Electricity

Current, potential difference and resistance; Ohm's law; resistors in series and parallel; electric power and the heating effect.

Key ideas

Electric current is the rate of flow of charge, measured in amperes with an ammeter connected in series.
Potential difference is the work done per unit charge, measured in volts with a voltmeter connected in parallel.
Ohm's law: at a constant temperature, the current through a conductor is proportional to the potential difference across it.
Resistance increases with length, decreases with thickness, and depends on the material (resistivity).
In series the current is the same through each resistor; in parallel the potential difference is the same across each.
Resistors in parallel give a total resistance smaller than the smallest one.
Current produces heat in a resistor (used in heaters, irons and fuses).

Remember

Current	I = Q ÷ t
Ohm's law	V = IR
Resistance of a wire	R = ρL ÷ A
Series / parallel	R = R₁ + R₂ + R₃; 1/R = 1/R₁ + 1/R₂ + 1/R₃
Power	P = VI = I²R = V² ÷ R
Heating	H = I²Rt; 1 kWh = 3.6 × 10⁶ J

Watch out

Connecting the voltmeter in series (it goes in parallel).
Adding resistances in parallel as if they were in series.
Confusing kW (power) with kWh (energy).

Worked example

Two bulbs are rated 40 W and 200 W, both at 220 V. Find their resistances. Which has the higher resistance? (Give the answer to 2 decimal places where needed.)

Working: R = V²/P: 220² ÷ 40 and 220² ÷ 200; the lower-power bulb has more resistance

Answer: 1,210 Ω and 242 Ω; the 40 W bulb