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Cambridge IGCSE Physics 0625: Current, Voltage and Resistance Notes
Electric current is the flow of charge, voltage is what pushes it round a circuit, and resistance is what opposes it. They are linked by V = IR. These notes explain electricity and circuits for the Cambridge IGCSE Physics (0625) syllabus: circuit symbols, current and charge, voltage, resistance, series and parallel circuits, I–V graphs, and electrical power and energy, with circuit calculators, a quiz, worked examples and exam tips.
Try the circuit calculatorsBook a 1-to-1 lessonWritten by Ms. Humaira Asim, a science teacher with 15+ years of teaching experience. Written for the Cambridge IGCSE Physics 0625 syllabus.
Circuit symbols
Current and charge
Electric current is the rate of flow of electric charge. In metal wires, the charge is carried by electrons. Current is measured in amperes (A) with an ammeter, which is connected in series.
Conventional current flows from the positive terminal to the negative terminal. Electrons actually flow the opposite way, from negative to positive.
Voltage (p.d. and e.m.f.)
Voltage is measured in volts (V) with a voltmeter, which is connected in parallel (across a component).
- The electromotive force (e.m.f.) of a cell or battery is the energy it gives to each unit of charge that passes through it. (Extended)
- The potential difference (p.d.) across a component is the energy transferred by each unit of charge passing through it. (Extended)
Resistance and V = IR
Resistance opposes the flow of current. It is measured in ohms (Ω).
| A wire has more resistance if it is… | Why |
|---|---|
| longer | Electrons have further to travel and more collisions (R is proportional to length) |
| thinner | There is less space for electrons to flow (R is inversely proportional to cross-sectional area) |
| hotter (for a metal) | The atoms vibrate more and get in the way of electrons |
You can measure resistance by connecting an ammeter in series and a voltmeter in parallel with the component, then using R = V ÷ I.
Series circuits
- The current is the same at every point.
- The voltages add up to the supply voltage.
- Total resistance = R₁ + R₂ + …
- If one lamp breaks, the circuit is broken and all the lamps go out.
Parallel circuits
- The voltage across each branch is the same as the supply voltage.
- The branch currents add up to the total current.
- The total resistance is less than the smallest resistance in the circuit.
- Extended: 1/R = 1/R₁ + 1/R₂
- Each branch can be switched on and off separately, which is why house lights are wired in parallel.
I–V graphs (Extended)
Electrical power and energy
The cost of electricity is worked out in kilowatt-hours (kWh): energy (kWh) = power (kW) × time (hours). Then cost = kWh × price per kWh.
Worked examples
Example 1: using V = IR
A current of 2.0 A flows through a 6.0 Ω resistor. Calculate the voltage across it.
- V = IR = 2.0 × 6.0
- = 12 V
Example 2: finding resistance
A 9.0 V battery makes a current of 0.30 A flow through a lamp. Calculate the resistance of the lamp.
- R = V ÷ I = 9.0 ÷ 0.30
- = 30 Ω
Example 3: charge (Extended)
A current of 0.50 A flows for 2.0 minutes. How much charge passes?
- Convert time: 2.0 min = 120 s
- Q = It = 0.50 × 120
- = 60 C
Example 4: series circuit
A 4 Ω and an 8 Ω resistor are connected in series to a 6 V battery. Find the current and the voltage across each resistor.
- Total R = 4 + 8 = 12 Ω
- I = V ÷ R = 6 ÷ 12 = 0.5 A (the same everywhere)
- V across 4 Ω = 0.5 × 4 = 2 V; across 8 Ω = 0.5 × 8 = 4 V
- Check: 2 + 4 = 6 V ✓
Example 5: parallel circuit
A 6 Ω and a 3 Ω resistor are connected in parallel to a 6 V battery. Find the current in each branch, the total current and the combined resistance.
- Each branch has 6 V across it
- I₁ = 6 ÷ 6 = 1 A; I₂ = 6 ÷ 3 = 2 A
- Total current = 1 + 2 = 3 A
- Combined R = V ÷ I = 6 ÷ 3 = 2 Ω (less than both resistors)
Example 6: power and energy
A kettle runs on 230 V and draws 10 A. Calculate its power, and the energy it uses in 3.0 minutes.
- P = IV = 10 × 230 = 2300 W
- t = 3.0 × 60 = 180 s
- E = Pt = 2300 × 180 = 414 000 J
Try it yourself: circuit calculators
Use these to check your answers to the practice questions.
V = IR
Fill in any two boxes.
Two-resistor circuit
Power and energy
P = IV and E = IVt
Quick quiz
Quick quiz: circuits
Choose an answer for each question. You get feedback straight away.
1. How should an ammeter be connected?
2. How should a voltmeter be connected?
3. In a series circuit, the current is…
4. In a parallel circuit, the voltage across each branch is…
5. What happens to the total resistance when you add a resistor in parallel?
6. A 12 V supply drives 3 A through a resistor. What is its resistance?
7. What is the unit of electrical charge?
8. Why does the resistance of a filament lamp increase as the current increases?
Common mistakes
- Putting meters the wrong way round. Ammeters go in series; voltmeters go in parallel.
- Saying current is “used up”. Current is the same all the way round a series circuit. Energy is transferred, not current.
- Adding parallel resistances. In parallel, the total resistance is less than the smallest one.
- Forgetting to convert minutes to seconds in Q = It and E = IVt.
- Mixing up W and kW, or J and kWh in cost questions.
Practice questions
Try each question first, then open it to check your answer.
Q1. A 12 V battery drives a current of 0.4 A through a resistor. Calculate its resistance.
R = 12 ÷ 0.4 = 30 Ω
Q2. What current flows through a 15 Ω resistor with 3.0 V across it?
I = 3.0 ÷ 15 = 0.20 A
Q3. Resistors of 10 Ω and 20 Ω are connected in series. What is the total resistance?
30 Ω
Q4. Two 10 Ω resistors are connected in parallel. What is the combined resistance?
1/R = 1/10 + 1/10 = 2/10, so R = 5 Ω
Q5. A current of 3.0 A flows for 20 s. How much charge passes? (Extended)
Q = 3.0 × 20 = 60 C
Q6. Calculate the power of a 12 V lamp that draws 2.0 A.
P = 2.0 × 12 = 24 W
Q7. A 2.0 kW heater is used for 3 hours. Electricity costs 30p per kWh. Calculate the cost.
Energy = 2.0 × 3 = 6.0 kWh. Cost = 6.0 × 30 = 180p (£1.80)
Q8. Why are the lights in a house wired in parallel?
So that each light gets the full supply voltage and can be switched on and off separately; if one bulb breaks, the others stay on.
Exam tips
- Write the formula, substitute with units, then give the answer with its unit (A, V, Ω, C, W, J).
- For series and parallel questions, first find the total resistance, then the total current, then each part.
- Check your answers: in series, voltages should add to the supply; in parallel, currents should add to the total.
- Draw circuit symbols neatly with a ruler and leave no gaps in the wires.
Frequently asked questions
What is the difference between current and voltage?
Current is the flow of charge around a circuit, measured in amperes. Voltage is the energy given to or transferred by each unit of charge, which pushes the current round, measured in volts.
Is current the same in a parallel circuit?
No. In a parallel circuit, the current splits between the branches. The branch currents add up to the total current from the supply.
What does V = IR stand for?
Voltage (V, in volts) = current (I, in amperes) × resistance (R, in ohms). It is sometimes called Ohm’s law.
Are these notes suitable for KS3 or Year 8 science?
The sections on circuit symbols, current, voltage, and series and parallel circuits suit Year 7 to 9 (KS3) science. Q = It, I–V graphs and the parallel resistance formula are GCSE and IGCSE content.
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Book a 1-to-1 lessonTry the Physics calculatorRelated notes: Speed, velocity and acceleration · Forces, resultant force and vectors. This topic is also part of GCSE and IGCSE Combined Science: see all Combined Science notes. These notes follow the Cambridge IGCSE Physics 0625 syllabus. Always check the current syllabus on the Cambridge International website.