Contents
1. Charge and Current
The rate of flow of electric charge. Conventional current flows from positive to negative; electrons flow from negative to positive.
A charge of 90 C flows through a wire in 3 minutes. Find the current.
t = 3 × 60 = 180 s → I = 90 ÷ 180 = 0.5 A
Current is the same everywhere in a series circuit. In a parallel circuit, current splits at each junction — the sum of branch currents equals the total current entering the junction.
2. Voltage (Potential Difference)
The energy transferred per unit charge between two points in a circuit. Measured in volts (V). 1 V = 1 J/C.
In a series circuit, voltages across components add up to the supply voltage. In a parallel circuit, all branches have the same voltage as the supply.
A voltmeter is always connected in parallel across the component being measured. An ammeter is always connected in series.
3. Resistance and Ohm's Law
The opposition to current flow. Measured in ohms (Ω).
A 12 V battery drives a current of 4 A through a resistor. Find the resistance.
R = V ÷ I = 12 ÷ 4 = 3 Ω
Factors affecting resistance of a wire
| Factor | Effect on resistance |
|---|---|
| Length ↑ | Resistance ↑ (proportional) |
| Cross-sectional area ↑ | Resistance ↓ |
| Temperature ↑ (for metals) | Resistance ↑ |
| Material (resistivity) | Different materials have different resistances |
Ohmic vs non-ohmic conductors
An ohmic conductor obeys Ohm's law — V/I is constant at constant temperature (e.g. metal wire). A non-ohmic conductor does not have a constant V/I ratio (e.g. filament lamp: resistance increases as it heats up; diode: only conducts in one direction).
4. Series and Parallel Circuits
| Series | Parallel | |
|---|---|---|
| Current | Same throughout: I_total = I₁ = I₂ | Splits: I_total = I₁ + I₂ |
| Voltage | Splits: V_total = V₁ + V₂ | Same across each branch: V_total = V₁ = V₂ |
| Resistance | R_total = R₁ + R₂ + … | 1/R_total = 1/R₁ + 1/R₂ + … |
| Fault effect | One break stops all components | Other branches continue to work |
Two resistors of 6 Ω and 3 Ω are connected in parallel. Find the combined resistance.
1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2
R = 2 Ω
Note: the combined parallel resistance is always less than the smallest individual resistor.
5. Electrical Power and Energy
A 230 V kettle draws 8 A. Find (a) the power, (b) the energy used in 3 minutes.
(a) P = IV = 8 × 230 = 1840 W
(b) E = Pt = 1840 × 180 = 331 200 J ≈ 331 kJ
6. Domestic Wiring and Safety
| Wire | Colour | Function |
|---|---|---|
| Live | Brown | Carries the alternating high voltage (230 V in Singapore) |
| Neutral | Blue | Completes the circuit; at ~0 V |
| Earth | Green/yellow stripe | Safety wire — connected to metal casing of appliance |
Fuse and circuit breaker
A fuse contains a thin wire that melts if current exceeds the rated value, breaking the circuit. A circuit breaker does the same using an electromagnet or bimetallic strip, but can be reset. Both protect against overheating and fire.
If a fault causes the live wire to touch the metal casing, the earth wire provides a low-resistance path. A large current flows → the fuse blows → circuit breaks → user is protected from electric shock.
- Ammeter: in SERIES (to measure current). Voltmeter: in PARALLEL (to measure p.d.).
- Series: same current, voltages add, resistances add (R_total = R1+R2).
- Parallel: same voltage, currents add, total resistance LESS than smallest branch (1/R = 1/R1 + 1/R2).
- Ohm's Law V = IR applies only to ohmic conductors at constant temperature.
- P = IV. Energy = Pt. Cost = Power (kW) x time (h) x rate (cost per kWh).
- Live wire: brown (dangerous, 230V). Neutral: blue (0V). Earth: green-yellow (safety).
7. Common Exam Traps
Adding resistors in parallel always reduces total resistance below the smallest individual value. If your answer is larger than the smallest resistor, you have made an error.
Voltmeter → parallel (across component). Ammeter → series (in line). Swapping them will give wrong readings or damage the meter.
Use P = IV when both I and V are given. Use P = I²R when I and R are given. Use P = V²/R when V and R are given. Memorise all three forms.
Conventional current flows from + to −. Electrons flow from − to +. In most circuit questions, use conventional current direction unless specifically asked about electron flow.
Key Terms — Flashcard Review
Tap each card to reveal the definition.
🎯 Practice Quiz — Test Yourself
8 O Level-style questions on this topic. Select an answer to see instant feedback.
Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.