Third law of thermodynamics (introductory) is a core Thermodynamics concept in Physics. This guide explains what it is, walks through a fully worked example, and lists the key equations you need — with a short quiz to test yourself.

Key equations and worked example

1 mol of gas at 300 K in a 24.6 L cylinder: P = nRT/V = (1×8.314×300)/0.0246 ≈ 101 kPa. Heat it to 600 K at fixed volume and the pressure doubles to ≈ 202 kPa — the molecules hit the walls twice as hard. Push the temperature slider and watch the particles speed up and redden.

• <code>Ideal gas law: P·V = n·R·T</code>
• <code>First law: ΔU = Q − W</code>
• <code>Carnot efficiency: η = 1 − Tc/Th</code>
• <code>Mean molecular KE: ⟨KE⟩ = 3kT/2</code>

Third law of thermodynamics (introductory) in detail

Third law of thermodynamics (introductory) is one of the central ideas in Thermodynamics, and it appears in Physics curricula under Laws of thermodynamics. It is worth learning deeply because it connects to so many other topics in this section.

Temperature measures the average kinetic energy of molecules (½m⟨v²⟩ = 3kT/2). Heating a gas makes molecules move faster, raising pressure at fixed volume. The first law (ΔU = Q − W) tracks energy: heat in minus work done by the gas. No heat engine beats the Carnot limit.

For exams, the pattern is predictable: first a definition or statement of the result, then a direct numerical application of one of the equations above, then a "why" question — why the formula takes that form, or what changes when a variable is doubled or halved. The worked example and quiz below cover exactly that progression.

Quick self-check:

• Q: A gas is heated from 300 K to 600 K at constant volume. What happens to its pressure?<br />A: It doubles — P ∝ T at fixed V (Gay-Lussac&#39;s law).
• Q: State the first law of thermodynamics.<br />A: ΔU = Q − W: the change in internal energy equals heat added minus work done by the system.
• Q: Why can no engine be 100% efficient?<br />A: The Carnot limit η = 1 − Tc/Th is always below 1 since Tc &gt; 0 — some heat must be rejected.