How does charge build up by friction, and what is an electric field?
Static electricity: charging by friction, the forces between charges, sparking, and the idea of an electric field around a charged object (separate physics only).
A focused answer to AQA GCSE Physics 4.2.6, covering how insulators are charged by friction through electron transfer, attraction and repulsion between charges, how sparks form, and the concept of an electric field around a charged object.
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What this dot point is asking
AQA wants you to explain how an insulator becomes charged by friction through the transfer of electrons, describe the forces between charged objects, explain how a spark forms, and describe the electric field around a charged object. This is topic 4.2.6 of the AQA GCSE Physics (8463) specification and is separate physics only.
Charging by friction
For example, rubbing a polythene rod with a duster transfers electrons to the rod, making it negative; the duster is left positive. Whether a material gains or loses electrons depends on the pair of materials involved: an acetate rod rubbed with the same cloth tends to lose electrons and become positive. The reason only insulators hold static charge is that in a conductor any extra charge would simply flow away to earth, whereas in an insulator the electrons cannot move freely, so the charge stays where it was deposited.
Forces between charges
Sparking
If charge builds up on an object, the potential difference between it and an earthed conductor can become very large. When it is big enough, electrons jump across the gap, ionising the air and producing a spark. This is why charged objects can discharge suddenly to earth.
Static electricity has useful applications too, such as in electrostatic paint sprayers, where the paint droplets are charged so they spread out (mutual repulsion) and are attracted to an oppositely charged object, giving an even coat that reaches hidden surfaces.
Electric fields
Field lines point away from a positive charge and towards a negative charge, and a stronger field is shown by lines that are closer together.
Try this
Q1. Explain how a polythene rod becomes negatively charged when rubbed with a cloth. [2 marks]
- Cue. Electrons are transferred from the cloth to the rod, so the rod gains electrons and becomes negative.
Q2. State what happens when two negatively charged objects are brought close together. [1 mark]
- Cue. They repel each other.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA student rubs a polythene rod with a dry cloth. The rod becomes negatively charged. Explain, in terms of the movement of electrons, how the rod becomes negatively charged and what charge is left on the cloth.Show worked answer →
When the rod and cloth are rubbed together, the friction transfers electrons from one material to the other (1 mark). Electrons move from the cloth onto the polythene rod (1 mark). The rod now has more electrons than protons, so it has a net negative charge (1 mark). The cloth has lost electrons, so it now has more protons than electrons and is left with an equal positive charge (1 mark). Markers reward stating that only electrons move (not protons), the direction of transfer, and that the positive charge is due to a shortage of electrons. A frequent error is to say protons are transferred.
AQA 20213 marksDescribe the electric field around a small positively charged sphere, and explain why a charged object placed in this field experiences a force that depends on its position.Show worked answer →
A positively charged sphere is surrounded by a radial electric field, with field lines pointing directly away from the sphere (1 mark). The field is strongest close to the sphere, shown by the field lines being closest together there, and it gets weaker with distance as the lines spread apart (1 mark). A charged object placed in the field experiences a non-contact electrostatic force; the force is larger when the object is closer to the sphere because the field is stronger there, and smaller further away (1 mark). Markers reward the radial pattern, the direction of the lines (away from positive), and the link between field strength, line spacing, and the size of the force.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)