Magnetism and Electromagnetism flashcards

Like poles repel and unlike poles attract. Cutting a magnet gives two smaller magnets, each with its own N and S pole; a lone pole cannot be isolated.

Point the right thumb along conventional current and curl the fingers: they show the circular field direction. Current out of the page (dot) gives counterclockwise; into the page (cross) gives clockwise.

Ampere-turns $=N\times I$ (turns times current), the coil's magnetizing strength. Add turns, raise the current, or add a soft iron core. 300 turns at 0.2 A equals 30 turns at 2 A.

$F=BI\ell \sin \theta$: B in teslas, I in amperes, $\ell$ the length in the field in meters, F in newtons. Maximum when the wire crosses the field; zero when parallel. Force is perpendicular to both.

$\Phi =BA\cos \theta$, in webers (1 Wb = 1 T times 1 m${}^2$), where $\theta$ is between the field and the surface normal. 0.5 T straight through 0.04 m${}^2$ gives 0.02 Wb.

$\mathcal{E}=N\frac{\Delta \Phi }{\Delta t}$: N turns, flux change in webers, time in seconds, emf in volts. Faster change or more turns gives more emf; a steady field induces nothing.

The induced current always flows so its own magnetic field opposes the change in flux that caused it. This reflects energy conservation: work must be done to get electrical energy out.

$W=\frac{1}{2}L{I}^2$ joules (L in henries, I in amperes), so doubling current quadruples energy. $\tau =L/R$ seconds; current reaches about 63% after one $\tau$, essentially full after five.