Effects of Current
This domain tests electric current, potential difference, resistance, household circuits, the heating effect, and the magnetic effect of current. Expect numerical problems, circuit reasoning, and questions on motors, generators and safety devices.
Core rules and concepts
- Ohm's law: V = IR at constant temperature for a metallic conductor.
- In series, current is the same and resistances add; in parallel, voltage is the same and 1/R = 1/R1 + 1/R2.
- Electrical power P = VI = I²R = V²/R; energy in kWh = power in kW × time in hours.
- The heating effect depends on I²R and is used in heaters, fuses and filament lamps.
- A current-carrying conductor creates a magnetic field; a solenoid behaves like a bar magnet.
- Fleming's left-hand rule gives the force on a conductor in a motor; the right-hand rule relates field and induced current in a generator.
- A fuse and the switch are placed in the live wire for safety.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 6 V supply. The equivalent resistance is 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current I = V/R = 6/2 = 3 A. The branch currents are 6/6 = 1 A and 6/3 = 2 A; they add to 3 A. Total power is VI = 6 × 3 = 18 W, or 18 J per second.
Traps that cost marks
- Connecting an ammeter in parallel or a voltmeter in series.
- Adding parallel resistances directly instead of adding reciprocals.
- Using kWh as a unit of power rather than energy.
- Saying a fuse protects by increasing resistance; it melts and breaks the circuit.
- Reversing the rules for motor and generator effects.