Theme D · Fields

Webers (Wb); max when field ⟂ surface.

EMF = rate of change of flux linkage.

Induced current opposes the change that made it — energy conservation.

A stationary magnet in a coil induces nothing; flux must be *changing*.

Rod length $L$ moving at $v$ across field $B$.

Step up/down AC voltage via turns ratio.

Step UP voltage ⇒ step DOWN current (power conserved if 100% efficient).

Units Wb (= T m²); the bigger this changes per second, the bigger the EMF.

Current in a field feels a force; reverse current OR field to reverse it.

$\Phi$: Wb · $B$: T · $A$: m² · $\varepsilon, V$: V · $N$: (no unit) · $\tfrac{d\Phi}{dt}$: Wb s⁻¹ = V.

Dropping the minus sign / forgetting Lenz; a steady (unchanging) flux gives ZERO EMF; using $\theta$ from the surface instead of the normal in $\Phi=BA\cos\theta$.