Explain the concept of resistance.

1. When the free electrons flow from one part to the other part of conductor, they collide with other electrons and also with the atoms and ions present in the conductor.
2. Due to these collisions there is some obstruction or opposition to the flow of electron through the conductor.
3. The property of the conductor due to which it opposes flow of current through it is called resistance.
4. It is numerically equal to the ratio of potential difference across it ends to the current flowing through it.