Electric charge is a property of protons and electrons. Like charges repel and unlike charges attract. Charge moves freely in conductors but not in insulators, and it is additive, conserved and quantised.
On a dry day, pulling off a woollen sweater produces small crackling sparks. A balloon rubbed on hair sticks to a wall. Both effects are caused by electric charge, a property of the particles that make up matter.
There are two kinds of charge, called positive and negative. The names are a convention. Experiment shows one rule between them: like charges repel, unlike charges attract.
In an atom, each proton carries a positive charge and each electron carries a negative charge of the same size. Most objects have equal numbers of protons and electrons, so their total charge is zero. They are neutral.
When two different materials are rubbed together, electrons move from one surface to the other. The material that loses electrons has more protons than electrons and is left positively charged. The material that gains electrons becomes negatively charged. No charge is created; it is moved from one object to the other.
In a conductor, such as a metal, some electrons are not bound to individual atoms and can move through the material. Charge placed on a conductor spreads over it. The human body and the Earth are also conductors.
In an insulator, such as glass, rubber or plastic, the electrons are bound to their atoms and almost cannot move. Charge placed on an insulator stays where it was placed.
A plastic rod rubbed with wool stays charged. A metal rod rubbed the same way, while held in your hand, does not. Why?
Both rods gain charge when rubbed.
Plastic is an insulator. The charge cannot move through it, so it stays on the rod.
Metal is a conductor, and so is your body. The charge moves through the metal, through you, and into the Earth. The Earth is so large that this charge makes no noticeable difference to it. So the metal rod is left with almost no charge.
If the metal rod is held by an insulating handle, the path to the Earth is blocked and the rod stays charged.
The total charge of an object is the algebraic sum of all the charges on it: the charges are added with their signs. An object with +3 units and −5 units of charge has total charge +3 + (−5) = −2 units.
The total charge of an isolated system is constant. Charge can move from one object to another, but the total does not change. When wool and plastic are rubbed together, the negative charge gained by one equals the negative charge lost by the other. This is the law of conservation of charge. No experiment has ever found it broken.
Two identical metal spheres carry +6 units and −2 units of charge. They are touched together, then separated. What charge does each have?
Each has +2 units.
By additivity, the total charge is +6 + (−2) = +4 units. By conservation, the total is still +4 units after contact.
While touching, the two spheres form one conductor, and charge moves between them. Because the spheres are identical, the charge divides equally by symmetry: +4 ÷ 2 = +2 units each.
Every charge found on an isolated object is a whole-number multiple of a smallest unit, e, the size of the charge on one electron or one proton:
q = ne, where n is an integer
An object can carry 1e, 2e or 10¹⁰e, but not 1.5e. This is the quantisation of charge.
The SI unit of charge is the coulomb (C). Since 2019, e is defined as exactly 1.602176634 × 10⁻¹⁹ C. So 1 C is about 6.24 × 10¹⁸ electron charges. In everyday charging, the number of electrons moved is so large that the steps of size e cannot be detected, and charge appears continuous.
Inside protons and neutrons there are particles called quarks, with charges of ±e/3 and ±2e/3. Quarks are never found on their own, so every charge measured on a free particle or object is still a whole multiple of e.
1Two charges of the same sign…
2A rod rubbed with a cloth becomes negatively charged. What happened?
3A conductor differs from an insulator because, in a conductor,…
4An object has +3 units and −5 units of charge. Its total charge is…
5Conservation of charge means that…
6Quantisation of charge means that the charge q on any object equals…
Next: Coulomb’s law, which gives the size of the force between two charges.
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