Ohm’s Law | What is Ohm’s Law

 

Ohm’s Law | What is Ohm’s Law :- In 1826, the German scientist Dr. George Simon Ohm expressed the relationship between the potential difference across a conductor and the current flowing in it by a law, which is called Ohm’s law.

According to Ohm’s law, “If the physical state of a conductor (such as temperature, length, area, etc.) is kept unchanged, then the ratio of the potential difference across its ends to the current flowing through it remains constant.”

Or

“If the physical conditions of a conductor such as temperature, pressure, length, area, etc., remain constant, then the applied potential difference across its ends is proportional to the current flowing through it.”

That is, if I current flows through the conductor by applying V potential difference, then by Ohm’s law

V/I = Constant

Here this constant is called the electrical resistance of the conductor and is expressed by R.

Electrical Resistance (R) :- The property of a conductor by which it opposes the flow of charge flowing through it, is called resistance.

S.I. unit of resistance :- Volt/Ampere or Ohm

Definition of 1 Ohm : – If one volt potential difference applied between the ends of a conductor results in one ampere current through it, then the resistance of the conductor is said to be one ohm.

1 Ω = 1 V/1 A

Formula of Ohm’s Law :-

V = IR

Verification of Ohm’s Law :-

To verify Ohm’s law, a battery (B), a rheostat (Rh), an ammeter (A) and a conductor wire PQ are taken as shown in the figure. The voltmeter is connected in parallel with the wire PQ and the ammeter is connected in series with the wire.

By changing the value of current with the help of rheostat, the corresponding potential difference for the value of different currents is found with a voltmeter.

A straight line graph is obtained between the readings of the voltmeter (V) and the ammeter (I).

The slope (tanθ) of the above V-I graph is the resistance (R) of the conducting wire PQ.

 

Limitations of Ohm’s Law

Ohm’s law has been found valid over a large class of materials, but there are some materials and devices used in electric circuits where the proportionality of V and I does not hold. The main deviations observed are :

1. V ceases to be proportional to I

In above graph the dashed line represents the linear Ohm’s law and the solid line is the voltage V versus current I for a good conductor.

2. The relation between V and I depends on the sign of V, i.e., if I is the current for a certain V, then reversing the direction of V keeping its magnitude fixed, does not produce the same current I in the opposite direction. This happens, for example, in a semiconductor diode.

Characteristic curve of a semiconductor diode.

3. The relation between V and I is not unique, i.e., there is more than one value of V for the same current I. For example in V-I graph for GaAs, we get more than one value of V for the same current I.

 

Ohmic and Non-Ohmic Conductors

(i) Ohmic Conductors : – The materials which obey Ohm’s law or for which the graph between V and I is a straight line, are called Ohmic conductors.

(ii) Non-Ohmic Conductors :- Those materials which do not obey Ohm’s law or for which the graph between V and I is not a straight line but a curved line, are called non-ohmic resistance.

 

Note :-

• If the temperature of the conductor increases, the amplitude of the vibrations of the positive ions in the conductor also increase. Due to this, the free electrons collide more frequently with the vibrating ions and as a result, the average relaxation time decreases. This results in an increase in resistance of the conductor.
• At different temperatures V–I curves are different.

 Slope (m) of V-I graph = tanθ = V/I = R

Here  tan θ₁ > tan θ₂        So R₁ > R₂      i.e. T₁ > T₂

 

 

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