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Defination of Resistance(R).

Resistance is the opposition to flow of electrons due to bonds between electrons and protons and also collision between electrons themselves as they make their way through the complex structure of a conductor.

A device designed with a known value of resistance or made to introduce a known value of resistance into a circuit is called a Resistor. In designing a resistor the factor explained below are taken into account.

In a purely resistive load, resistance determine the amount of current that flow in a circuit in respect to the applied voltage.

Factors affecting resistance.

They are basically four factors which affect resistance of a material, which are cross sectional area of a material, length of a material, temperature and type of material(resistivity).

1. Length (L).

Resistance of a material increase with increase in length of material or resistance varies directly with length of a material, that is if the length of a material is doubled resistance is also doubled.

This is because as the length of a material is increased, the added material has also resistance, so this resistance will be added to that of the material before increased.

2. Cross sectional area (A).

Resistance of a material varies inversely with cross sectional area of a material or resistance decreases as the cross sectional area increases, that is if the cross sectional area of a material is doubled resistance is halfed.

This is because on large cross-sectional area more electrons can pass through at the same time whilst on small cross sectional area, few electrons can pass through at the same time.

More electrons flowing at same time means less resistance and few electrons flowing at same time means more resistance when other factors and supply voltage kept the same.

3. Type of material (ρ).

Resistance depends on type of material (resistivity). Resistivity is that resistance measured between two opposite faces of a 1m cube of a material e.g copper.

Since resistance depends on resistivity, then resistivity is a constant. By putting resistivity in the combined above formulas, resistance is equal to resistivity multiplied by length all over cross sectional area.

The above formula is a formula used to find resistance.

4. Temperature

Resistance of a material change with change in temperature, for example resistance of conductors increase with increase in temperature whilst resistance of semiconductor decreases with increase in temperature.

This is because electrons gain energy in form of heat and light to break from the nucleus and became free. If heat is increased the electrons will break from the nucleus and became free for conduction.

In semiconductors, an increase in temperature result in decrease in resistance. This is because as temperature increases, more electrons in semiconductor material became available for conduction.

In conductors, an increase in temperature result in increase in resistance. This is because a conductor already have free electrons in it, as the temperature increase too much electrons will become available for conduction which lead to much collision of electrons as the move resulting in increase in resistance in conductors.

If resistance change with change in temperature, resistivity also change with change in temperature.

If the resistance in the previous formula was for resistivity of a material at room temperature(28°) and then if the temperature changes, to find resistance of a material at new temperature we then use the following formula.

In the formula, temperature coefficient of resistance is the increase in resistance of a material, when a material is subjected to a temperature increases of 1°. For example, for copper it is 0,0043/°C.

How resistance is measured.

Resistance is measured in Ohms using an instrument called an Ohmmeter. An ohmmeter is connected to the component being measured. An ohmmeter has its own battery, after connecting it, the ohmmeter supply current at a certain voltage.

The amount of current which then flows is used to divide the supplied voltage and the value of resistance is obtained. Before measuring resistance, first disconnect the component to be measured from the supply otherwise this may cause destruction of the ohmmeter battery.