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What is electric charge, and how do objects become charged by friction?

Describe positive and negative charge, charging by friction, and the forces between charges

A focused answer to the O-Level Physics outcome on static electricity. Positive and negative charge, charging by friction through electron transfer, the law of force between charges, and everyday effects and hazards of static.

Generated by Claude Opus 4.87 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to describe the two kinds of electric charge, to explain how objects become charged by friction through the transfer of electrons, to state that like charges repel and unlike charges attract, and to recognise everyday effects of static electricity. The big idea is that charging is about moving electrons, not creating charge.

The answer

Two kinds of charge

There are two types of electric charge, positive and negative. An atom is normally neutral because it has equal numbers of positive protons and negative electrons. Charge is measured in coulombs (C\text{C}).

Charging by friction

When two suitable materials are rubbed together, electrons are transferred from one to the other. Electrons are negatively charged and are the only particles that move in charging by friction (the protons stay fixed in the nuclei).

  • The material that gains electrons becomes negatively charged.
  • The material that loses electrons becomes positively charged.

Charge is conserved: the electrons gained by one object are exactly those lost by the other, so no charge is created or destroyed.

Forces between charges

Charged objects exert forces on one another:

like charges repel,unlike charges attract\text{like charges repel}, \qquad \text{unlike charges attract}

Two negatives repel, two positives repel, and a positive and a negative attract. A charged object can also attract small light neutral objects (such as bits of paper) by inducing charge in them.

Conductors and insulators

Charge can move freely through conductors (metals) but not through insulators (plastics). This is why a charged plastic rod keeps its charge, while charge on a metal would flow away to earth through your hand.

Examples in context

Example 1. A balloon sticking to a wall. Rub a balloon on your hair and it gains electrons, becoming negatively charged. Held against a wall, it induces a positive charge on the wall's surface, and the unlike charges attract, holding the balloon in place against gravity, a clear demonstration of charging by friction and induction.

Example 2. Fuelling and static hazards. When fuel flows through a pipe it can build up static charge by friction. A spark from this charge near flammable vapour could cause a fire, so fuel tankers are earthed with a conducting wire to let the charge flow safely away to the ground before it can build up.

Try this

Q1. State whether an object that has gained electrons is positively or negatively charged. [1 mark]

  • Cue. Negatively charged, because electrons are negative.

Q2. State the law describing the force between two charges. [1 mark]

  • Cue. Like charges repel; unlike charges attract.

Q3. A cloth is rubbed on a plastic rod and the rod becomes negative. Explain what happens to the cloth and why. [2 marks]

  • Cue. The cloth loses electrons to the rod, so it has more protons than electrons and becomes positively charged.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA plastic rod is rubbed with a dry cloth and becomes negatively charged. (a) Explain, in terms of electrons, how the rod became negatively charged. (b) State the charge on the cloth and explain why.
Show worked answer →

(a) Rubbing transfers electrons from the cloth to the rod. The rod gains extra electrons, and since electrons are negative, the rod becomes negatively charged.

(b) The cloth is left positively charged. It has lost electrons to the rod, so it now has more positive protons than negative electrons, giving it a net positive charge.

Markers reward charging by the transfer of electrons (not protons), the rod gaining electrons to become negative, and the cloth losing the same electrons to become positive.

Original4 marks(a) State the law describing the force between two electric charges. (b) Two light balls hang on threads. When given the same type of charge, they swing apart. Explain this observation.
Show worked answer →

(a) Like charges repel and unlike charges attract.

(b) Both balls carry the same type of charge, so they are like charges. Like charges repel, so each ball pushes the other away, and they swing apart on their threads until the repulsion is balanced by gravity and tension.

Markers reward the law (like repel, unlike attract), and applying it: same charge means like charges, which repel and push the balls apart.

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