Thermodynamics is the study of heat, work, temperature, and energy transfer. Its laws explain why engines run, why hot objects cool down, why refrigerators need electricity, and why no machine can be perfectly efficient. These ideas connect everyday experiences, such as melting ice and warming food, to deep rules about energy and matter.
The four laws give a compact framework for predicting what is possible in physical systems.
Key Facts
- Zeroth Law: If A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then A is in thermal equilibrium with C.
- First Law: ΔU = Q - W, where ΔU is change in internal energy, Q is heat added to the system, and W is work done by the system.
- Second Law: In an isolated system, entropy never decreases, so ΔS ≥ 0.
- Heat engines convert some input heat into work: efficiency e = Wout / Qin.
- No heat engine can be 100 percent efficient because some energy must be rejected as waste heat.
- Third Law: As temperature approaches absolute zero, the entropy of a perfect crystal approaches zero, so S → 0 as T → 0 K.
Vocabulary
- Thermal equilibrium
- A condition in which objects in contact have the same temperature and no net heat flows between them.
- Internal energy
- The total microscopic kinetic and potential energy of the particles inside a system.
- Heat
- Energy transferred from one object or system to another because of a temperature difference.
- Work
- Energy transferred when a force moves something, such as a gas pushing a piston.
- Entropy
- A measure of energy spreading or the number of microscopic arrangements possible in a system.
Common Mistakes to Avoid
- Confusing heat with temperature is wrong because heat is energy transferred, while temperature measures average particle motion.
- Writing the First Law with the wrong sign is wrong because ΔU = Q - W assumes W is work done by the system, not work done on the system.
- Thinking entropy means only disorder is incomplete because entropy is more precisely about energy spreading and possible microscopic arrangements.
- Assuming absolute zero can be reached in a real experiment is wrong because the Third Law says it can be approached but not reached by a finite process.
Practice Questions
- 1 A gas absorbs 500 J of heat and does 180 J of work on a piston. What is the change in the gas's internal energy?
- 2 A heat engine takes in 1200 J of heat from a hot reservoir and rejects 750 J to a cold reservoir. How much work does it do, and what is its efficiency?
- 3 Explain why a refrigerator does not violate the Second Law of Thermodynamics even though it moves heat from a cold interior to a warmer room.