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The free electrons in a metal are in constant random motion. As they move about they collide with each other and with the atoms of the metal. If a potential difference is now applied across the metal the electrons tend to move towards the positive connection. As they do so their progress is interrupted by collisions. The ease with which the free electrons can move through the material is called its conductance. If the temperature of the metal is raised the atoms vibrate more strongly and the electrons make more violent collisions with them and so the conductance of the metal decreases.

The conductance of any conducting material depends on the following factors:
(a) the material itself (actually how many free electrons there are per metre cubed)
(b) its length
(c) its cross-sectional area and (d) its temperature

The conductance of a given piece of material is connected to the current flowing through it and the potential difference between its ends by the equation:

Conductance (G) = Current (I)/Potential Difference (V)

The units of conductance are amperes volts-1 (AV-1) known as siemens.

A specimen has a conductance of 1 siemen if a current of 1A flows through it when a potential difference of 1 V is applied between its ends.

Example problem
Calculate the conductance of a specimen if a current of 50 mA flows in it when a voltage of 6 V is applied across it.

Conductance = I/V = 0.005/6 = 8.33x10-4 siemens = 12 millisiemens

© Keith Gibbs 2011