What are the Laws of Thermodynamics?
The laws of thermodynamics govern how energy moves and transforms in every physical system, from engines to living cells. They explain why heat flows one way, why perpetual motion machines are impossible, and how energy is always conserved.
There are four laws: the zeroth law defines thermal equilibrium, the first law states energy is conserved (ΔU = Q − W), the second law says entropy in an isolated system never decreases, and the third law states entropy approaches zero as temperature approaches absolute zero.
- 1↓Zeroth LawIf two systems are each in thermal equilibrium with a third, they are in equilibrium with each other — this defines temperature.
- 2↓First LawEnergy cannot be created or destroyed: ΔU = Q − W.
- 3↓Second LawThe entropy of an isolated system never decreases; heat flows spontaneously from hot to cold.
- 4Third LawAs temperature approaches absolute zero, the entropy of a perfect crystal approaches zero.
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Step-by-step worked examples
A gas absorbs 500 J of heat and does 200 J of work on its surroundings. Find ΔU.
ΔU = Q − W = 500 − 200 = 300 J
An engine releases 150 J of heat while 400 J of work is done on it. Find ΔU.
Q = −150 J (heat leaves the system) W = −400 J (work is done on the system, so work done BY the system is negative) ΔU = Q − W = −150 − (−400) = 250 J
In a fully isolated system undergoing a spontaneous process, what happens to the total entropy?
By the second law, ΔS ≥ 0 for an isolated system Entropy increases or stays the same — it never decreases
Flashcards
Quick quiz
Q1.Which law states that energy cannot be created or destroyed?
Q2.A system absorbs 400 J of heat and does 150 J of work. What is ΔU?
Q3.According to the second law, the entropy of an isolated system...
Q4.What does the zeroth law establish?
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Common mistakes
Thinking energy is destroyed in friction or heat loss. — Correct: Energy is conserved overall — it converts to heat rather than disappearing (first law).
Believing entropy can decrease anywhere. — Correct: The entropy of an isolated system as a whole never decreases; a subsystem's entropy can decrease if compensated elsewhere.
Confusing heat and temperature. — Correct: Heat (Q) is energy transferred; temperature measures average kinetic energy.
Assuming perpetual motion machines are just hard to build. — Correct: They violate the first or second law and are physically impossible.
FAQ
What are the laws of thermodynamics?
Four laws governing energy, heat, and entropy: zeroth (temperature), first (energy conservation), second (entropy), and third (absolute zero).
What is the formula for the first law of thermodynamics?
ΔU = Q − W, where ΔU is the change in internal energy, Q is heat added, and W is work done by the system.
What are examples of the laws of thermodynamics?
A car engine converting fuel to work and waste heat, a refrigerator moving heat against its natural flow using work, and ice melting toward equilibrium.
How do you calculate change in internal energy using the first law?
Subtract the work done by the system from the heat added: ΔU = Q − W.