Electric Charges & Charging By Rubbing
Key idea: O Level static electricity: positive/negative charge, coulombs, and charging by rubbing (electron transfer).
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The core idea
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Learning objectives
- state that there are positive and negative charges and that charge is measured in coulombs
- state that unlike charges attract and like charges repel
- Describe an electric field as a region where charge experiences force
- Recall that the field lines of an isolated point charge give the direction of the force on a positive test charge
- Draw the directed electric field of an isolated point charge
- Interpret the electric-field pattern between two isolated point charges
- Draw the electric-field pattern between two isolated point charges
- Explain charging by rubbing as electron transfer
- Describe experiments that charge a conductor by induction
- Describe potential electrostatic-charging hazards
- Describe an electrostatic precipitator and transfer the principle
1. Definition
A. Electric charge
Electric charge can be positive or negative and is measured in coulombs (C).
B. Charging by rubbing
Charging by rubbing is the process where electrons are transferred from one object to another, leaving the objects with equal and opposite charges.
2. Key Ideas
- Like charges repel; unlike charges attract.
- In solids, only electrons move during charging. Protons are fixed in the nucleus.
- An object becomes:
- negative if it gains electrons
- positive if it loses electrons
- A neutral object has equal amounts of positive and negative charge (not “no charge”).
- Unit: charge q (C).
You should know that charge can be positive or negative, is measured in coulombs, and that charging by rubbing transfers electrons.
3. Detailed Explanations
A. What happens when you rub two insulators?
When two insulating materials are rubbed together, electrons can be pulled from one surface to the other:
- the material that loses electrons becomes positively charged
- the material that gains electrons becomes negatively charged
The two objects end up with equal and opposite charges (charge is conserved overall).
B. Everyday examples
- plastic ruler rubbed with a dry cloth can attract small pieces of paper
- comb rubbed through dry hair can attract hair strands
- balloon rubbed on a shirt can stick to a wall
C. Why does a charged object attract small neutral objects?
A charged object can cause charges in a nearby neutral object to shift slightly (polarisation). The nearer side becomes oppositely charged, so the attraction can be stronger than repulsion.
Charging conductors without contact is covered here: Ways to Charge Conductors.
4. Common Mistakes
- Saying protons move from one object to another (in solids, protons do not move).
- Saying “neutral means no charge” (neutral means equal positive and negative charge).
- Mixing up which sign corresponds to gaining/losing electrons.
5. Exam Tips
- In explanations, explicitly use the word electrons:
- “electrons are transferred from A to B”
- “A loses electrons → positive”
- “B gains electrons → negative”
- When asked about attraction/repulsion, state: “like repels, unlike attracts”.
6. Worked Examples
Modelled example 1
Predicting the charges
Problem
Study the worked solution
Use the rod's sign to infer electron motion
Method
State that the rod gained electrons.Reason
A negative net charge means an excess of electrons relative to protons.Working
Electrons: cloth → plastic rod.Track the other object and conserve charge
Method
State that the cloth lost electrons and became positive.Reason
The electrons transferred between the two objects rather than being created.Working
Rod: negative; cloth: equal and opposite positive charge.
Guided practice 2
Like vs unlike
Problem
Use the force direction
Hints
Hint 1: recall the force rule
Hint 2: keep both possibilities
View solution step by step
Infer what is certain
Method
State that both objects have the same type of charge.Reason
Only like charges repel in this comparison.Working
Both positive or both negative.State what remains unknown
Method
Do not assign a particular sign without another reference charge.Reason
Repulsion distinguishes like from unlike, but not positive from negative.Working
Same sign is determined; the sign itself is not.
Common misconception 3
“Positive charge” misconception
Learner response
Identify the mobile charge carrier
View solution step by step
Reject proton motion
Method
State that protons remain bound within atomic nuclei in the solid.Reason
Charging by rubbing transfers surface electrons between materials.Working
No proton transfer.Explain positive net charge
Method
State that the object lost electrons and has an electron deficit.Reason
Its unchanged positive charge then exceeds its remaining negative charge.Working
Electron loss → positive net charge.
Examiner practice 4
Neutral object attraction
Examination question
Describe redistribution, near-side sign and net force
View solution step by step
Polarise the paper
2 marksMethod
Describe a slight redistribution of charge so the nearer side becomes oppositely charged.Reason
The comb’s electric field shifts charges within the neutral paper.Working
Neutral overall, but separated charge distribution.Explain the net attraction
1 markMethod
State that the nearer opposite-charge attraction exceeds the farther like-charge repulsion.Reason
Electric force is stronger at the shorter separation.Working
Near attraction > far repulsion → net attraction.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark redistribution, near-side sign and net attraction.
Challenge 5
Electron transfer direction
Changed-material transfer
Work backwards from the observed rod sign
Hints
Hint 1: interpret positive
Hint 2: follow the transferred electrons
View solution step by step
Infer electron direction
Method
Move electrons from the glass rod to the silk.Reason
The glass’s positive charge shows it has an electron deficit.Working
Electrons: glass → silk.Assign the silk charge
Method
State that the silk becomes negatively charged.Reason
It gains the electrons lost by the glass, conserving total charge.Working
Glass: positive; silk: negative.
7. Mind Stretchers
Mind stretcher 1: Conservation of chargeExtension
When you rub two objects together, do you “create” charge? Explain.
Show Answer
No. Charge is not created; electrons are transferred from one object to the other. One becomes negative and the other becomes positive with equal and opposite charges overall.
Mind stretcher 2: Dry vs humid airExtension
Static effects are stronger on a dry day than a humid day. Why?
Show Answer
Moist air allows charge to leak away more easily because water molecules help charges move. In dry air, charge stays on insulating surfaces longer, so effects are stronger.
8. Practice and next step
- Complete the Static Electricity quiz and check that every charge explanation names electron gain or loss.
- Continue to Insulators and Conductors, then Charging by Induction.
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027