How the circuits are made
Every circuit is built from real parts: E12 resistor values, 1.5 V cells, 6 and 9 V batteries, 12 and 24 V power supplies, and the mains for running costs. The values are chosen so most answers come out as exact decimals, and every answer is worked out with exact fractions. The key reduces the circuit step by step: equivalent resistance, the current from the supply, how it splits, the p.d. across each part, then the power.
Six levels
- Series circuits: two to four resistors.
- Parallel circuits: two or three branches.
- Series and parallel together.
- Ammeter and voltmeter readings, and what each one does when a resistor or lamp is removed, added or switched off.
- Power, energy in joules and kilowatt-hours, charge, and the cost of running a mains heater.
- A mix of all of these, plus cells with internal resistance (emf, terminal p.d., lost volts).
An example from set GCSE26
R1 (12 Ω) is in series with R2 (15 Ω) and R3 (10 Ω) in parallel, on a 9 V battery. The parallel pair is 15 × 10 ÷ 25 = 6 Ω, so the total is 18 Ω and the current is 9 ÷ 18 = 0.5 A. R1 takes 0.5 × 12 = 6 V, leaving 3 V across the pair: 0.2 A through R2 and 0.3 A through R3, which add back up to 0.5 A. The power is 3 W in R1, 0.6 W in R2 and 0.9 W in R3, 4.5 W in all. Open set GCSE26
Questions
How do you find the total resistance of a combination circuit?
Work from the inside out. Add resistors that are in series. Replace each group in parallel by one resistor: for two, product over sum; for more, 1/R = 1/R1 + 1/R2 + 1/R3. Repeat until one resistor is left.
What is a V-I-R-P table?
A grid with a row for each resistor and one for the whole circuit, and columns for voltage V, current I, resistance R and power P. Start with the total row, then use V = I R, the series and parallel rules and P = V I to fill the rest.
Why is the total resistance in parallel less than the smallest resistor?
Each branch is another path for the current, so the supply drives more current through the group than through any one branch alone. More current from the same p.d. means less resistance.
What do the ammeters and voltmeters read?
An ammeter is in series and reads the current through its own wire. A voltmeter is connected across a part and reads the p.d. across it. The meters here are ideal: they do not change the circuit.
How accurate do my answers need to be?
Give 3 significant figures. Anything within 1% of the exact value is marked right. Within 5% is marked close, which usually means a number was rounded too early. You can type units and prefixes, like 45 mA or 2.2 kΩ.
Which circuit symbols does it use?
UK and IEC symbols by default: a box for a resistor and a circle with a cross for a lamp, as in the AQA GCSE list. Switch to US zigzag resistors, or to the NCERT symbols with a zigzag resistor, plug key and filament bulb.
What does the AQA option change?
The AQA GCSE Physics specification says students do not have to calculate the total resistance of resistors in parallel. With the option on, parallel totals are never asked: you compare with the smallest branch instead, and add branch currents. Level 6 also leaves out internal resistance, which is not in the AQA specification.
How do I use this with a class?
Share with class sends a link to the same circuits. To stop copying, print a class set: each student gets a different set, and the keys follow, labelled by copy number.
Checked 26 October 2026: AQA GCSE Physics 8463 specification (PDF), sections 4.2.1 to 4.2.4; NCERT Science class 10, chapter 11 Electricity (PDF), Table 11.1 and sections 11.6 and 11.8; AP Physics 2 course page, Unit 11 Electric Circuits.