The diagram shows equipotential lines due to two objects.

The two objects could be

A.     electric charges of the same sign only.

B.     masses only.

C.     electric charges of opposite sign only.

D.     masses or electric charges of any sign.

C

Candidates had presumably seen a similar diagram showing the magnetic field lines around two anti-parallel currents. We can only assume that the majority chose A, as they ‘saw’ the two objects repelling each other and assumed they must be charges of the same sign.

Teachers should emphasise the different information that equipotentials and field lines give. They are related inasmuch as the potential gradient is the field strength and so field lines are always perpendicular to the equipotentials. Any candidate sketching the field lines onto the diagram will see immediately that the only answer can be C. Alternatively they could have seen that approaching the pair of objects from below is possible without crossing any equipotentials. Hence they must be opposite charges.