Question

The diagram shows two points P and Q which lie. ${90}^{\circ }$ apart, on a circle of radius r. A positive point change at the circle creates an electric field of magnitude E at both P and Q. Which expression gives the work done a unit positive charge from P to Q?

• A. 0
• B. $E\times r$
• C. $E\times \left(\frac{\pi r}{2}\right)$
• D. $E\times \left(\pi r\right)$

Answer 1

The correct option is A 0

The force on the charge when it moved from $P$ to $Q$ does not change as it is moving along the circle. The forces is always at right angles to the motion, So no work is done.

The problem can also be considered in terms of the electrical potential. The charges $P$ and $Q$ are at the same potential (from the $(+Ve)$ charge at the centre) and so zero work done in the correct answer.