Variable Resistance
Physics: Cambridge International GCSE Supplement, topic 4.5.6
Wires and Resistance
The resistance of a wire depends on the wire’s length and cross-sectional area of the circular end of the wire. Resistance (in ohms) is proportional to (α) the length (in metres) divided by the cross-sectional area (in metres squared).
Size of a wire
- Increasing the length of a wire, increases the resistance of the wire.
- Decreasing the cross-sectional area of the wire increases its resistance.
- So short, wide wires have less resistance than long, narrow wires.
Wire thickness
- The cross-sectional area (A) of a circular wire is equal to π times its radius (r) squared.
- Therefore, doubling the thickness of a wire quadruples its cross-sectional area.
Test yourself on Variable Resistance
How will stretching a wire affect its resistance?
What effect does increasing the thickness of a wire have on its resistance?
A filament lamp is connected to a power supply with a 4.7 m of wire. The lamp is dimmer than its specifications suggest. What could we do to the wire to make the lamp brighter?