A charge 2μC is taken from infinity to any point P(say) in an electric field, without changing its velocity. If workdone against electrostatic forces is −40μJ, then the electric potential at that point will be:
A
+20V
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B
−10V
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C
+10V
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D
−20V
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Solution
The correct option is D−20V 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=VP−V∞
Since, we take V∞=0
⇒Wext(∞→P)q=VP....(1)
Since the work done against electrostatic forces is −40μJ, it suggests that Wext=−40μJ
From Eq.(1),
−40μJ2μC=VP
∴VP=−20V
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=−20V)