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An external agent brings a charge of -5 C from infinity to a certain point in an electrostatic field, doing 100 J of work in the process. What is the electric potential at that point?

1. +10 V
2. -10 V
3. +20 V
4. -20 V

Correct answer: -20 V

Solution

Since W_ext = q*V (with V at infinity zero), dividing 100 J by the charge -5 C gives -20 V. The negative charge requiring positive external work means it is being pushed toward a region of negative potential.

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