Chapter 1 · Physics
Electric Charges and Fields: Chapter Introduction
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A rubbed comb lifts paper bits without touching them. Rubbing moves electrons, a charged body attracts even neutral paper by induction, and every charge reaches out through its electric field.
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Electric Charge: Like Charges Repel, Unlike AttractThose sparks when you pull off a sweater come from electric charge. How rubbing moves electrons, what positive and negative mean, and why like charges repel.Physics · simulation
Key points
- Rubbing transfers electrons: a plastic comb run through dry hair gains electrons and becomes negative.
- Two kinds of charge: like charges repel, unlike charges attract.
- A charged body attracts a neutral one by induction: the nearer opposite charge wins.
- Coulomb's law F = kq₁q₂/r²: double the distance and the force becomes one-fourth. Each charge sets up an electric field; field lines start on + and end on −.
Why the paper jumps
- Rubbing moves electrons from the hair to the comb: the comb becomes negatively charged, the hair positively.
- The paper is neutral, but near the comb its charges shift slightly: positive charge nearer the comb, negative charge farther away.
- The nearer (attractive) charge feels a stronger force than the farther (repulsive) one, so the net force pulls the paper to the comb. This is induction.
Coulomb's law
F = k q₁q₂ / r², k = 1/(4πε₀) ≈ 9 × 10⁹ N m² C⁻²
| Change | Effect on F |
|---|---|
| Double one charge | F doubles |
| Double the distance r | F becomes F/4 |
| Halve the distance r | F becomes 4F |
Electric field and field lines
A charge q sets up an electric field E around it; another charge q₀ placed there feels F = q₀E. The field is how a force acts without contact.
- Field lines start on positive charges and end on negative charges.
- The tangent to a field line gives the direction of E; closer lines mean a stronger field.
- Two field lines never cross.
What the chapter covers
- Electric charge, conductors and insulators, charging by induction, basic properties of charge.
- Coulomb's law and forces between multiple charges.
- Electric field, field lines, electric dipole.
- Electric flux and Gauss's law, with its applications.
Quick quiz
4 questions. Choose an answer for each, then submit to see your score and the explanations.