor experience a magnetic force, causing charge separation along it.

A straight conductor of length $l$ moves at speed $v$ perpendicular to a uniform magnetic field of flux density $B$. The conductor is perpendicular to both the field and its motion. What is the magnitude of the induced emf?

$\varepsilon = Blv$

What happens to the polarity of the emf induced in a moving conductor if either its direction of motion or the magnetic field direction is reversed?

The polarity reverses.

Why does a uniformly rotating coil in a magnetic field have an alternating induced emf?

Its magnetic flux linkage changes as it rotates.

A coil of $N$ turns and area $A$ rotates at angular speed $\omega$ in a uniform field of flux density $B$. Its normal is initially aligned with the field. What is its flux linkage at time $t$?

$NBA\cos(\omega t)$

A coil of $N$ turns and area $A$ rotates at angular speed $\omega$ in a uniform field of flux density $B$. Its normal is initially aligned with the field. What is its induced emf at time $t$?

$\varepsilon = BAN\omega\sin(\omega t)$

The sign depends on the chosen terminal orientation.

What is the maximum induced emf magnitude for a coil of $N$ turns and area $A$ rotating at angular speed $\omega$ in a uniform field of flux density $B$?

$BAN\omega$

When is the induced emf zero in relation to the flux linkage of a rotating coil?

When the flux linkage is momentarily at a maximum or minimum.

When does a rotating coil's induced emf have maximum magnitude in relation to its flux linkage?

When the flux linkage is changing most rapidly.