Electric current, voltage, resistance, Ohm's law $V = IR$, series and parallel circuits, electric power $P = VI$, and household electricity

What this dot point is asking

QCAA wants Year 11 students to apply Ohm's law and energy conservation to analyse electric circuits.

Charge, current, voltage, resistance

**Charge $q$** (coulombs C). Elementary charge $e = 1.6 \times 10^{-19}$ C.

**Current $I$** ($I = q/t$, amperes A). Rate of charge flow.

**Voltage / potential difference $V$** ($V = E/q$, volts V). Energy per unit charge.

**Resistance $R$** (ohms $\Omega$). Opposition to current.

Ohm's law

For ohmic conductors. Equivalent: $I = V/R$, $R = V/I$.

Series circuits

Same current, voltages add.

$R_{\text{total}} = R_1 + R_2 + R_3 + \ldots$

Parallel circuits

Same voltage, currents add.

$\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \ldots$

For two: $R_p = R_1 R_2 / (R_1 + R_2)$.

Mixed circuits

Simplify step by step: identify parallel combinations, replace with equivalent; identify series sums; apply Ohm's law.

Electric power

Watts (W) = J/s.

Energy

$E = Pt$ (joules or kWh). Household billing in kWh.

1 kWh = 3.6 MJ.

Household electricity

Australian household supply: 230 V AC at 50 Hz. The 230 V is RMS; peak is 325 V.

Safety: fuses/circuit breakers limit current; earth wire for fault paths; RCDs detect imbalances.

Common errors

Adding parallel resistances directly. Use reciprocal formula.

Wrong power formula. Match to your knowns.

Voltage vs current confusion. Series: same I. Parallel: same V.

In one sentence

Electric circuits obey Ohm's law $V = IR$, with series resistances adding and parallel combinations following $1/R = 1/R_1 + 1/R_2 + \ldots$; power $P = VI = I^2 R = V^2/R$, energy $E = Pt$; household electricity in Australia is 230 V AC at 50 Hz and energy is billed in kilowatt-hours.