This topic connects three quantities: potential difference (p.d.), current and resistance. You learn to calculate one from the other two, predict how current and p.d. share out in series and parallel circuits, read what a current-voltage graph says about a component, and place meters correctly on a diagram.
The exact equations and the level of detail depend on your syllabus year, including which resistor combination formulas your route requires. Check the current Cambridge IGCSE Physics 0625 page for your exam year. The Physics learning guide shows where this module sits in the whole subject.
What do I need to know first?
You need the ideas in charge and current: current is the rate of flow of charge, it is measured in amperes, and it is the same at every point in a series loop. You also need basic unit skills, including milli- and kilo- prefixes, from measurement and quantities.
Comfort with rearranging a three-term equation matters here. Most lost marks in this topic come from unit conversion and from reading a diagram too quickly.
One orienting worked example
A 12 V supply is connected to a 10 Ω resistor and a 20 Ω resistor in series. (Invented example data.) Find the current and the p.d. across each resistor.
Step 1, total resistance: in series the resistances add, so R = 10 + 20 = 30 Ω.
Step 2, current: I = V ÷ R = 12 ÷ 30 = 0.40 A. The same current flows through both resistors.
Step 3, p.d. across each: V = I × R gives 0.40 × 10 = 4.0 V and 0.40 × 20 = 8.0 V.
Step 4, check: 4.0 + 8.0 = 12 V, which matches the supply. The larger resistor takes the larger share.
Every lesson in this module is a variation on those moves: list the quantities with units, choose the relationship, calculate, then check against the circuit.
In what order should I study the lessons?
- Use a resistance relationship with units: the core equation R = V ÷ I and the unit habits everything else depends on.
- Analyse current in series and parallel branches: how current and p.d. share out, and how to find a combined resistance.
- Interpret a current-voltage graph: reads resistance from a point on a graph, including curved lines.
- Distinguish a component law from a universal rule: separates the definition of resistance from Ohm’s law for a particular component.
- Explain meter placement on a diagram: where an ammeter and a voltmeter go, and what happens when they are swapped.
Then test yourself with the potential difference, resistance and circuits practice set.
Which traps catch most students?
- Leaving current in milliamperes while using volts and ohms.
- Adding resistances in parallel as if they were in series.
- Saying the current is “used up” as it passes round a series circuit.
- Reading resistance as the gradient of a curved current-voltage graph instead of V ÷ I at a point.
- Claiming a filament lamp “breaks” V = I × R, when what fails is the proportionality of current and p.d.
- Connecting a voltmeter in series, or an ammeter across a component.
How should I use the practice set?
Write every equation and the unit on each answer line, then open the worked answer. Use the error-routing section at the end to return to the right lesson. Afterwards, log any repeated slip in the mistake log and retest queue so it comes back later as a fresh question.
The circuit reasoning simulator is useful for checking a prediction before you look at the answer. Students who know every equation yet still lose marks at the diagram or the graph can gain from a teacher watching the working as it happens. That is what our online one-to-one Physics tuition is designed to offer.