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Resistance Definition and Units of Electrical Resistance

The electrical resistance is a property of a piece of substances. Let us apply an electric potential difference between two points in a substance. As a result, there is an electric current. This current will flow from one point to another. This current may be anything from zero to infinity. Obviously, this value depends on the capability of the substance to carry the current.  Conversely, every substance has some capability to prevent the movement of electrons causing a current.

Some substances have a strong capability of opposing the flow of electrons (charge).  So, the current through those substances for a certain applied voltage is low. On the other hand, if the said capability is low the current through it will be high. Hence, for a certain applied voltage, a substance can carry a certain current.  The current through the substance depends on the property of resisting the flow of current. We call this typical property of a substance as resistance.


This is the property of the substance. But this is not the property of the material by which the substance is made of.


At a constant temperature, the current through a certain substance is directly proportional to the voltage applied across the substance. The proportionality constant of this relationship is the resistance.

Here, R is the resistance. 

Physics behind Electrical Resistance

We can explain the resistance very easily by considering a pure metallic substance. In metal, there are plenty of free electrons available in it. Even these free electrons are available at room temperature also. Now, we apply a voltage across the metal. Then the free electrons tend to propagate towards the higher potential end of the metal. This is because of the electrostatic force. During their journey, they collide with atoms of the metal. Due to these collisions, the electrons get difficulty to propagate one point to another. This phenomenon causes resistance against the flow of electrons. Undoubtedly, this opposition to the flow of electrons is resistance. 


At a constant temperature, the current is directly proportional to the voltage. If V is the voltage and I is the current then,

Now if we consider the voltage across the resistor is 1 volt and current through the resistor is 1 ampere, then R is 1 volt/ampere. This is the unit of resistance.

So the quantity of resistance which causes one-ampere current for one-volt voltage is its unit.  We have named this unit ohm.  The name Ohm comes after the name of great German physicist George Simon Ohm. We generally denote the ohm with Ω.

Georg Simon Ohm
Georg Simon Ohm

Other Units

The resistance may be very large as well as very small depending on the nature of a substance and its uses. For example, an insulator offers very large resistance. Whereas a metallic conductor offers very low resistance. Depending on the value, we often express the unit of resistance megaohm, kiloohm, milliohm, and microohm, etc.

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