The electric potential at point A is 20 V and at B is `-40` V. Find the work done by an external force and electrostatic force in moving an electron slowly from B to A.

Explanation

Here, the test charge in an electron,
i.e. `q_(0)=-1.6xx10^(-19)C`
`V_(A)=20V` and `V_(B)=-40V`
`:.` Work done by external force,
`(W_(B-A))_("external force")=q_(0)(V_(A)-V_(B))`
`=(-1.6xx10^(-19))[(20)-(-40)]`
`=-9.6xx10^(-18)J`
`:.` Work done by electric force,
`(W_(B-A))_("electric force")=-(W_(B-A))_("external force")`
`=-(-9.6xx10^(-18)J)=9.6xx10^(-18)J`