Electricity

In the year 1820 a French physicist, Andre Ampere made many discoveries on the nature of electricity. While a German physicist, George Ohm showed that the flow of  electric current through a wire depended on its 'resistance' and the potential difference between its ends.

Activity

Take a nichrome wire of about 50 cm length and apply a potential difference of 1.5 V from a battery. Measure the current flowing through the wire using an ammeter connected to it in series, and note the potential difference across the resistance using a voltmeter connected across it. Note down and tabulate the set of readings for I and V, by increasing the number of cells.

A graph drawn between the voltmeter readings and ammeter readings, shows a straight line pattern. The straight line indicates a linear relationship V and I.

This relationship between V and I is known as ohm's law.

Mathematically this relationship can be expressed as,

V I

V = RI

Where R is the constant of proportionality and gives the resistance offered by a conductor.

Or = constant

If the above activity is repeated with another wire or a torch bulb, the current values are found to be different in each case. This shows that the current through the circuit is not decided by the battery but by the component through which it passes. Some materials allow larger current through them than others. In other words certain materials offer more resistance to the flow of current than others. The property by virtue of which a material opposes or limits the flow of electric current through it, is called its 'electric resistance'.

Resistance

The resistance of a conductor may be defined in terms of current passing through it and the potential difference across its ends. Resistance of a wire is the ratio of the potential difference 'V' across its ends and the current 'I' flowing through it.

R =

The unit of resistance is called the ohm, named after George Simon ohm, who found the relationship between current and potential difference. This law is called 'ohm's law'.

Statement of Ohm's Law

The current through a metallic element is proportional to the potential difference between its ends, provided the temperature of the element remains constant.

Unit of resistance is 'ohm'

Ohm is the SI unit of resistance. The Greek letter omega() is used to represent resistance.

Definition of Ohm

One ohm is the resistance of a conductor through which a current of 1 ampere flows when the potential difference across its ends is 1 volt.