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Question

A charge 2 μC is taken from infinity to any point P (say) in an electric field, without changing its velocity. If workdone against electrostatic forces is 40 μJ, then the electric potential at that point will be:

A
+20 V
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B
10 V
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C
+10 V
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D
20 V
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Solution

The correct option is D 20 V
Let the charge q (2μC) is taken from to point P, thus the relation for work done by external force is given by

Wext(P)q=VPV

Since, we take V=0

Wext(P)q=VP ....(1)

Since the work done against electrostatic forces is 40 μJ, it suggests that Wext=40 μJ

From Eq.(1),

40 μJ2 μC=VP

VP=20 V

So, (d) is the correct choice.

Why this question?Tip: Here work done against electrostatic forces is -ve thus work done by external force will be -ve. It indicates that the charge is moved along the direction of electric field by electric forces. Thus its potential has decreased(Vi=0, Vf=20 V)

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