Year 13 · Science · Physical sciences
Electric circuits and power
Students apply Ohm's law, rules for series and parallel circuits and electrical power calculations to circuits and household appliances.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
These lessons were written around Australian Curriculum strands and matched to New Zealand year levels by age (NZ Year 1 is the first year of school). They are not yet mapped to The New Zealand Curriculum, so check them against your own learning programme.
Learning goals
Learners will
- Define current, potential difference, resistance and power, with their units.
- Apply Ohm's law and power equations.
- Calculate equivalent resistance in series and parallel circuits.
Success looks like
- I can use V = IR and P = VI to solve problems.
- I can calculate total resistance for resistors in series and in parallel.
- I can calculate the current drawn by an appliance from its power rating.
The big idea
Current (I) is the rate at which charge flows, in amperes. Potential difference (V), or voltage, is the energy given to each coulomb of charge, in volts. Resistance (R) opposes current, in ohms.
Ohm's law: V = IR. For an ohmic conductor at constant temperature, current is directly proportional to potential difference.
In a series circuit the current is the same everywhere, and resistances add: R_total = R₁ + R₂ + … The supply voltage is shared between components.
In a parallel circuit each branch has the full supply voltage, the branch currents add up to the total current, and 1/R_total = 1/R₁ + 1/R₂ + … The total resistance is always less than the smallest branch resistance.
Electrical power is P = VI. Combining with Ohm's law gives P = I²R and P = V²/R. Energy used is E = Pt; electricity bills use kilowatt-hours (1 kWh = 3.6 million joules).
Australian household mains supply is nominally 230 V AC. Household circuits are wired in parallel so each appliance receives the full voltage and can be switched independently. Circuit breakers and safety switches (residual current devices) protect people and wiring.
Mains electricity can kill. All calculations in this lesson use appliance labels and battery devices only; never open or modify mains-powered equipment.
Worked examples
Ohm's law
A 12 V battery is connected across a 4 Ω resistor. What current flows?
- I = V ÷ R.
- I = 12 ÷ 4 = 3 A.
Answer: 3 A.
Series and parallel
Resistors of 6 Ω and 3 Ω are connected (a) in series and (b) in parallel to a 9 V supply. Find the total resistance and total current in each case.
- (a) Series: R = 6 + 3 = 9 Ω. I = 9 ÷ 9 = 1 A.
- (b) Parallel: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so R = 2 Ω.
- I = 9 ÷ 2 = 4.5 A.
Answer: (a) 9 Ω, 1 A; (b) 2 Ω, 4.5 A.
Appliance current
A kettle is rated at 2300 W on a 230 V supply. What current does it draw, and how much energy does it use in 3 minutes?
- I = P ÷ V = 2300 ÷ 230 = 10 A.
- E = Pt = 2300 W × 180 s = 414 000 J.
- In kilowatt-hours: 2.3 kW × 0.05 h = 0.115 kWh.
Answer: 10 A; 414 000 J (0.115 kWh).
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
What is the unit of resistance?
Ohm — Resistance is measured in ohms (Ω).
A current of 2 A flows through a 5 Ω resistor. What is the potential difference across it?
10 V — V = IR = 2 × 5 = 10 V.
Two 10 Ω resistors are connected in parallel. What is the total resistance?
5 Ω — 1/R = 1/10 + 1/10 = 2/10, so R = 5 Ω.
Why are household appliances connected in parallel?
So each receives the full supply voltage and can be switched independently — Parallel wiring gives each appliance the full voltage and independent control.
A 60 W device runs for 5 hours. How much energy does it use?
0.3 kWh — E = 0.06 kW × 5 h = 0.3 kWh.
Resistors of 4 Ω, 6 Ω and 10 Ω are in series. What is the total resistance in ohms? (number)
20 — 4 + 6 + 10 = 20 Ω.
A 1150 W toaster runs on 230 V. What current does it draw in amperes? (number)
5 — I = P ÷ V = 1150 ÷ 230 = 5 A.
A 3 A current flows through a 4 Ω resistor. What power is dissipated in watts? (number)
36 — P = I²R = 3² × 4 = 9 × 4 = 36 W.
Watch out for
- Students often think current is 'used up' as it passes through components. Current is the same before and after a component in series; energy is transferred, not charge.
- Some think adding resistors in parallel increases total resistance. It decreases, because there are more paths for current.
- Some confuse power (W) with energy (J or kWh). Power is the rate of energy transfer.
Tips for parents
- Collect power ratings from appliance labels and have your student calculate the current and the cost of running each for an hour using your electricity tariff.
- Encourage neat circuit diagrams with standard symbols before calculating.
- Safety: never open, modify or probe mains-powered equipment or power points, and read appliance labels only when the appliance is switched off and unplugged.
Go further
- Estimate your household's daily electricity use from appliance ratings and hours of use, then compare with a recent bill.
- Research how rooftop solar panels and home batteries are connected to the grid in Australia.
- Derive P = I²R and P = V²/R from P = VI and V = IR.