Energy, electricity and electronics
What the domain tests
Energy, electricity and electronics tests safe circuit reasoning and energy calculations. Expect forms and conversions of energy, power and efficiency, Ohm's law, series and parallel circuits, component symbols and polarity, resistors for LEDs, simple sensors and transistor switching, and renewable sources.
Core rules and concepts
- Ohm's law: V = IR. Electrical power: P = VI = I²R. Energy in kWh = power in kW × time in hours.
- In series, current is the same through each component and voltages add. In parallel, voltage is the same across branches and currents add.
- A diode conducts one way; an LED needs a series resistor to limit current. A transistor can act as an electronic switch, with a small base current controlling a larger collector current.
- Mains electricity is dangerous; school work should use low-voltage supplies. Check polarity, insulation and correct fuse rating.
- Efficiency = useful output energy ÷ input energy × 100%.
Worked example
A 12 V supply drives an LED with a forward voltage of 2 V at 20 mA. The resistor must drop 12 − 2 = 10 V. R = V ÷ I = 10 ÷ 0.020 = 500 Ω. The nearest preferred value is 560 Ω. Resistor power = I²R = 0.020² × 560 = 0.224 W, so a 0.5 W resistor is a safe choice; a 0.25 W resistor would run too close to its rating. The LED polarity and resistor position in series must be checked.
Traps that cost marks
Using the full supply voltage across the LED, forgetting to convert mA to A, choosing a resistor wattage that is too low, or assuming current is the same in parallel branches are common errors. Never connect an LED without a current-limiting resistor. Do not treat a short circuit as a valid test.