Ohms Law

What is Ohms Law?

Keeping physical conditions such as temperature etc constant, the potential difference (V) between the ends of the conductor is directly proportional to the current (I) flowing through it i.e.

V ∝ I

V = RI

V/I = R

Where ‘R’ is constant of proportionality and is called as the resistance of the conductor. It depends upon the length, area and nature of the material of the conductor.

Experimental Verification:

Take a wire AB. Connect a battery B, key K, ammeter A and rheostat Rh in series with AB. Also, connect a voltmeter V in parallel with AB.

First of all, with the help of Rheostat, keep the current at 1.0 amp and note the corresponding value of potential difference. Now, with the help of Rheostat, keep the current at 2.0 amp and note the corresponding value of potential difference and so on.

Now, draw the graph between current (I) and potential difference (V). We will get a straight line as shown. This clearly shows that V ∝ I. Hence Ohms law is verified.

Deduction of Ohms Law from Drift Velocity:

We know, I = nAe x ν_d ………….(i)

But ν_d = eEτ/m ……………(ii)

Put (ii) in (i)

I = nAe (eEτ/m)

I = nAe²Eτ/m

But E = V/l

I = nAe²Vτ/ml

I/V = nAe²τ/ml

V/I = ml/nAe²τ

Put ml/nAe²τ = R = resistance of the conductor = constant

V/I = R

This is Ohm’s Law.

Limitations of Ohm’s Law:

The limitations of the Ohm’s law are-

• It is not applicable to the nonlinear devices such as diodes, zener diodes, voltage regulators etc.

• It does not hold good for non-metallic conductors such as silicon carbide. The law for such conductors is given by,