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Electricity and Magnetism

Resistance

The resistance of a conductor is a measure of the opposition it offers to the flow of electric current.

The resistance of a component causes electrical energy to be converted into heat.

Resistance is defined as follows
.....

Units of resistance VA-1 or Ohms, W.

Therefore, the resistance of a conductor is the voltage needed to give a current of 1A.

George Ohm investigated the resistance of metals.

He first tried to find how the resistance of a piece of metal depends on its dimensions. He found that resistance depends on

i) the length of the piece of metal,
ii) the cross-sectional area of the piece of metal, A
iii) the type of metal.

His results showed that
and

R a 1/A
therefore,

The constant is called the resistivity of the metal, r, units Wm.

Ohm’s Law

Ohm also investigated the relation between the voltage across a given piece of metal and the current flowing through it. His results gave what is now called Ohm’s law, which is stated as follows:

For a metal conductor at constant temperature, the current flowing through it is directly proportional to the voltage across it.

As voltage divided by current is resistance, this law tells us that the resistance of a piece of metal (at constant temperature) is constant.

It can easily be shown that the resistance of a piece of metal increases as its temperature increases.

 

© David Hoult 2008