P2.3 Electric circuits and resistance · KS3 Science

Resistance

Resistance is how much a component opposes the current. It is measured in ohms (Ω).
For the same voltage, a higher resistance means a smaller current.
Variable resistors change the current, for example in dimmer switches and volume controls.

Potential difference = current × resistance (V = I × R), so resistance = potential difference ÷ current.
A resistor with 6 V across it and 2 A through it has a resistance of 6 ÷ 2 = 3 Ω.
For a fixed resistor at a constant temperature, the current is directly proportional to the voltage: double the voltage and the current doubles, giving a straight line through the origin on a graph.
To measure resistance, use an ammeter in series and a voltmeter in parallel across the component, then calculate V ÷ I.

A longer wire has more resistance. A thinner wire has more resistance. Metals have more resistance when they are hot.
In series, resistances add up: 4 Ω and 6 Ω make 10 Ω. Adding resistors in parallel lowers the total resistance, because there are more paths for the current.
High-resistance wires get hot, which is useful in kettles, toasters and heaters.
A thermistor's resistance falls as it gets warmer. A light-dependent resistor's (LDR's) resistance falls as the light gets brighter.

Resistance: How much a component opposes the flow of current, measured in ohms (Ω).
Ohm: The unit of resistance.
Resistor: A component that reduces the current in a circuit.
Variable resistor: A resistor whose resistance can be changed.
Ohm's law: For a fixed resistor, the current is directly proportional to the potential difference.
Directly proportional: When one quantity doubles, the other doubles too.
Thermistor: A resistor whose resistance falls as its temperature rises.
Light-dependent resistor: A resistor whose resistance falls as the light gets brighter.
Current: The flow of electric charge, measured in amps (A).
Potential difference: The push that makes current flow, measured in volts (V).