What is electric charge?
Everything is made of atoms. An atom has a tiny centre called the nucleus. The nucleus holds protons, which have a positive (+) charge. Around the nucleus move electrons, which have a negative (−) charge.
A normal atom has as many protons as electrons. The plus and minus cancel, so the atom is neutral (no overall charge).
Charge is measured in coulombs (C). One electron carries a very tiny charge: about 1.6 × 10⁻¹⁹ C.
Charging by rubbing (friction)
When you rub two different insulators (materials that do not let charge flow, like plastic, rubber, wool or glass), some electrons move from one to the other.
- The object that gains electrons becomes negative.
- The object that loses electrons becomes positive.
Only electrons move. Protons are locked inside the nucleus. So a positive object is one that has lost electrons, not one that gained protons.
Examples: a plastic rod rubbed with wool becomes negative; a glass rod rubbed with silk becomes positive.
This charge stays still on the insulator. That is why it is called static (not moving) electricity.
Conservation of charge
Rubbing does not make charge. It only moves it. If the balloon gains 3 electrons, the cloth loses exactly 3. Before rubbing: 0 + 0 = 0. After: (−3) + (+3) = 0. The total charge stays the same. This is the law of conservation of charge.
Forces between charges
Like charges repel (push apart): + and +, or − and −. Unlike charges attract (pull together): + and −.
This force works without touching. It is a non-contact force. It gets stronger when the charges are bigger and weaker when they are further apart. Double the distance and the force drops to about one quarter.
Charging by induction and conduction
Induction means charging (or pulling) an object without touching it. Bring a negative balloon near a neutral wall. The wall's electrons are pushed back a little. The side near the balloon is now slightly positive. Unlike charges attract, so the balloon sticks. This is also why a charged comb picks up neutral bits of paper.
Conduction means charging by touching. When a charged object touches a metal, some charge flows onto the metal. Both end up with the same sign of charge.
A gold-leaf electroscope shows charge: its thin metal leaves get the same charge, repel each other and spread apart.
Electric fields
Every charged object has an electric field around it. This is the region where another charge feels a force. We draw it with field lines: they point away from a positive charge and towards a negative charge. Where the lines are close together, the field is strong. The field is strongest near the object and gets weaker further away.
Sparks, earthing, dangers and uses
When a lot of charge builds up, the field becomes very strong. It can tear electrons off air particles and a spark jumps across the gap. Lightning is a giant spark between a cloud and the ground.
Earthing means joining a charged object to the ground with a conductor (a metal wire). The extra charge flows away safely. Fuel tankers are earthed before refuelling a plane, because a spark near fuel vapour could start a fire. Tall buildings have a lightning conductor: a metal rod joined to the ground.
Useful static: photocopiers and laser printers, paint sprayers that make paint stick evenly, and dust filters in factory chimneys.
Key formulas and definitions
- Rubbing moves electrons: gains electrons → negative; loses electrons → positive
- Total charge before = total charge after (conservation of charge)
- Like charges repel; unlike charges attract
- Charge of one electron e ≈ −1.6 × 10⁻¹⁹ C; charge Q = n × e
- Force gets weaker with distance: double the distance → about ¼ of the force
- Unit of charge: coulomb (C)
Worked examples
1. A plastic ruler is rubbed with a wool cloth and becomes negative. What happened to the cloth, and why?
Electrons moved from the wool to the ruler. The ruler gained electrons (negative). The wool lost the same number of electrons, so it became positive by the same amount.
2. A balloon gains 5 × 10¹² electrons. What is its charge? (e = 1.6 × 10⁻¹⁹ C)
Q = n × e = 5 × 10¹² × 1.6 × 10⁻¹⁹ = 8 × 10⁻⁷ C. It gained electrons, so the charge is −8 × 10⁻⁷ C.
3. Rod A repels rod B. Rod B attracts rod C. Rod C is positive. What are A and B?
B attracts a positive rod, so B is negative (or neutral — but it also repels A, and only charged things repel). So B is negative, and A repels B, so A is also negative.
4. Why does a charged comb attract a neutral bit of paper?
Induction. The comb's charge pushes the paper's electrons slightly to one side. The near side gets the opposite charge, so the pull on the near side is bigger than the push on the far side.
5. Why must a fuel tanker be earthed before refuelling an aircraft?
Fuel rubbing through the pipe can build up static charge. A spark could light the fuel vapour. Earthing lets the charge flow safely to the ground, so no spark forms.
Common mistakes
- Saying protons move when you rub. Only electrons move; protons stay in the nucleus.
- Thinking a positive object gained positive charge. It lost electrons.
- Believing rubbing creates charge. It only moves charge; the total stays the same.
- Thinking only charged objects can be attracted. A charged object also attracts neutral objects by induction.