11.1P–11.7PStatic charge
Physics only — not on the Combined Science specification
When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.
Read that carefully, because the whole sub-topic depends on it: only electrons move. Protons stay in the nuclei, fixed in the material. A positive charge is never produced by adding positive charge — it is what is left behind when electrons leave.
When two electrically charged objects are brought close together they exert a force on each other. Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract. Attraction and repulsion between two charged objects are examples of non-contact force.
Non-contact means the objects do not have to touch for the force to act — which is why this sub-topic needs the idea of a field in the next section.
You should be able to describe the production of static electricity, and sparking, by rubbing surfaces; describe evidence that charged objects exert forces of attraction or repulsion on one another when not in contact; and explain how the transfer of electrons between objects can explain the phenomena of static electricity.
The evidence for non-contact forces is the standard demonstration: suspend a charged rod so it can swing freely, and bring a second charged rod near it without touching. The suspended rod moves — towards the second rod if the charges are different, away if they are the same. It moves before the two touch, so the force must act at a distance.
Worked example
A balloon is rubbed on a jumper and then sticks to a wall. Explain, in terms of electrons, what has happened.
- Rubbing transfers electrons between the two insulating surfaces.
- The balloon gains electrons from the jumper, so the balloon becomes negatively charged and the jumper is left equally positive.
- When the charged balloon is held near the wall, the electrons in the surface of the wall are repelled by the balloon's negative charge and move slightly away.
- That leaves the surface of the wall closest to the balloon slightly positive.
- Opposite charges attract, and the force acts without contact, so the balloon is held against the wall.
Answer: The balloon gains electrons and becomes negative; it repels electrons in the wall's surface, leaving that surface positive, and the attraction between opposite charges holds the balloon there.
Common mistake
Never say that positive charge moved. Only electrons are transferred. An object becomes positively charged by losing electrons, and the object that gained them is negative by exactly the same amount.
- Static charge
- the charge left on an insulating material when electrons are rubbed off one material onto another; the material gaining electrons becomes negatively charged and the one losing them is left equally positive
Before you practise
- Rubbing two insulators transfers ELECTRONS — never protons, which cannot move.
- The material that gains electrons becomes negative; the one that loses them becomes positive.
- Like charges repel and unlike charges attract, and a charged object attracts an uncharged one.
- Earthing gives electrons a path to or from the ground, removing the excess charge.
- Uses: insecticide sprayers and paint sprayers, where the charged droplets repel each other and spread evenly, and are attracted to the target.
- Dangers: sparking when refuelling, and shocks from a car door or a slide.
- A spark happens when the potential difference is large enough to make the air conduct.