Physics / Unit 9: Electricity and Circuits
9.1 Electric Charge
Notes
Matter contains protons (positive) and electrons (negative). An object is charged when it has extra or missing electrons.
- Like charges repel; opposite charges attract.
- Charge is measured in coulombs (C). One electron has charge $-e = -1.6 \times 10^{-19} \text{ C}$.
- Conservation of charge: charge moves from one object to another but is never created or destroyed.
- Conductors (metals) let electrons move easily; insulators (rubber, plastic, glass) do not.
- Charging by friction (rubbing), conduction (touching) or induction (bringing a charge near without touching).
- Example: rubbing a balloon on hair moves electrons to the balloon, so the balloon becomes negative and the hair positive.
Practice
A rubber rod becomes negatively charged after being rubbed with wool. What happened?
Only electrons move in ordinary charging; the rod gained them.
An object has a charge of $-4.8 \times 10^{-19} \text{ C}$. How many extra electrons does it have?
$\displaystyle \frac{4.8 \times 10^{-19}}{1.6 \times 10^{-19}} = 3$.
Which material is the best conductor?
Metals like copper have electrons that move freely.
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