# State Ohm's law and mention its limitations.

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2014-12-07T14:45:59+05:30

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Ohm’s law:

The current I passing through a conducting element (or resistor) is directly proportional to the potential difference across its ends.

I ∞ V or,          I = V/R,

Here, R is called as the resistor of the given conductor.  It is a physical property of the material and its geometrical shape.

Limitations

There is a range of voltages over which the resistance is fairly constant. Due to dissipation of heat, the resistance is heated and its resistance changes.

The resistance does not obey near the breakdown voltage of the materials. The materials become super conductors at very high voltages.

Some materials do not obey ohm's law below a certain temperature. the material behaves differently.

Ohmic resistors only follow ohm’s law.  Non-ohmic resistors like diodes, semi-conductors or insulators, do not obey ohm’s law.

Resistance depends on the temperature.  It increases with temperature.

Resistance is also altered, there is a presence of magnetic field or electric field, if that induces an emf in the conductor.  That is, if there is an electromagnetic effect.

Every real conductor or resistor has an inherent capacitance and inductance.  That could depend on the geometrical shapes too.  It is not a pure resistance.

For gases and liquids, the law is perhaps not valid exactly.  For ionic substances, the law may be invalid exactly.

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2014-12-07T14:54:46+05:30
Ohms law states that the current flowing between two points in a conductor is ohms law states that current flowing between two points in a conductor is directly proportional to the potential difference between the two points provided temperature is kept constant.
I  is directly proportional to  V
V/I= R (constant)the constant is resistance.

limitations of ohms law:
it is valid for metal conductors provided temperature and other physical conditions remain constant. the resistance of the material changes with temperature. it is not applicable for a gaseous conductor. it is not applicable to semiconducters like germanium and silicon.