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Flip to reveal answersHow many kinds of electric charge are there, and how do they interact?
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All 14 Flashcards — Coulomb's law and charging
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Question
How many kinds of electric charge are there, and how do they interact?
Answer
**Two** — positive and negative. **Like charges repel** (push apart); **unlike charges attract** (pull together). Unit: the coulomb (C).
Question
State Coulomb's law.
Answer
The force between two point charges is $F = k\dfrac{q_{1}q_{2}}{r^{2}}$ — proportional to each charge and to the inverse square of the distance r between them.
Question
What is the Coulomb constant k?
Answer
**k = 8.99 × 10⁹ N m² C⁻²** — given in the data booklet. It sets the strength of the electric force.
Question
Halve one of the two charges — what happens to the Coulomb force?
Answer
It **halves** — F is proportional to each charge, so halving q_{1} (or q_{2}) halves F.
Question
Double the separation between two charges — what happens to the force?
Answer
It is divided by **2² = 4** — F is proportional to 1/r² (the inverse-square law).
Question
What moves when an object is charged?
Answer
**Electrons** (tiny negative particles). Gaining electrons makes an object negative; losing them makes it positive.
Question
Name the three ways to charge an object.
Answer
**Friction** (rubbing), **contact** (touching a charged object), and **induction** (bringing a charge near and grounding — no contact).
Question
What sign of charge does induction leave?
Answer
The **opposite** sign to the charge brought near — and it never needs contact.
Question
State the law of conservation of charge.
Answer
Charge is never created or destroyed, only **moved**. If one object gains −q, another is left with +q, so the total is unchanged.
Question
Two charges of +3 μC and −5 μC sit close together. Attractive or repulsive?
Answer
**Attractive** — they have opposite signs, so they pull together.
Question
How does the Coulomb force depend on the distance r?
Answer
It is **inversely proportional to r²** (inverse-square): double r → quarter F; triple r → one-ninth F.
Question
Two identical conducting spheres with +Q and −2Q touch. What charge does each end with?
Answer
**−Q/2** each: the total (−Q) is shared equally.
Question
How is k related to the permittivity of free space?
Answer
**k = 1/(4πε₀)**; in a medium use its permittivity ε, which gives a smaller force.
Question
In charging by induction, why is the earth removed before the rod?
Answer
So the electrons that flowed away cannot flow back; the sphere keeps the opposite charge to the rod.
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Electric and magnetic fields
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