Charges
High-Yield Summary
- Protons carry positive charge, electrons negative. Unlike charges attract, like charges repel.
- A neutral atom has equal protons and electrons; gaining/losing electrons makes it an ion.
- Law of Conservation of Electric Charge: total charge in an isolated system is constant — charge transfers, never created/destroyed.
- SI unit of charge: coulomb (C). Elementary charge (magnitude on one proton or electron): 1.6×10⁻¹⁹ C.
- Insulators (e.g., rubber) keep charge localized; conductors (e.g., copper) let charge move freely across the material.
Key Terms
- Ion
- An atom with a net charge, from gaining or losing electrons.
- Elementary charge
- 1.6×10⁻¹⁹ C — the magnitude of charge on a single proton or electron.
- Insulator
- A material that resists charge movement; charge stays localized (e.g., rubber).
- Conductor
- A material that lets charge move freely throughout it and across its surface (e.g., copper, aluminum).
Insulators vs. Conductors
| Insulator | Conductor |
|---|---|
| Charge stays put where applied | Charge spreads freely across the material |
| Example: rubber | Example: copper, aluminum |
| Useful to prevent charge flow (wire coating) | Useful to enable charge flow (wires, circuits) |
Common MCAT Trap
- 'Positive' and 'negative' are arbitrary labels (Benjamin Franklin's choice) — don't read physical meaning into the names themselves.
- Conservation of charge applies to the total across an isolated system, not to each object individually — charge redistributes between touching objects, it doesn't vanish from one.
Quick Recall
What turns a neutral atom into an ion?
What is the elementary charge, and what carries it?