## OHM'S LAW AND RESISTANCE

According to ohm’s law the current (I) flowing through a conductor is directly proportional to the potential difference (V) across the ends of the conductor, provided temperature of the conductor is kept constant, i.e.

I V

or V I

or V = R X I

Where R is constant and is called resistance.

**Resistance of a Conductor**

The resistance of a conductor is the property of a conductor to oppose the flow of current passing through it. The resistance of a conductor may be defined as the ratio of potential difference (V) across the ends of the conductor to the current (I) flowing through it, i.e.

Resistance =

or R =

The S.I. unit of a resistance is ohm. It is represented by the symbol **omega (Ω)**

**1 ohm (Ω)**

1 ohm is the resistance of a conductor through which a current of 1 ampere flows when a potential difference of 1 volt is applied across its ends ; e.g.

1 Ohm =

**Factors Affecting the Resistance of a Conductor**

The various factors that affect the resistance of a conductor are:

1. **Length of the Conductor**

The resistance of a conductor is directly proportional to the length of the conductor, i.e.

R l

Therefore the resistance of the conductor becomes double it the length of a conductor is doubled, and the resistance of the conductor becomes half if the length of a conductor is halved.

2. **Area of Cross Section of the Conductor**

The resistance of a conductor is inversely proportional to area of cross-section of the conductor, i.e.

R

Therefore the resistance of the conductor becomes half when the area of cross-section of a wire is doubled, and the resistance of the conductor becomes double when the area of cross-section of wire is halved. This means that a thick wire has less resistance than a thin wire.

3. **Nature of Material of the Conductor**

The resistance of a conductor depends on the nature of the material of which it is made.

4. **Effect of Temperature**

The value resistance of a conductor increases on raising the temperature and decreases on lowering the temperature.

Question:- Why electrician wear rubber hand gloves while working with electricity?Answer:- Electrician wear rubber hand gloves while working with electricity because rubber is bad conductor of electricity and protects them from electric shocks. |

**Specific Resistance or Resistivity**

We know that R I (1)

and R (2)

By combining both equation we get

R

or R = ρ X (3)

Where ρ (rho) is a constant and is know as** specific resistance or resistivity.**

By re-arranging equation (3) we get

ρ = R X (4)

if A = 1m^{2} and l = 1m then from equation (4), we get

ρ = R

Thus the **specific resistance or resistivity** of a conductor is resistance of a conductor which is 1 meter long and 1 square meter in cross section. S.I. Unit of specific resistance or resistivity is ohm-m or Ω-m.

**Test Your Understanding and Answer These Questions:**

- What is ohm’s law?
- What is definition of resistance?
- What is specific resistance?
- What is S.I. unit of specific resistance?
- What is S.I. unit of resistance of a conductor?
- What are the factors effecting resistance of a conductor?
- Why electrician wear rubber hand gloves while working with electricity?

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