Module 4: Electricity and Magnetism

For many components the current is proportional to the voltage. This is Ohm's law, the single most useful equation in circuit work.

In a series circuit the components sit one after another on a single loop. The same current flows through every component, the voltages across them add up to the supply voltage, and the resistances simply add: $R_T = R_1 + R_2 + \ldots$

In a parallel circuit the components sit on separate branches. Each branch has the full supply voltage across it, the currents in the branches add to the total, and the combined resistance is found from $\dfrac{1}{R_T} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \ldots$ (always less than the smallest branch).

Power is the rate at which a circuit transfers energy, in watts (W). Combining it with Ohm's law gives three handy forms.

Every magnet has a north and a south pole, and like poles repel while unlike poles attract. The magnetic field is drawn with field lines that run from north to south outside the magnet, and never cross. Poles always come in pairs: break a magnet in half and each piece has its own north and south.

A current-carrying wire creates a magnetic field that circles around it. The right-hand grip rule gives the direction: point your right thumb along the current, and your fingers curl the way the field points. The field is strongest close to the wire and weakens with distance.

Coiling a wire into a solenoid lines up the field of every loop, producing a strong, uniform field inside that looks just like a bar magnet, with a north and south pole at its ends. Adding a soft-iron core makes an electromagnet, whose strength you can control with the current and switch off entirely.

Only a few materials, called ferromagnetic (iron, nickel, cobalt), can be strongly magnetised. Inside them are tiny regions called domains, each a small magnet. In an unmagnetised sample the domains point every which way and cancel out; in a magnetised one they line up.