Electrostatics

Revision notes

Matter can have positive or negative net charge. Static charging is produced by electron transfer; protons remain bound in nuclei. Like charges repel and unlike charges attract, and charged objects can exert forces without contact. Static charge builds readily on insulators and can produce sparks. Electric current is a rate of flow of charge and requires a closed circuit with a source of potential difference.

Charging by electron transfer

Electrostatic charging involves an imbalance of electrons. An object that gains electrons becomes negatively charged, while an object that loses electrons becomes positively charged. Protons remain bound in atomic nuclei.

Insulators tend to hold charge in one region, whereas conductors allow charge to move more freely.

Forces between charges

Like charges repel and unlike charges attract. A charged object produces an electric field in the surrounding space. Field direction is defined by the force that would act on a positive test charge.

A neutral object can still be attracted to a charged object because charges within it can redistribute.

Exam technique

Use electron-transfer language accurately. A positive charge means the object has lost electrons; protons do not move. Keep static charge separate from continuous current in a circuit.

Attraction of neutral objects

A charged object can attract a neutral object because charges inside the neutral material can redistribute slightly. The opposite charge can become effectively closer to the charged object than the like charge, producing a net attraction even though the neutral object has no overall charge.

Discharge and sparks

If charge builds up sufficiently, a large potential difference can cause air to become conducting and a rapid discharge can occur. This produces a spark. In practical situations, the importance of a spark depends on the surroundings and whether a discharge could create a hazard.

When describing charging, state which object gains or loses electrons. When explaining attraction, distinguish an overall neutral object from the local redistribution of charge within it.