ZIMSEC O Level · Physics · Form 4
Charge, current and voltage
- Current I = charge ÷ time (I = Q/t), measured in amperes with an ammeter in series.
- Potential difference (voltage) is energy per unit charge, measured in volts with a voltmeter in parallel.
- Resistance R = V ÷ I, in ohms (Ω). Ohm's law: for a metal at constant temperature, V ∝ I.
Series and parallel
- Series: same current; voltages add; R = R₁ + R₂.
- Parallel: same voltage; currents add; 1/R = 1/R₁ + 1/R₂ (total is less than the smallest).
Worked example: Find the total resistance of 6 Ω and 3 Ω in parallel.
1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 → R = 2 Ω
Power and energy
P = VI = I²R. Energy = Pt. Electricity is sold in kilowatt-hours (kWh).
Safety
Fuses and circuit breakers cut off the supply if current is too high; earthing protects people from metal cases becoming live. Use the correct fuse rating: just above the normal current.
Exam tip: Draw circuit symbols exactly – a cell, a resistor and a voltmeter each have standard symbols that are marked.
Check yourself
- A charge of 30 C flows in 10 s. Find the current.
- A 240 V heater draws 5 A. Find its power and its resistance.
- Which fuse (3 A, 5 A or 13 A) suits a 600 W, 240 V appliance?
Show answers
- 3 A
- 1 200 W; 48 Ω
- Current = 2.5 A, so the 3 A fuse.
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