Series and parallel circuits, resistance, potential difference, power — every electricity calculation at GCSE with worked examples.
Electricity is one of the most calculation-heavy topics in GCSE Physics, and one where students most consistently lose marks through formula confusion and unit errors. This guide covers every circuit concept at GCSE — from what current actually is, through Ohm's law, to the differences between series and parallel circuits and how to calculate power and energy transfer.
Electric current is the rate of flow of charge. It is measured in amperes (A) and flows when there is a complete circuit and a potential difference (voltage) to drive it. Conventional current flows from positive to negative — opposite to the direction of electron flow, which is from negative to positive. This historical convention causes confusion but you must use it in exam answers.
Potential difference (PD) is the energy transferred per unit charge between two points in a circuit. It is measured in volts (V). You can think of it as the "push" driving current around the circuit — a higher PD pushes more current through for a given resistance.
Resistance opposes the flow of current. It is measured in ohms (Ω). The higher the resistance, the lower the current for a given PD.
An ohmic conductor obeys Ohm's law — its resistance stays constant regardless of the current through it (at constant temperature). A resistor at constant temperature is ohmic. Its I-V graph is a straight line through the origin.
Non-ohmic components have resistance that changes with conditions. Two key examples:
A thermistor is a resistor whose resistance decreases as temperature increases. Used in temperature sensors and thermostats. An LDR (light-dependent resistor) has resistance that decreases as light intensity increases. Used in automatic lighting circuits and burglar alarms.
In a series circuit, components are connected in a single loop. All current must flow through every component. The rules:
Adding more resistors in series always increases total resistance and therefore decreases current. If one component in a series circuit fails (breaks the circuit), all components stop working — this is why Christmas lights used to all go out when one bulb failed.
In a parallel circuit, components are connected in separate branches. Current splits between branches. The rules:
Adding more resistors in parallel always decreases total resistance and therefore increases total current drawn from the supply. Household circuits are wired in parallel — each appliance has the full mains voltage across it, and one failing does not affect the others.
❌ The most common parallel circuit error: using R_total = R₁ + R₂ (the series formula) instead of the parallel formula. Remember — parallel resistance is always less than the smallest individual resistance. If your answer is larger than the smallest resistor, you've used the wrong formula.
Power is the rate of energy transfer — how many joules per second. P = IV. For a 230 V mains supply and a 2 A current: P = 230 × 2 = 460 W.
Energy transferred: E = Pt = IVt. If a 100 W bulb is on for 3600 seconds (1 hour): E = 100 × 3600 = 360,000 J = 360 kJ.
The kilowatt-hour (kWh) is a unit of energy used by electricity companies. 1 kWh = 3,600,000 J = 3.6 MJ. Cost = power (kW) × time (hours) × price per kWh.
Cells and batteries supply direct current (DC) — current flows in one direction only. The mains electricity supply in the UK is alternating current (AC) at 230 V and 50 Hz. Alternating current reverses direction 50 times per second.
The three-pin plug: live wire (brown) — carries the high voltage. Neutral wire (blue) — completes the circuit at near zero voltage. Earth wire (green and yellow) — safety wire connected to the metal casing of appliances. The fuse is in the live wire — it melts if current is too high, breaking the circuit before cables overheat.
The AQA electricity specification is at the AQA GCSE Physics specification page.
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