ZIMSEC O Level · Combined Science · Form 3, Form 4
Circuits
- Current (I) is the flow of charge, measured in amperes with an ammeter connected in series.
- Voltage (V) is measured in volts with a voltmeter connected in parallel.
- Resistance (R) opposes current, measured in ohms (Ω). Ohm's law: V = IR.
- Series circuit: the same current flows everywhere; resistances add (R = R₁ + R₂).
- Parallel circuit: the voltage across each branch is the same; if one bulb blows, the others stay on (house wiring).
Worked example: A 12 V battery is connected to a 4 Ω resistor. Find the current.
I = V ÷ R = 12 ÷ 4 = 3 A
Electrical power and cost
- Power P = VI (watts).
- Energy used (kWh) = power (kW) × time (h). Cost = units × price per unit.
Safety
Use fuses and circuit breakers, earth wires on metal appliances, and never touch switches with wet hands. Don't overload sockets.
Magnetism
- Like poles repel; unlike poles attract. Magnetic materials include iron, nickel and cobalt.
- A current in a wire produces a magnetic field. An electromagnet (a coil around an iron core) can be switched on and off – used in electric bells, relays and scrap-metal cranes.
Exam tip: Ammeters go in series; voltmeters go in parallel. This is tested almost every year.
Check yourself
- Find the total resistance of 3 Ω and 6 Ω resistors in series.
- A 2 kW stove is used for 3 hours. How many units (kWh) does it use?
- Why are house lights connected in parallel?
Show answers
- 9 Ω
- 6 kWh
- Each light gets the full voltage and can be switched on and off on its own; if one fails the others still work.
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