IGCSE Physics: Electricity and Circuits — The Complete Guide to Scoring Full Marks

Electricity and circuits typically account for 20-25% of the marks on IGCSE Physics Paper 4 (Extended) and Paper 2 (Core). That is more than any other single topic. Yet year after year, examiner reports from Cambridge highlight the same issues: students confuse current in series and parallel circuits, misapply Ohm's law, and lose marks on straightforward power calculations because they pick the wrong formula or forget to convert units. The good news is that electricity questions follow predictable patterns. Once you understand the underlying logic of how charge moves through circuits, most problems become almost mechanical. The bad news is that partial understanding is worse than no understanding — students who have a vague sense of "voltage goes up in parallel" but cannot explain why will consistently lose marks on multi-step problems where the examiner is testing conceptual clarity, not just formula recall. This guide is structured around the three areas that account for the vast majority of lost marks: Ohm's law and V-I characteristics, series versus parallel circuit analysis, and power and energy calculations. Master these three areas and you have covered roughly 80% of what the electricity section will throw at you.

Every IGCSE Physics student can write V = IR. Far fewer can explain what it actually means, and this is where marks are lost. Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant. The "provided temperature remains constant" part is critical — it means Ohm's law is NOT a universal law of electricity. It is a description of how ohmic conductors (like metal wires at constant temperature) behave. A filament lamp is not ohmic: as current increases, the filament heats up, its resistance increases, and the V-I graph curves upward. A diode is not ohmic: it conducts in one direction only, producing a characteristic hockey-stick curve. A thermistor is not ohmic: its resistance decreases as temperature increases. Cambridge examiners love asking students to sketch, identify, or interpret V-I characteristic graphs. The three you must know perfectly are: a straight line through the origin for an ohmic resistor, an upward curve for a filament lamp, and the forward-bias exponential curve for a diode. When asked to explain the shape, always link it to resistance. The filament lamp curve bends because resistance increases with temperature. The diode curve shows negligible current in reverse bias because resistance is extremely high, then a sharp increase in current once the threshold voltage (about 0.7V for silicon) is reached. A common exam trap is asking students to "determine the resistance at a specific point on a V-I graph." Many students try to calculate the gradient of the curve. This is wrong. Resistance at a point is V/I at that point — read the values directly from the axes and divide. The gradient gives you 1/R only for a straight line through the origin.

If you learn nothing else from this guide, learn these six rules — they are sufficient to answer almost every IGCSE circuit question. Series circuits: (1) The current is the same at every point in the circuit. Current does not get "used up" by components — this is the single most common misconception in IGCSE Physics. A 2A current entering a resistor is a 2A current leaving that resistor. (2) The total voltage of the supply equals the sum of the voltages across each component. If a 12V battery powers three resistors, and the first two have 4V and 5V across them, the third must have 3V. (3) The total resistance is the sum of individual resistances: R_total = R1 + R2 + R3. Parallel circuits: (4) The voltage across each branch is the same and equals the supply voltage. If a 12V battery has three parallel branches, each branch has 12V across it. (5) The total current from the supply equals the sum of the currents through each branch. (6) The total resistance is found using 1/R_total = 1/R1 + 1/R2 + 1/R3, which always gives a total resistance LESS than the smallest individual resistor. Here is the conceptual key that makes all six rules intuitive: think of current as water flow and voltage as water pressure. In a series circuit, there is only one path, so all the water flows through every component (same current everywhere), and the pressure drops across each component (voltages add up). In a parallel circuit, the water can split into different paths, so the pressure is the same in each path (same voltage), but the total flow is the sum of flows in each path (currents add up). When you face a complex circuit with both series and parallel elements, the strategy is always the same: identify which components are in series, which are in parallel, simplify the parallel sections first into equivalent single resistors, then solve the resulting series circuit.

Power questions are among the easiest marks available in IGCSE Physics — IF you pick the correct formula. There are three power equations: P = IV (power equals current times voltage), P = I²R (power equals current squared times resistance), and P = V²/R (power equals voltage squared divided by resistance). All three are mathematically equivalent — they are just rearrangements of P = IV combined with V = IR. The skill is choosing the one that matches the variables you already know. If the question gives you voltage and current, use P = IV. If it gives you current and resistance, use P = I²R. If it gives you voltage and resistance, use P = V²/R. Never calculate a missing variable first and then use P = IV — this introduces an extra step where you can make errors. Go directly to the formula that uses what you have. For energy calculations, E = Pt (energy equals power times time) is the master formula. But remember: time must be in seconds. A question that says "the heater runs for 5 minutes" requires you to convert to 300 seconds before multiplying. This unit conversion error appears in examiner reports every single year as one of the top reasons students lose marks on otherwise correct calculations. The alternative energy formula E = QV (energy equals charge times voltage) is used when you know the charge flowing rather than the current and time. Since Q = It, the two formulas are equivalent: E = QV = ItV = Pt. Understanding this connection means you never need to memorise formulas in isolation — you can derive any one from the others.

When you face a circuit problem in the exam, follow this method every single time. Step 1: Identify whether the circuit is series, parallel, or a combination. If combination, identify which parts are in series and which are in parallel. Step 2: Write down what you know — list every value the question gives you (voltage, current, resistance, power) next to the relevant component on the diagram. Step 3: Apply the appropriate circuit rules. In series: current is the same everywhere, voltages add up, resistances add up. In parallel: voltage is the same across branches, currents add up, use 1/R formula for resistance. Step 4: Calculate what the question asks for, showing every step. Write the formula first, then substitute the numbers, then calculate. Never skip steps — method marks are awarded for the working even if the final answer is wrong. Step 5: Check your answer makes physical sense. If you calculated a current of 500A through a small resistor connected to a 6V battery, something is wrong. If you calculated a total resistance of 200 ohms for two 50-ohm resistors in parallel, something is wrong (it should be less than 50). This method takes about 15 seconds to set up and prevents the majority of errors I see students make. The students who lose marks are not the ones who cannot do the maths — they are the ones who rush into calculations without first establishing what type of circuit they are dealing with.