Static Electricity and Electric Fields
Static electricity is about electrons moving from one object to another. Everything on this page follows from one fact: only electrons move — protons stay put in the nucleus.
1. Charge
There are two types of charge: positive and negative.
LIKE charges REPEL. UNLIKE charges ATTRACT.
| Particle | Charge |
|---|---|
| Proton | positive |
| Electron | negative |
| Neutron | neutral |
Charge is measured in coulombs (C).
Describe charge as “positive” or “negative”, not as “one” or “minus one”. Those are relative charge values used in nuclear physics; when a question asks for the charge on a proton, the answer is positive. Both slips were recorded.
2. Charging by friction
When two insulating materials are rubbed together, ELECTRONS are transferred from one to the other.
- The material that GAINS electrons becomes NEGATIVELY charged
- The material that LOSES electrons becomes POSITIVELY charged
Classic example: rubbing a polythene rod with a cloth — the rod gains electrons and becomes negative. Rubbing an acetate (perspex) rod — it loses electrons and becomes positive.
A LACK of electrons makes an object POSITIVE, not negative. This exact confusion was recorded. Removing negative charge leaves an excess of positive charge behind — so fewer electrons means more positive.
Only ELECTRONS move. Protons never transfer, because they are held in the nucleus. Any explanation involving “protons moving to the cloth” is wrong.
Charge is conserved: one object gains exactly what the other loses, so the two are equal and opposite.
Charging by friction only works for INSULATORS. A conductor would let the charge flow away, especially through your hand.
3. Attraction and repulsion
Repulsion is the only conclusive test for charge, exactly as with magnetism.
A charged object will attract an uncharged one, because it causes induced charge: the nearer surface acquires the opposite charge, so attraction always results. That is why a charged rod picks up small pieces of paper.
Attraction alone doesn’t prove an object is charged — it may simply be uncharged and polarised.
4. Electric fields
An electric field is a region in which a charge experiences a force.
Electric field strength is the force per unit charge at a point.
An electric field is not “an area charged by electrons”. That was a recorded definition error. The field is the region of influence around a charge, where another charge would feel a force.
Drawing field lines — the conventions:
- Field lines point AWAY from positive charges and TOWARDS negative ones
- The direction is that of the force on a POSITIVE test charge
- Lines never cross
- Closer lines mean a stronger field
- Between parallel plates, the field is uniform — parallel, equally spaced lines
Patterns to know: an isolated positive charge (lines radiating outwards), an isolated negative charge (lines pointing inwards), and two opposite charges (lines running from + to −).
The direction of force on an ELECTRON is OPPOSITE to the field line, because the electron is negative. Misunderstanding the direction of the force on an electron in a field was a recorded error. Field lines show the force on a positive charge; a negative charge feels the reverse.
5. Conductors and insulators
Conductors (metals, graphite) contain free electrons, so charge flows through them easily. Insulators (plastic, rubber, glass, dry air) have no free electrons, so charge stays where it is put.
Earthing:
Earthing provides a path for excess charge to flow to the ground, safely discharging the object.
If a negatively charged object is earthed, electrons flow from the object to earth. If it is positively charged, electrons flow from earth to the object.
Note the direction carefully — in both cases it is electrons that move, never protons.
6. Uses and hazards
Uses:
- Photocopiers and laser printers — charged drum attracts toner
- Electrostatic spray painting — charged paint droplets repel each other, spreading evenly, and are attracted to the oppositely charged object
- Dust precipitators in chimneys — charged plates attract dust particles
Hazards:
- Refuelling aircraft or tankers — a spark could ignite fuel vapour, so the vehicle is earthed (bonded) first
- Lightning — charge builds in clouds and discharges to earth; lightning conductors provide a safe path
- Electronic components can be damaged by static discharge
The standard safety answer is “earthing” — providing a conducting path so charge leaks away gradually instead of building up to a spark.
7. Mistakes that cost marks
Saying protons move during charging.
Saying a lack of electrons makes something negative.
Giving the charge as “one” rather than positive or negative.
Saying attraction proves an object is charged.
Defining an electric field as a charged area rather than a region where a charge feels a force.
Field lines pointing towards a positive charge.
Field lines that cross.
Getting the force direction on an electron backwards.
Trying to charge a conductor by friction without insulating it.
Frequently asked questions
What are the two types of charge? Positive and negative.
What is the rule for charges? Like charges repel; unlike charges attract.
How does an object become charged? By friction — electrons are transferred between the two materials.
Which particle moves? Only the electron. Protons stay in the nucleus.
What makes an object positive? Losing electrons.
What is an electric field? A region where a charge experiences a force.
Which way do field lines point? Away from positive, towards negative — the direction of force on a positive charge.
Which way is an electron pushed? Opposite to the field lines, because it is negative.
What is earthing? Providing a conducting path for excess charge to flow to the ground.
Why is a fuel tanker earthed? To prevent a build-up of static that could spark and ignite the fuel vapour.
Quick revision checklist
- I know like charges repel and unlike attract
- I know the charges of protons, neutrons and electrons
- I know charging happens by transferring electrons
- I know losing electrons makes an object positive
- I know protons never move
- I know charge is conserved
- I know only repulsion proves an object is charged
- I can define an electric field correctly
- I can draw field lines for single and paired charges
- I know field lines show the force on a positive charge
- I know an electron is pushed the opposite way
- I can explain earthing and which way electrons flow
- I can give uses and hazards of static electricity
These notes cover static electricity and electric fields in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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