George Simon Ohm (1787–1854), a German physicist, is credited with finding the relationship between current and voltage for a resistor. This relationship is known as Ohm’s law.

Ohm’s law can be stated as follows :
The ratio of potential difference between any two points of a conductor to the current flowing through it, is constant provided the physical conditions (i.e., temperature etc.) do not change.

In other words, Ohm’s law states that in a closed circuit, when a voltage V is applied across a conductor, then current I flowing through it is directly proportional to the applied voltage,
i.e., I ∝ V or
V   = constant
 I
V = R ohms
 I
V = IR volts
I =V amp.
     R
where, V = Voltage between two points,
I = Current flowing in the circuit, and
R = The resistance of the conductor.
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The units of voltage, current and resistance are volt (V), ampere (A) and ohm (Ω) respectively.

Conditions for applying Ohm’s law.

Ohm’s law is applicable under the following
conditions :
1. Ohm’s law can be applied either to the entire circuit or to a part of the circuit.
2. When Ohm’s law is applied to a part of a circuit, part resistance and the potential difference
across that part resistance should be used.
3. Ohm’s law can be applied to both D.C. and A.C. circuits. However, in case of A.C. circuits,
impedance Z, is used in place of resistance

>> The linear relationship (I ∝ V) does not apply to all non-metallic conductors. For example,
for silicon carbide, the relationship is given by :
V = k I^x
where, k and x are constants and x is less than unity.

Limitations of Ohm’s Law:

1. Ohm’s law is not applicable to non-linear elements like diode, transistor etc.
2. Ohm’s law is not applicable for non-metallic conductors like silicon carbide.
3.Ohm’s law does not apply under the following
conditions :
    (a). Electrolytes where enormous gases are produced on either electrode.
    (b). Non-linear resistors like vacuum radio valves, semiconductors, gas filled tubes etc.
    (c). Arc lamps.
    (d). Metals which get heated up due to flow of current.
    (e). Appliances like metal rectifiers, crystal detectors, etc. in which operation depends on the
direction of current.
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Kirchhoff’s laws

Kirchhoff’s laws, two in number, are particularly useful
(a) in determining the equivalent resistance of a complicated network of conductors and (b) for calculating the currents flowing in the various conductors Read more>>