How do objects become charged by friction, and what are the uses and hazards of static electricity?
Charging insulators by friction through the transfer of electrons, attraction and repulsion between charges, electric fields around charges, and the uses and hazards of static electricity.
A focused answer to OCR Gateway GCSE Physics A topic P3 on static electricity, covering charging insulators by friction through electron transfer, attraction and repulsion of charges, electric fields, and the uses and hazards of static electricity.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
OCR wants you to explain how insulators are charged by friction, describe attraction and repulsion between charges, understand the electric field around a charge, and give the uses and hazards of static electricity. This is topic P3.1 of the OCR Gateway Physics A (J249) specification.
Charging by friction
This only happens with insulators, because in a conductor any charge would simply flow away. For example, rubbing a polythene rod with a cloth transfers electrons onto the rod, leaving it negative and the cloth positive. The two objects always end up with equal and opposite charges, because the electrons removed from one are exactly the electrons gained by the other.
Attraction and repulsion
These forces act at a distance, without the objects touching, which is why a charged comb can pick up small pieces of paper or bend a stream of water.
Electric fields
When two charged objects are brought close, their fields overlap and the force between them depends on the size of the charges and how close they are: the closer they are, the stronger the force. This field picture explains both the attraction and repulsion of charges and how a spark can jump when the field becomes strong enough to ionise the air.
Uses and hazards of static electricity
Uses include inkjet printers and electrostatic paint sprayers (charged droplets spread evenly and are attracted to the surface), photocopiers (charge attracts toner to the paper), and electrostatic dust precipitators in chimneys (charged plates attract dust particles to clean the gases).
Hazards include sparks when charge builds up near flammable vapours (for example, refuelling an aircraft or tanker), which could cause a fire or explosion, and electric shocks from touching a charged object. Both are reduced by earthing: connecting the object to the ground with a conductor so the charge flows safely away instead of building up.
Try this
Q1. State what happens to two objects that both carry a negative charge when they are brought close. [1 mark]
- Cue. They repel (push apart), because like charges repel.
Q2. Explain why a cloth used to charge a rod is left with the opposite charge to the rod. [2 marks]
- Cue. The electrons gained by the rod are exactly those lost by the cloth, so the cloth is left with an equal and opposite charge.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20183 marksA polythene rod is rubbed with a dry cloth and becomes negatively charged. Explain, in terms of electrons, how the rod becomes charged and what charge the cloth is left with.Show worked answer →
A P3 Explain question worth three marks. When the rod is rubbed, electrons are transferred from the cloth to the polythene rod (1 mark). The rod gains electrons, so it has more negative charge than positive and becomes negatively charged (1 mark). The cloth has lost electrons, so it is left with an equal positive charge (1 mark). Markers reward electron transfer (not proton transfer), the rod gaining electrons to become negative, and the cloth becoming positive. A common error is to say protons or charge "flows" rather than electrons being transferred.
OCR 20213 marksStatic electricity can be both useful and hazardous. Describe one use and one hazard of static electricity, and explain how the hazard can be reduced.Show worked answer →
A P3 question worth three marks. A use: in an inkjet printer or paint sprayer, charging the droplets makes them spread evenly and stick to the surface, or a photocopier uses charge to attract toner to the paper (1 mark). A hazard: when refuelling an aircraft or tanker, charge can build up and a spark could ignite the fuel vapour, or you can get an electric shock from a charged object (1 mark). This is reduced by earthing (connecting to the ground with a conductor) so the charge flows safely away rather than building up (1 mark). Markers reward a valid use, a valid hazard, and earthing as the way to reduce it.
Related dot points
- Circuit symbols and how to build circuits, the rules for current and potential difference in series and parallel circuits, and how total resistance changes when components are added in series or in parallel.
A focused answer to OCR Gateway GCSE Physics A topic P3 on circuits, covering standard circuit symbols, the rules for current and potential difference in series and parallel circuits, and how the total resistance changes when components are added in series or in parallel.
- Electric current as the rate of flow of charge, the charge equation, potential difference as energy transferred per unit charge, resistance and the equation linking potential difference, current and resistance.
A focused answer to OCR Gateway GCSE Physics A topic P3 on current, potential difference and resistance, covering current as the rate of flow of charge, the charge equation, potential difference as energy per unit charge, resistance, and the equation linking them.
- The I-V characteristics of an ohmic resistor, a filament lamp and a diode, ohmic and non-ohmic behaviour, and how the resistance of a thermistor and an LDR varies with temperature and light.
A focused answer to OCR Gateway GCSE Physics A topic P3 on I-V characteristics, covering the graphs for an ohmic resistor, a filament lamp and a diode, ohmic and non-ohmic behaviour, and how the resistance of a thermistor and an LDR changes with temperature and light.
- Permanent and induced magnets, magnetic materials, attraction and repulsion between poles, the magnetic field around a bar magnet and the Earth, and how a compass shows field direction.
A focused answer to OCR Gateway GCSE Physics A topic P4 on magnets, covering permanent and induced magnets, magnetic materials, attraction and repulsion between poles, the magnetic field around a bar magnet and the Earth, and how a compass shows field direction.