Zeroth Law of Thermodynamics
High-Yield Summary
- Temperature measures the average kinetic energy of particles in a substance.
- Heat flows from higher to lower temperature until thermal equilibrium (no net heat flow) is reached.
- The Zeroth Law: two systems each in equilibrium with a third system are in equilibrium with each other — this is what makes thermometers work.
- Three temperature scales: Celsius (0–100°C for water), Fahrenheit (32–212°F for water), Kelvin (starts at absolute zero, SI standard).
- Thermal expansion: solids primarily undergo linear expansion (ΔL = αLᵢΔT); liquids primarily undergo volumetric expansion (ΔV = βVᵢΔT).
Temperature Scales
| Scale | Freezing / Boiling Point of Water |
|---|---|
| Celsius | 0°C / 100°C |
| Fahrenheit | 32°F / 212°F |
| Kelvin (starts at absolute zero, −273.15°C) | 273 K / 373 K |
Temperature Conversions and Thermal Expansion
C = 5/9(F − 32) | F = 9/5C + 32 | K = C + 273
- ΔL = αLᵢΔT = Linear expansion (solids): α = coefficient of linear expansion, Lᵢ = initial length
- ΔV = βVᵢΔT = Volumetric expansion (liquids): β = coefficient of volumetric expansion, Vᵢ = initial volume
Common MCAT Trap
- The Zeroth Law is the reason thermometers work at all — a thermometer is the "third system" that reaches equilibrium with the object being measured. Don't overlook it as trivial; it's foundational to the concept of measurable temperature.
- Solids expand linearly (length), liquids expand volumetrically (volume) — matching the right expansion formula to the right phase of matter is a common point of confusion.
Quick Recall
What does the Zeroth Law of Thermodynamics state?
What does temperature actually measure at the particle level?