The three key quantities
Current (I) is the rate of flow of electric charge, measured in amperes (A) with an ammeter connected in series. Voltage or potential difference (V) is the energy transferred per unit charge, measured in volts (V) with a voltmeter connected in parallel. Resistance (R) opposes the current and is measured in ohms (Ω).
Key idea
V = I R, so R = V / I and I = V / R. Ohm's law states that, for a metallic conductor at constant temperature, the current is directly proportional to the voltage across it.
Series and parallel circuits
- Series: the current is the same everywhere; the voltages add up; total resistance R = R₁ + R₂ (resistances add).
- Parallel: the voltage across each branch is the same; the currents add up; the total resistance is less than the smallest single resistor.
The resistance of a wire increases with its length, decreases with a larger cross-sectional area, and depends on the material.
Worked example
A current of 3 A flows through a lamp when the voltage across it is 12 V. Its resistance R = V / I = 12 ÷ 3 = 4 Ω. If a second, identical lamp is added in series, the total resistance becomes 4 + 4 = 8 Ω.
Remember
- Ammeter in series, voltmeter in parallel.
- Series: current same; parallel: voltage same.
- V = I R links all three quantities.