Revision notes · Electricity
Static electricity (physics only)
Static charge4.2.5.1
Definition: Charge is a property of matter. Like charges repel; opposite charges attract. An insulator cannot conduct electricity because its electrons are fixed in place and cannot flow through the material; a conductor can conduct electricity because it has delocalised electrons that are free to flow.
When two insulating materials are rubbed together, electrons are transferred from one to the other — the object that loses electrons becomes positively charged, and the object that gains electrons becomes negatively charged. Which object loses or gains electrons depends on the materials involved. If two conductors were rubbed together instead, electrons would simply flow between them and cancel out, so conductors do not build up static charge this way.
- •A spark occurs when enough charge builds up between two objects that are close together but not touching — charge jumps through the air from the highly negatively charged object to the highly positively charged one, balancing the charges out.
- •Charged objects exert an electrostatic force (of attraction or repulsion) on each other — this is a non-contact force, since it acts even when the objects are not touching. A greater charge, or a smaller distance between the objects, produces a greater force.
⚠️ Common mistake: Forgetting that only insulators build up static charge from rubbing — charge on a conductor flows away (through the conductor, and to earth if it is connected) rather than staying in one place.
Electric fields4.2.5.2
Definition: A charged object creates an electric field in the space around it — a region where another charged object experiences a force.
- •Electric field lines point in the direction a positive charge placed in the field would move: away from a positive charge, and towards a negative charge.
- •The field is strongest close to the charged object, shown by field lines that are closest together near the object and spread further apart with distance.
- •The greater the charge on the object, the stronger the field (and the greater the force felt by another charge placed in it).
🧠 Remember: Field lines: out of positive, into negative — and always closer together where the field is stronger.
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