Resistance and Factors Affecting Resistance Explained

What is Resistance

Resistance is the opposition to the flow of electrons due to bonds between electrons and protons, and the collisions between electrons as they move through a material.

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 factors explained below are considered.

In a purely resistive circuit, resistance determines the amount of current that flows in a circuit with respect to the applied voltage. A purely resistive circuit is a circuit that doesn't have capacitance and inductance.

Factors Affecting Resistance

There are four factors that affect the resistance of a material: cross-sectional area, length, temperature and type of material (resistivity).

1. Length (L)

Resistance of a material increases with an increase in length; or resistance is directly proportional to the 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 also has its resistance. This resistance is added to that of the material before the increase.

2. Cross-Sectional Area (A)

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

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

More electrons flowing at same time means low resistance, while few electrons flowing at the same time means high resistance.

3. Type of Material (ρ)

Resistance depends on the type of material (resistivity). Resistivity is the resistance measured between two opposite faces of a 1m cube of a material such as copper.

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

4. Temperature

Resistance of a material changes with a change in temperature. For example, resistance of conductors increases with an increase in temperature, while resistance of semiconductors decreases with an increase in temperature.

This is because electrons gain energy in the form of heat and light to break from the nucleus and became free. If the temperature of a material is increased, electrons break free from the nucleus and become available for conduction.

In semiconductors, an increase in temperature results in a decrease in resistance. This is because as temperature increases, more electrons in the semiconductor material become available for conduction.

In conductors, an increase in temperature results in an increase in resistance. This is because a conductor already has free electrons in it. As the temperature increases, too many electrons become available for conduction, leading to excessive collisions among electrons as they move, resulting in an increase in resistance in conductors.

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

If we are to find the resistance of a material at the same temperature where resistivity was measured, there is no need to include temperature in the formula of resistance as in the previous formula.

But if we are to find the resistance at a different temperature that where resistivity was measured, we then include temperature as in the following formula.

In the previous formula, the temperature coefficient of resistance is the increase in resistance of a material, when a material is subjected to a temperature increase of 1°. For example, for copper it is 0.0043/°C.

How Resistance is Measured

Resistance is measured in ohms by an instrument called an Ohmmeter. When measuring resistance, the ohmmeter supplies current to a component or circuit being measured at a certain voltage.

The amount of current that 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 or battery.

If you have made it this far, you have made it. Consider visiting our YouTube channel; we have helpful electrical tutorials there.