What is Electrical Resistance?

Electrical resistance is the opposition to electric current flowing through a conductor. Simply put, it blocks or reduces the movement of electrons through a conductor.

When an electric current flows through a conductor, electrons drift from one end to the other end of the conductor. During this drifting, the moving free electrons collide with atoms and other electrons in their way. As a result, these collisions create resistance. Therefore, the current flowing through the conductor attains a limited value.

In other words, we can say that if the resistance of a conductor increases, the current flowing through the conductor decreases. An increase in resistance means the enhancement of opposition offered by the conductor to the flow of free electrons.

We quantify this opposition to current flow offered by the conductor with the unit ohm. In simple words, the ohm is the unit of resistance.

Unit of Electrical Resistance

Moreover, Ohm’s law explains the relationship between voltage, current, and resistance of a conductor. This law states that the voltage across a conductor is directly proportional to the current flowing through it. If we consider this relation as linear, then the constant of proportionality is the resistance offered by the conductor.

That means: Voltage = Current × Resistance (V = IR) or R = V/I.

One ohm equals one volt per ampere. Obviously, we represent this unit (volt per ampere) as the ohm (Ω). In practice, we often encounter larger resistance values. Consequently, engineers use these common multiples:

• Kilohm (kΩ): Equals 1,000 ohms
• Megohm (MΩ): Equals 1,000,000 ohms

For example, a typical resistor might have 10kΩ or 2.2MΩ resistance. Conversely, some applications need smaller units. Thus, we use:

• Microohm (μΩ): Equals 0.000001 ohms
• Milliohm (mΩ): Equals 0.001 ohms

Factors Affecting Electrical Resistance

The value of resistance of a particular conductor depends upon four conditions.

Length of the Conductor

Firstly, it depends on the length of the conductor. If we increase the length of a conductor without changing its cross-section, the opposition offered by the conductor to the free electrons will also increase.

This is because each of the free electrons participating in the flow of current will have to drift along a longer path. In other words, each of the drifting electrons has to travel a greater distance to constitute current. Hence, it has to face more obstructions on the way.

The simple relation between the resistance and the length of a conductor is linear. That is, the resistance of a conductor is directly proportional to its length.

Cross-Sectional Area

Now think about the cross-sectional area of the conductor. If we increase the cross-sectional area of the conductor, each free electron participating in drifting will get a wider area to move through.

That means the electrons face less obstruction from the internal structure of the conductor. As a result, the resistance offered by the conductor to the drifting electrons decreases.

This relation is also linear, which simply means the resistance of a conductor is inversely proportional to its cross-sectional area.

Resistance Formula

So we can write, \[R \propto \frac{L}{A}\]
Where L is the length and A is the cross-sectional area of the conductor. So, we can rewrite this as,\[R = \rho \frac{L}{A}\]Where, \(\rho\) is the constant of proportionality. For this relation, we call this \(\rho\) the resistivity of the conductor.

Material Type (Resistivity)

This resistivity differs with the material of the conductor. The resistivity is an electrical property of any material. Obviously, it has no direct relation to the size and shape of the conductor.

Temperature

There is another parameter on which the resistance of the conductor depends. Lastly, this is the temperature of the conductor.

Effect of Temperature on Metallic Conductors

If the temperature increases, the atomic vibration increases in a conductor. This increased random vibration of the conductor atoms offers more obstructions to the drifting electrons. Therefore, the conductor provides more resistance to the current.

Effect of Temperature on Semiconductor Materials

When the temperature increases, more and more covalent bonds are broken. These breakdowns of covalent bonds release free electrons in the crystal structure. These cause more available free electrons to drift to form an electric current. Therefore, with increasing temperature, the resistance of a semiconductor decreases.

We will concentrate our discussion on the effect of temperature on resistance in our article as well.