IGCSE Physics Electricity Notes
Electricity is charge flowing round a circuit. Get current, voltage and resistance clear and the whole topic makes sense.
Key concepts
- Current (I)
- The rate of flow of charge in amps (A). Measured with an ammeter in series.
- Voltage / p.d. (V)
- Energy transferred per unit charge in volts (V). Measured with a voltmeter in parallel.
- Resistance (R)
- Opposition to current in ohms (Ω). Higher resistance means less current for the same voltage.
- Current is the same everywhere in a series circuit.
- Charge (Q) = current (I) × time (t).
- Electrical energy transferred = power × time.
Key formulas
Ohm's law
V = I × R
Charge
Q = I × t
coulombs = amps × seconds.
Electrical power
P = V × I
also P = I²R.
Required practicals
- Investigate how the current through a resistor varies with voltage (I–V characteristics).
- Investigate how resistance depends on the length of a wire.
Worked example
A 230 V supply drives 2 A through a heater. Find the power.
- 1Write the equation: P = V × I.
- 2Substitute: P = 230 × 2.
- 3Calculate: P = 460 W.
- 4State the unit: watts (W).
Answer: P = 460 W
Common mistakes
✗ Connecting an ammeter in parallel or a voltmeter in series.
✓ Ammeter in series, voltmeter in parallel.
✗ Confusing current and voltage.
✓ Current is flow of charge (A); voltage is energy per charge (V).
✗ Forgetting units in the answer.
✓ Quote A, V, Ω or W as appropriate.
Quick check
Try these, then reveal the answers.
What is the unit of charge?Reveal answer
The coulomb (C).
A current of 3 A flows for 10 s. How much charge passes?Reveal answer
Q = 30 C
How is a voltmeter connected?Reveal answer
In parallel with the component.
Exam tips
- Rearrange equations before substituting to secure the method mark.
- For I–V graphs, a straight line through the origin means constant resistance (ohmic).
- Track units carefully throughout the calculation.
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Frequently asked questions
What is the difference between current and voltage?+
Current is the rate of flow of charge (amps); voltage (p.d.) is the energy transferred per unit charge (volts).
Why does a filament lamp's resistance increase with current?+
As current heats the filament, its ions vibrate more and oppose the flow of charge, raising resistance.
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