Static ElectricityAQA GCSE Physics: Revision notes
Section 1
How is static charge produced by friction between insulators?
Static electricity is produced when two insulating materials are rubbed together. When this happens, electrons are transferred from one material to the other, leaving one material positively charged and the other negatively charged.
Key points about friction and charge production:
- Insulators do not allow electrons to flow freely, so charge builds up on the surface
- The material that loses electrons becomes positively charged
- The material that gains electrons becomes negatively charged
- The more friction applied, the greater the charge transferred
- Common examples include: rubbing a balloon on hair, rubbing a cloth on a polythene rod, or walking across a carpet
The charge remains on the material until it is discharged (released) in a spark or through contact with another conductor.
Think of friction like a tug-of-war between atoms: one material pulls electrons away from the other, leaving one side with excess electrons (negative) and the other short of electrons (positive).
Section 2
How does electron transfer explain static charge?
Static charge is explained entirely by the transfer of electrons between materials. All materials are made of atoms containing protons (positive) and electrons (negative).
The electron transfer process:
- When two insulators are rubbed together, friction provides energy
- This energy allows electrons to break free from their atoms
- Electrons transfer from one material to the other
- The material that gains electrons becomes negatively charged (excess electrons)
- The material that loses electrons becomes positively charged (deficit of electrons)
Important principle: Only electrons move, never protons. Protons remain fixed in the nucleus.
Charge is always conserved – the total positive charge gained by one object equals the total negative charge gained by the other object. If object A loses 10 electrons, it gains +10 charge; if object B gains those 10 electrons, it gains −10 charge.
Examiners expect you to state clearly that 'only electrons transfer, not protons' and to explain charge in terms of gain or loss of electrons, not just 'becoming charged'.
Common error: saying 'protons transfer' or 'the object gains protons'. Only electrons move. Protons are fixed in the nucleus.
Section 3
How do charged objects interact with each other?
The behaviour of charged objects follows two fundamental rules:
| Interaction | Description | Example |
|---|---|---|
| Like charges repel | Two objects with the same type of charge push each other away | Two negatively charged balloons repel each other |
| Unlike charges attract | Two objects with opposite charges pull each other together | A negatively charged balloon attracts a positively charged rod |
Key points:
- Like charges means both positive or both negative – these repel
- Unlike charges (or opposite charges) means one positive and one negative – these attract
- The force between charges is non-contact – it acts at a distance without the objects touching
- The force increases when charges are larger or closer together
Why this happens: Charged objects create electric fields around them. When another charged object enters this field, it experiences a force.
Like charges are like two magnets with their north poles facing each other – they push apart. Unlike charges are like opposite magnetic poles – they pull together.
Section 4
What is an electric field and how is it represented?
An electric field is the region around a charged object in which another charged object will experience a force.
Properties of electric fields:
- Every charged object creates an electric field around it
- The field is strongest closest to the charged object
- The field is weaker further away from the charged object
- The field exists even in empty space (it is non-contact)
Representing electric fields using field lines:
- Electric field lines show the direction and pattern of the electric field
- Field lines point away from positive charges (the direction a positive test charge would be pushed)
- Field lines point towards negative charges (the direction a positive test charge would be pulled)
- Closer together field lines indicate a stronger field
- Further apart field lines indicate a weaker field
- Field lines never cross
Pattern around a single charged object:
- Lines radiate outward (positive charge) or inward (negative charge)
- The density of lines decreases with distance
Pattern between two opposite charges:
- Field lines go from the positive charge towards the negative charge
When describing field patterns, always state the direction of field lines relative to the charges: lines radiate OUT from positive charges and IN towards negative charges.
Section 5
What are the risks and uses of electrostatic charge?
Static electricity has both dangers and useful applications.
Risks and dangers of electrostatic charge:
| Risk | How it occurs | Consequence |
|---|---|---|
| Sparking | Charge builds up, then suddenly discharges through air | Fire or explosion in flammable environments; electric shock |
| Electric shock | Large static charge discharges through a person | Pain, injury, or in extreme cases, harm |
| Ignition of flammable materials | Sparks from static discharge ignite gases, vapours, or dusts | Explosion in petrol stations, chemical plants, grain silos |
Practical examples of risks:
- Refuelling vehicles – charge builds up on fuel being pumped; spark can ignite petrol vapour
- Handling explosive materials – any spark from static could be dangerous
- In aircraft – static builds up during flight; must be discharged safely
Precautions to reduce risk:
- Use earthing straps to safely discharge charge to the ground
- Ensure good electrical conductivity between parts (use conductive materials)
- Use antistatic mats and clothing to prevent charge accumulation
- Keep flammable materials away from sources of static discharge
Uses of electrostatic charge:
| Use | How it works |
|---|---|
| Electrostatic spray painting | Paint droplets are negatively charged; the object to be painted is positively charged; attraction ensures even coating and minimal waste |
| Photocopiers and laser printers | Static charge on a drum attracts toner powder to create an image |
| Air purifiers | Static attracts dust particles out of the air |
Electrostatic spray painting in detail:
- Paint is given a negative charge as it leaves the nozzle, breaking it into fine droplets
- The object to be painted is earthed (grounded) so it is positively charged
- Electrostatic attraction pulls the negatively charged paint droplets onto the positive object
- This ensures even coverage, reduces overspray, and minimises waste
- It is particularly useful for coating car bodies and complex shapes
At a petrol station: fuel being pumped gains a negative charge. If the nozzle, hose, and vehicle are not earthed, a spark could jump and ignite the petrol vapour. An earthing strap from the vehicle to ground prevents charge accumulation, making sparking safe.
Section 6
What is the force on a charged object in an electric field? (Higher Tier)
A charged object placed in an electric field experiences a force. The size of this force depends on both the charge of the object and the strength of the electric field.
The relationship between field strength and force:
The force on a charged object is directly proportional to:
- The charge on the object – a larger charge experiences a larger force
- The electric field strength – a stronger field produces a larger force
Mathematical relationship (Higher Tier):
- Force = Charge × Electric Field Strength
- F = Q × E
- Where F is force (in newtons, N), Q is charge (in coulombs, C), and E is field strength (in newtons per coulomb, N/C)
Direction of force:
- A positive charge experiences a force in the same direction as the field lines
- A negative charge experiences a force in the opposite direction to the field lines
Field strength (E):
- Field strength is defined as the force per unit charge: E = F/Q
- Field strength is measured in newtons per coulomb (N/C) or volts per metre (V/m)
- A stronger field (larger E) means a larger force on a given charge
- Field strength decreases with distance from the charged object
Worked example: A charge of 2 coulombs is placed in an electric field with strength 5 N/C. Force = Q × E = 2 × 5 = 10 N in the direction of the field (if charge is positive).
For Higher Tier, you must know the equation F = QE and be able to rearrange it. Always state the direction of force: positive charges follow field lines; negative charges experience force opposite to field lines.
A negative charge of 4 C is in a field of strength 3 N/C pointing right. Force = 4 × 3 = 12 N, but pointing LEFT (opposite to field lines because charge is negative).
Must Know
- Static charge is produced by friction: Rubbing insulating materials together causes electrons to transfer from one to the other, making one positive and one negative.
- Only electrons transfer, never protons – electrons are lost or gained; this charge transfer is the only explanation for static electricity.
- Like charges repel; unlike charges attract – identical charges push apart, opposite charges pull together; these are non-contact forces.
- Electric fields exist around charged objects and are represented by field lines: lines point out from positive charges and in towards negative charges; closer lines mean stronger field.
- Risks of static electricity include sparking and electric shock, especially near flammable materials; earthing straps prevent dangerous charge accumulation. Electrostatic spray painting uses attraction between opposite charges for even, efficient coating.
- (HT) Force on a charge = Q × E: A charge in an electric field experiences a force proportional to both the charge and field strength; positive charges move with field lines, negative charges move against them.
That's the notes covered.
Carry on to the next subtopic.