Resistance & Resistors
Resistance and the factors affecting it; specific resistance; the resistor colour code; conductance; positive and negative temperature coefficients; resistors in series and parallel; the Wheatstone bridge.
Resistance & resistors — summary notes
Free- Resistance depends on the material and the dimensions of the conductor: R = ρL / A. Longer means more resistance; thicker (greater area) means less.
- Resistivity ρ is a property of the material, measured in ohm-metres. Copper and silver have very low resistivity.
- Metals have a positive temperature coefficient — resistance rises as they get hotter. Carbon and semiconductors have a negative coefficient.
- Fixed resistors are marked with a 4-band or 5-band colour code; the last band is the tolerance.
- Variable resistors: a rheostat is a two-terminal current control, a potentiometer is a three-terminal voltage divider.
- ⚠ Exam trap: thicker wire means LOWER resistance. A common distractor reverses the area relationship.
- Resistance of a conductor
- R = ρL / A
- Temperature effect
- R₂ = R₁[1 + α(t₂ − t₁)]
- Wheatstone bridge at balance
- R₁/R₂ = R₃/R₄
A wire's length is doubled and its cross-sectional area is halved. What happens to its resistance?
R = ρL/A. Doubling L doubles R; halving A doubles R again. The resistance becomes 4 times the original.
Why does a filament lamp draw a large surge current when first switched on?
The tungsten filament is cold, and metals have a positive temperature coefficient, so its resistance is low. As it heats, resistance rises and current settles to its normal value.
The Wheatstone bridge
FreeResistance concept map
FreeResistance & Resistors
Resistance & resistors quiz
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