This is a core skill tested in Criteria A and C. You must know:

Series circuits:

- Current: same everywhere ($I = I_1 = I_2 = ...$).

- Voltage: splits across components ($V_{total} = V_1 + V_2 + ...$).

- Resistance: $R_{total} = R_1 + R_2$.

- Adding more resistors increases total resistance and decreases current.

Parallel circuits:

- Current: splits at junctions ($I_{total} = I_1 + I_2 + ...$).

- Voltage: same across each branch ($V_{total} = V_1 = V_2$).

- Resistance: $1/R_{total} = 1/R_1 + 1/R_2$, total resistance is always less than the smallest individual resistance.

- Adding more resistors in parallel decreases total resistance and increases total current from the battery.

Many questions combine Ohm’s law with circuit rules – always identify series/parallel first.

You must be able to recognise and sketch I–V graphs (current on y‑axis, voltage on x‑axis).

- Ohmic conductor (resistor): straight line through origin; constant resistance.

- Filament lamp: S‑shaped curve; resistance increases with current because the filament heats up.

- Diode: very low current in reverse bias (almost zero), steep increase in forward bias above threshold voltage.

Typical ‘explain’ question: “Why does the resistance of a filament lamp increase as the current increases?” – Because the filament gets hotter, causing the metal ions to vibrate more, making it harder for electrons to flow.