Mistakes Students Make in IGCSE Physics Electricity Questions

Electricity is one of the highest-mark topics in IGCSE Physics. Specific errors appear repeatedly.

Mistake 1: Confusing series and parallel

In series: current is the same through all components. Voltage is shared between components. In parallel: voltage is the same across all branches. Current is shared between branches.

Students frequently swap these, saying “voltage is the same in series.” Check by thinking about what stays the same on a single path (current) versus what is the same across branches that share two nodes (voltage).

Mistake 2: Adding resistance incorrectly

Series: R_total = R1 + R2 + R3 Parallel (two resistors): 1/R_total = 1/R1 + 1/R2

Students sometimes add parallel resistors directly (R_total = R1 + R2), which is the series formula. In parallel, the total resistance is always less than the smallest individual resistor.

Mistake 3: Using P = I^2 R incorrectly

P = I^2 R applies when you know the current through a specific component and its resistance. If you use the total circuit current with just one component’s resistance, you get the wrong answer.

Mistake 4: Ammeter in parallel

An ammeter must be connected in series. Connecting it in parallel creates a short circuit (ammeters have very low resistance). A voltmeter must be connected in parallel across the component you are measuring.

Mistake 5: Confusing EMF and terminal voltage

EMF is the total energy per coulomb supplied by the cell. Terminal voltage is the voltage across the cell’s terminals, which is less than the EMF when current flows because of the internal resistance. Many students assume the voltmeter across the cell reads the EMF; it only does so when no current flows.

Mistake 6: Forgetting that adding a resistor in parallel reduces total resistance

When a second lamp is added in parallel, the total resistance decreases. This means the total current from the supply increases. Students often assume each lamp gets the same current as before, but the total current changes.

Mistake 7: Not recognising a short circuit

If a wire connects directly across a component with no other resistance, all the current takes the wire (it has essentially zero resistance). The component carries no current. Students miss this when a switch or wire bypasses a component.