IGCSE Physics / Electrical resistance
Resistance and V = I × R
Edexcel International GCSE Physics (4PH1), chapter 8: Electrical resistance, specification point 2.13, 2.10. Free lesson.
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Resistance describes how much a component opposes current, and is measured in ohms (Ω). The equation V = I × R links voltage, current and resistance, so you can calculate any one of them when you know the other two.
What resistance does
Resistance is how much a component opposes the current. For the same voltage, increasing resistance makes the current smaller. For example, with a 6 V supply, 6 Ω gives a current of 1 A, while 12 Ω gives 0.5 A. Resistance is measured in ohms (Ω). So, at a fixed voltage, more resistance means less current.
How voltage, current and resistance are linked
Use V = I × R to find voltage, current or resistance. Voltage is measured in volts (V), current in amps (A), and resistance in ohms (Ω). Convert prefixes before substituting values. 1 mA = 0.001 A. 1 kΩ = 1000 Ω.
How to calculate current
Convert the resistance to ohms first, then substitute the known values into V = I × R. Rearrange the equation to make I the subject.
How to calculate resistance
Rearrange V = I × R to find resistance: R = V/I. The current must be in amps before you divide. With 6 V and 30 mA, convert 30 mA to 0.03 A. Then R = 6/0.03 = 200 Ω. Leaving the current as 30 would give 0.2 Ω, which is wrong because the current was not converted to amps.
How a variable resistor dims a lamp
A variable resistor changes the resistance in a series circuit. The battery voltage stays fixed. Increasing the variable resistance raises the total resistance, so the current falls and the lamp becomes dimmer. The ammeter measures the current in the series loop. The variable resistor is shown by a resistor symbol with an arrow through it.