Lesson List
Lesson: Electrical resistance || Electrical conductance || Conductors || Insulators || Semi-conductors

## Electrical resistance

• It is denoted by the symbol R’.
• It is measured in a unit called ohm (Ω) with the help of a Wheatstone bridge
• The electrical resistance is:
1. Directly proportional to its length, l
2. Inversely proportional to its area of cross section, A.

## Electrical conductance

• The free ions Na+ and Cl present in the solution are responsible for the conductance in a solution.
• The inverse of resistivity is termed as conductivity.
• Ω = 1/k
• It is represented by the symbol (Greek, kappa).
• It is measured in a unit called Sm–1.
• When length = 1m.

Cross sectional area = 1m2

Then conductivity becomes the conductance.

The conductivity of an electrolytic solution depends on:

2.Size of the ions produced and their solvation
3.Nature of the solvent and its viscosity
4.Concentration of the electrolyte
5.It increases with the increase of temperature.
6.Pressure

• Matters can be classified into conductors, insulators andsemiconductors depending on the magnitude of their conductivity.

Conductors:

• Solids with conductivities ranging between 104 to 107 ohm–1m–1 are conductors.
• Metals have conductivities in the order of 107 ohm–1m–1 is good conductors.
• For example, Iron, Copper, Aluminum.

Fig. Wood and plastics are also solids but are insulators

A man touching the electric pole with a metal rod will get an electric shock because metal rod is a conductor whereas a man touching the same with a wooden plank will be safe because wood is an insulator.

Semiconductors:

• Solids with conductivities in the intermediate range from 10–6 to 104 ohm–1 m–1.
• For example, Gallium, Germanium, Silicon

## Measurement of resistance of a solution of an electrolyte

PROBLEM:

THE RESISTANCE OF A CONDUCTIVITY CELL CONTAINING 0.001M KCL SOLUTION AT 298 K IS 1500 Ω. WHAT IS THE CELL CONSTANT IF CONDUCTIVITY OF 0.001M KCL SOLUTION AT 298 K IS 0.146 X 10-3 S CM-1.

SOLUTION:

Conductivity, k = 0.146 × 10 – 3 S cm – 1

Resistance, R = 1500 Ω

Cell constant = k × R = 0.146 × 10 – 3 × 1500 = 0.219 cm – 1

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