R = V/I

The unit of resistance is the ohm. A component has the resistance of 1 ohm if the current flowing through it is 1 amp and the voltage between its ends is 1 volt.

For metallic conductors, the resistance is constant providing the temperature of the conductor stays the same.

Resistance in Metals

Electrons collide with lattice ions; each collision changes the electron's velocity and impedes its drift. More energy from the power supply is needed to accelerate the electron again.

Variation of Resistance with Temperature

At low temperatures-

• Lattice ions have small amplitude and low frequency vibrations.
• Electrons are quite likely to pass the ions without interference.
• The resistance is low

• lattice ions vibrate faster and with greater amplitude
• electrons are more likely to collide with...