Energy, electricity and electronics
What this domain tests
Energy and electronics questions test energy accounting, circuit laws and safe component selection. Expect units, efficiency and a simple calculation.
Core rules
- Energy and power: energy (J) = power (W) × time (s); household energy in kWh; efficiency = useful output/input × 100.
- Ohm’s law: V = IR; power P = VI = I²R.
- Series: same current, voltages add, R_total = R1 + R2 + ...
- Parallel: same voltage, currents add, 1/R_total = 1/R1 + 1/R2 + ...
- Components: a resistor limits current; a diode conducts one way; an LED needs a series resistor; a transistor or relay switches a larger load; LDR and thermistor are sensors; a microcontroller executes a program.
- Supply: mains is 230 V a.c. in Malta, batteries are d.c.; fuse or breaker in the live conductor; earth for class I appliances.
Worked example
An LED with a forward voltage of 2.0 V and a current of 20 mA is run from a 9 V battery. Series resistor R = (9 − 2)/0.020 = 350 Ω. The nearest preferred value is 390 Ω, giving I = 7/390 = 17.9 mA, which is safe and bright enough. Resistor power = I²R = 0.0179² × 390 = 0.125 W, so a 0.25 W resistor is adequate with margin. The reasoning checks both current and power, not just the ohms.
Traps
- Forgetting the LED forward voltage.
- Mixing mA and A in the same calculation.
- Applying series rules to a parallel circuit.
- Connecting an ammeter across a component, or a voltmeter in series.
- Omitting the fuse rating or using the earth wire as a return.