State that there are positive and negative charges

4.2.1 Electric charge

⚡️ Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric field. There are only two kinds of charge: positive (+) and negative (−).

What is electric charge?

Imagine a crowd at a concert. Some people (positive charges) are standing on the left side, and others (negative charges) are on the right. When they move closer, the crowd feels a pull or push. This “pull” or “push” is the force between charges. The amount of people in each group represents the magnitude of the charge, written as $q$.

Positive and negative charges

• Positive charge (+) is carried by protons in the nucleus of an atom. • Negative charge (−) is carried by electrons that orbit the nucleus. • Like charges repel: $+ +$ or $- -$ push away. • Opposite charges attract: $+ -$ pull together.
🔋 Think of a magnet: north and south poles attract, but two north poles repel. Charges work the same way but with a different symbol.

  • Proton: $+e$ (positive elementary charge)
  • Electron: $-e$ (negative elementary charge)
  • Neutral atom: equal numbers of $+e$ and $-e$ → net charge $q = 0$

How charges are measured

The SI unit of charge is the coulomb (C). One coulomb equals the charge of about $6.24 \times 10^{18}$ elementary charges: $$1\,\text{C} = 6.24 \times 10^{18}\,e$$ In practice, we often use the symbol $q$ to represent any charge, positive or negative.

Exam tips

✔️ Remember:

  • Electrons carry a negative charge ($-e$).
  • Protons carry a positive charge ($+e$).
  • Use $q$ when the sign is not specified; use $+q$ or $-q$ when the sign matters.
  • When asked to identify the type of charge, look for the symbol: $+$ or $-$.
  • In diagrams, arrows pointing away from a charge indicate a positive charge; arrows pointing toward a charge indicate a negative charge.

Quick quiz

  1. Which particle carries a positive charge? (Answer: Proton)
  2. What is the symbol for a negative elementary charge? (Answer: $-e$)
  3. True or False: Like charges attract. (Answer: False)
  4. How many elementary charges are in 1 coulomb? (Answer: $6.24 \times 10^{18}$)

Summary table

Charge type Symbol Example particle Force with like charge Force with opposite charge
Positive $+q$ Proton Repel Attract
Negative $-q$ Electron Repel Attract

Revision

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