D.4 Induction (HL only)

Electromagnetic induction is the process by which a changing magnetic field generates an electromotive force (emf) , and if the circuit is closed, a current , in a conductor. This is not just a lab curiosity: it is the principle behind every electrical generator, transformer, and wireless charger in the modern world.

The key insight, discovered by Michael Faraday in 1831, is that it is change that matters. A static magnetic field through a loop does nothing. But as soon as that field changes , whether by moving the magnet, changing the field strength, or rotating the loop , electricity is produced.

Magnetic Flux: Magnetic flux $\Phi$ is a measure of the total magnetic field passing through a given area. It is defined as:
$\Phi =BA\cos \theta$
where $B$ is the magnetic flux density (T), $A$ is the area of the loop (m²), and $\theta$ is the angle between the magnetic field direction and the normal (perpendicular) to the loop surface.

The SI unit of magnetic flux is the weber (Wb), where $1\text{Wb}=1\text{T}\cdot {\text{m}}^2$.

Understanding the $\cos \theta$ factor is crucial:

• When the field is perpendicular to the loop ($\theta =0°$): $\Phi =BA$ , maximum flux.
• When the field is parallel to the loop ($\theta =90°$): $\Phi =0$ , no flux threads through the loop.