Static Electricity Notes
Edexcel GCSE Physics: Revision notes
Key facts
- Rubbing two insulators transfers electrons: the surface that gains them is negative, the one that loses them is positive. No charge is created.
- Like charges repel; unlike charges attract.
- Shocks, lightning and sticking balloons are explained by electron movement and induction.
- Earthing lets excess electrons flow away, neutralising an object safely.
- An electric field is where a charge feels a force; closer field lines mean a stronger field.
- 1
Rub
Two insulators are rubbed together.
- 2
Electrons transfer
From one surface to the other.
- 3
Opposite charges
One is negative, the other equally positive.
Charging by friction
Rubbing moves electrons from one insulator to the other, leaving equal and opposite charges.
When two insulators are rubbed together, electrons move from one surface to the other. Insulators cannot let the charge flow away, so it stays put (static).
The surface that gains electrons becomes negative; the one that loses them is equally positive. Only electrons move, so the charges are always equal and opposite. Conductors do not stay charged because electrons flow away through them.
- 1
Rub
Two insulators are rubbed together.
- 2
Electrons transfer
From one surface to the other.
- 3
Opposite charges
One is negative, the other equally positive.
A cloth rubbed on a rod becomes positively charged. What happened?
Attraction and repulsion
Like charges repel and unlike charges attract, without the objects touching.
Charged objects exert forces on each other without touching. That is why a charged balloon can be pulled towards a wall, or two charged rods of the same sign push apart.
Like charges
- and +, or − and −
- Repel
Unlike charges
- and −
- Attract
Two negatively charged balloons are brought close together. What happens?
Shocks, lightning and induction
Each is electron movement: a build-up of charge that discharges, or a charged object rearranging charge in a neutral one.
A shock from a door handle happens when charge built up by friction flows suddenly through an earthed conductor. Lightning is the same on a huge scale: friction between ice and water droplets separates charge in a storm cloud until a rapid spark discharge occurs.
In electrostatic induction, a charged object near a neutral conductor moves electrons within it, leaving an opposite charge on the near surface. The two then attract.
- 1
Balloon is charged
By friction.
- 2
Electrons shift
Electrons in the wall move away from or towards the balloon.
- 3
Near surface
Gains the opposite charge to the balloon.
- 4
Attraction
Unlike charges attract, so the balloon sticks.
Why does a charged balloon stick to a neutral wall?
Earthing, uses and dangers
Earthing lets excess charge flow safely to or from the ground, preventing sparks.
Earthing connects an object to the ground by a conductor so excess electrons flow away (or in), neutralising it. In an exam answer, link earthing to electrons flowing to or from earth.
A spark is a sudden discharge through the air. Earthing the tanker, nozzle and vehicle before fuelling stops a spark igniting fuel vapour.
- 1
Object holds excess charge
extra electrons or a shortage of them
- 2
Connect to earth
with a conductor, e.g. a metal wire
- 3
Electrons flow
to earth (or from earth) through the conductor
- 4
Neutral
no charge builds up, so no spark
Use
- Insecticide sprayer: droplets share the same charge, repel and spread into a fine mist
- The mist is attracted to the earthed plant, coating all surfaces
Danger
- Friction from flowing fuel builds up static charge
- A spark could ignite fuel vapour
Why are fuel tankers earthed during fuelling?
Electric fields
An electric field is the region around a charge where another charge feels a force; closer field lines mean a stronger field.
An electric field explains why charges push or pull without touching. Field lines show its direction and strength: they are closer together where the field is stronger.
Point charge
- Lines are radial
- Outwards from positive, inwards to negative
Parallel plates
- Lines parallel and evenly spaced
- From the positive to the negative plate (uniform field)
Which way do field lines point between charged parallel plates?
Try an exam question
A polythene rod becomes negatively charged when rubbed with a cloth. Explain how.
[4 marks]
- [1]Electrons are transferred from the cloth to the rod.
- [1]The rod gains electrons, so it is negative.
- [1]The cloth loses electrons, so it is positive.
- [1]Only electrons move, so the charges on the rod and cloth are equal and opposite.
That's the notes covered.
Carry on to the next subtopic.