The force gets stronger as the distance between the objects decreases.

The idea

Bring two charged objects closer and the force between them grows, because the second object moves into a region where the first object's field is stronger. The increase is steep rather than steady, since halving the separation does far more than double the force, which is why a charged rod feels like nothing at arm's length but snatches at a scrap of paper a centimetre away. The same rule governs sparking: as an earthed conductor is moved towards a charged dome, the field in the gap grows stronger until the air breaks down. Notice that the two objects always feel forces of the same size however different they are, so the small object does not feel a bigger force just because it is the one that moves.

What earns the marks

• The force increasing as the separation decreases.
• Explaining this as the second object moving into a stronger region of the field.
• The two objects always experiencing forces equal in size and opposite in direction.
• Linking the stronger field at small separation to the air breaking down and a spark forming.

Common mistakes and how to put them right

• Describing the increase as directly proportional to the decrease in distance.

• Saying the lighter object feels the larger force because it accelerates more.

• Claiming the force only appears once the objects touch.

• Confusing the strengthening field with an increase in the charge on the objects.

Exam tips

• Write 'increases' and give the field reason, because a bare 'gets bigger' may not earn the explanation mark.
• Remember the forces on the two objects stay equal in size even when only one of them moves.
• Link a spark question back to the field in the gap rather than to the distance alone.