4.2.5.1Static charge
Physics only — not assessed in Combined Science: Trilogy.
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
4.2.5.2Electric fields
Physics only — not assessed in Combined Science: Trilogy.
A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field.
A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases.
You should be able to draw the electric field pattern for an isolated charged sphere. Four conventions have to be right:
• the lines are radial — straight, and pointing directly towards or away from the centre • the arrows point away from a positive charge and towards a negative charge • the lines are evenly spaced all around the sphere • the lines are closer together near the sphere, where the field is strongest, and further apart away from it.
You should also be able to explain the concept of an electric field. A field is a region in which a charged object experiences a force. Drawing it as lines is a way of recording two things at once: the direction of the force on a positive charge placed there, and — through the spacing — how strong that force would be.
You should be able to explain how the concept of an electric field helps to explain the non-contact force between charged objects, as well as other electrostatic phenomena such as sparking.
The non-contact force: rather than saying one charge somehow reaches across empty space to push another, the field lets you say that the first charge fills the space around it with a field, and the second charge experiences a force because of the field where it actually is. The force is then explained locally, and the fact that it gets stronger as the objects get closer follows directly from the field being stronger nearer the charge.
Sparking: as charge builds up on an object, the potential difference between it and a nearby earthed object rises, and so does the field strength in the air between them. If the field becomes strong enough, it pulls electrons off the atoms in the air. The air is no longer an insulator — it now contains charged particles that can move — so charge flows through it suddenly, and that flow is the spark you see and hear.
Common mistake
Field lines must never cross, and they must be drawn with arrows. A pattern without arrows does not say whether the sphere is positive or negative, which is usually half the mark.
- Electric field
- the region around a charged object in which a second charged object experiences a force; the field is strongest close to the charged object and weaker further away
Before you practise
- This sub-topic is physics only — it is not assessed in Combined Science: Trilogy.
- Rubbing certain insulators transfers electrons: the material that gains them becomes negative, the one that loses them is left equally positive.
- Only electrons move — positive charge never does.
- Same charges repel, different charges attract, and both are non-contact forces.
- A charged object creates an electric field around itself, strongest close to the object and weaker further away.
- A second charged object in the field experiences a force, which grows as the separation decreases.
- Draw the field of an isolated charged sphere as radial lines, arrows away from positive and towards negative, closer together near the sphere.
- Sparking happens when the field is strong enough to pull electrons off atoms in the air, so charge can flow through it.