As the rate of flow of water through a pipe is directly proportional to the effective pressure (i.e., difference of pressure at two ends) and inversely proportional to the frictional resistance, similarly the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor and inversely proportional to the conductor resistance. This relation was discovered by Georg Simon Ohm and so it is known as Ohm’s law.
If I is the current flowing through a conductor of resistance R across which a potential difference V is applied then according to Ohm’s law.
Where V is in volts, R is in ohms and I is in amperes.
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Ohm’s law may be defined as follows:
Physical state i.e., temperature etc., remaining the same, the current flowing through a conductor is directly proportional to the potential difference applied across its ends.
Or
The ratio of potential difference applied across a conductor and current flowing through it remains constant provided physical state i.e., temperature etc. of the conductor remains unchanged.
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i.e., V/I = Constant = R …(2.2)
Where R is known as the resistance of the conductor.
Ohm’s law may be alternatively expressed as:
V = I R …(2.3)
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Equations (2.1), (2.2) and (2.3) give Ohm’s law in three forms with which the student should be familiar.
Ohm’s law cannot be applied to circuits consisting of electronic tubes or transistors because such elements are not bilateral i.e., they behave in different way when the direction of flow of current is reversed as in case of a diode. Ohm’s law also cannot be applied to circuits consisting of non-linear elements such as powdered carbon, thyrite, electric arc etc. For example, for silicon carbide, the relationship between applied voltage (and potential difference) V and current flowing I is given as V = K Im where K and m are constants and in is less than unity.