Physics A2 · Electric fields. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The electric potential at a point is the work done per unit positive charge in bringing a small test charge from infinity to that point. Potential is zero at infinity. It is a scalar, so it has a sign but no direction: positive near a positive charge and negative near a negative charge. Potentials from several charges simply add.
The electric field strength at a point equals the negative of the potential gradient there. The field points in the direction in which the potential falls.
Two point charges have electric potential energy because work was done to bring them together. It is positive for like charges (they repel) and negative for unlike charges (they attract).
Students use r2 in the potential formula, or drop the minus sign of a negative charge. Potential and potential energy vary as 1/r, and the signs of the charges must stay in the calculation.
Answer: +60 V.
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