Static charge: Two objects that carry different types of charge attract.

The idea

Unlike charges attract: a positive object and a negative object pull towards each other along the line joining them. The pull is a genuine non-contact force and grows as the separation shrinks. Attraction is a weaker test for charge than repulsion, because a charged object also attracts an uncharged one. Bring a charged rod near a thin stream of water or a scrap of paper and the charges inside the neutral object shift slightly, with the opposite charge drawn to the near side and the like charge pushed to the far side. The near side is closer, so attraction beats repulsion and the object moves towards the rod. To decide the sign of an unknown charge, test it against a rod of known charge and look for repulsion.

What earns the marks

• Identifying the force as a force of attraction.
• The reason that the two objects carry opposite types of charge.
• Stating that a charged object also attracts an uncharged object.
• Explaining the attraction of a neutral object by the shift of charge within it.

Common mistakes and how to put them right

• Concluding that an object is charged simply because it is attracted to a charged rod.

• Saying attraction happens because one object has more electricity than the other.

• Claiming the neutral object gains charge when attracted, rather than having its charge redistributed.

• Giving the correct force but reversing the signs in the reason.

Exam tips

• If asked to identify an unknown charge, design the test around repulsion.
• Explain paper or water attraction by movement of charge within the neutral object, not by transfer onto it.
• Name both signs in your reason, for example one is positive and one is negative.