Thermodynamics is the physics of heat, temperature and energy transfer — why a metal spoon feels colder than a wooden one at the same temperature, why a bicycle pump warms up as you compress it, and why no engine ever built can be 100% efficient.
This module walks you through the whole chain: how we measure temperature, how materials expand when heated, how to track energy with calorimetry, how gases behave under the gas laws, and how the First and Second Laws tie it all together into engines, refrigerators and entropy.
Almost every thermodynamics question on the CSCA exam is really an energy-bookkeeping question: work out what energy went in, what came out, and what is left over. Get the sign conventions right and this becomes one of the most reliable scoring sections on the paper.
Thermodynamics is worth roughly 12–15% of the CSCA Physics paper — about 6 to 8 of the 48 questions. The highest-yield topics are calorimetry with phase changes, the ideal gas law, the First Law applied to the four standard processes, Carnot efficiency, and reading work off a PV diagram. Entropy and kinetic theory typically add one or two questions each, and unit conversion to Kelvin appears somewhere in almost every one of them.
Temperature measures the average kinetic energy of the particles in a body. Heat is energy in transit from a hotter body to a cooler one. A bathtub of warm water is at a lower temperature than a spark, but it carries far more thermal energy — that is the difference between the two ideas.
T(K) = T(°C) + 273.15, so absolute zero is −273.15 °C = 0 K. Converting from Fahrenheit: T(°C) = (T(°F) − 32) × 5/9. Every gas law, radiation and entropy calculation must use Kelvin — Celsius will give a wrong answer because the scale does not start at zero energy.
If A is in thermal equilibrium with C, and B is also in equilibrium with C, then A and B are in equilibrium with each other. That is what makes a thermometer meaningful. Bodies in thermal equilibrium are at the same temperature, with no net heat flow between them.
Energy passes through a material by particle collisions and (in metals) by free electrons. Rate: Q/t = kA ΔT / L, where k is thermal conductivity, A the cross-sectional area and L the thickness. Metals have high k; air, wool and foam have very low k, which is why they make good insulators. Doubling the thickness halves the rate of heat loss.
Heat carried by the bulk movement of a fluid. Warmed fluid expands, becomes less dense and rises; cooler fluid sinks to replace it, setting up a convection current. Convection needs a fluid, so it cannot happen in a solid or a vacuum.
All bodies emit electromagnetic waves, and radiation is the only mechanism that works through a vacuum. Stefan–Boltzmann: P = εσA T⁴ with σ = 5.67 × 10⁻⁸ W/(m²·K⁴). Because of the fourth power, doubling the absolute temperature multiplies the radiated power by 16. Wien's displacement law gives the peak wavelength: λ_max = b/T with b = 2.90 × 10⁻³ m·K. Matt black surfaces are the best emitters and absorbers; shiny silver surfaces are the worst — which is why a vacuum flask has silvered walls and a vacuum gap to block all three mechanisms at once.
The rate of cooling is proportional to the temperature difference between the body and its surroundings. A hot drink cools quickly at first and then more and more slowly as it approaches room temperature.
💡Temperature measures average particle KE; heat is energy in transit. T(K) = T(°C) + 273.15. Three transfer modes: conduction (Q/t = kAΔT/L), convection (fluids only) and radiation (P = εσAT⁴, works in a vacuum).
📋 Key Formulas
📝 Worked Example 1
Example 1: A window pane has k = 0.8 W/(m·K), area 2 m², thickness 5 mm, with 20 °C across it. Find the rate of heat loss.
Step 1: Convert thickness: L = 0.005 m.
Step 2: Q/t = kA ΔT/L = 0.8 × 2 × 20 / 0.005 = 6400 W. Doubling the glass thickness would halve this.
📝 Worked Example 2
Example 2: A sphere of radius 0.1 m at 500 K radiates as a black body (ε = 1). Find the power radiated.
Step 1: A = 4πr² = 4π(0.1)² = 0.1257 m².
Step 2: P = εσAT⁴ = 5.67 × 10⁻⁸ × 0.1257 × 500⁴ = 446 W.
📝 Worked Example 3
Example 3: A body glows at 600 K. Find its peak emission wavelength.
λ_max = b/T = 2.90 × 10⁻³ / 600 = 4.8 × 10⁻⁶ m (4.8 μm, in the infrared — which is why it feels hot but does not look bright).
📝 Worked Example 4
Example 4: Convert 68 °F to Celsius and Kelvin.
T(°C) = (68 − 32) × 5/9 = 20 °C, so T(K) = 20 + 273.15 = 293 K.
🧠Convert to Kelvin before any radiation, gas law or entropy calculation — Celsius silently gives a wrong answer.
🧠Radiated power goes as T⁴, so a factor-of-2 temperature rise is a factor-of-16 power rise. This ratio comes up almost every year.
🧠Thermos flask questions want all three mechanisms: vacuum stops conduction and convection, silvering stops radiation.
⚠️Treating heat and temperature as the same thing: temperature is average particle energy; heat is the energy that flows because of a temperature difference.
⚠️Leaving thickness in millimetres: L must be in metres in Q/t = kAΔT/L, or the answer is out by a factor of 1000.
⚠️Assuming convection happens in solids: convection needs a fluid that can flow, so a solid metal bar transfers heat by conduction only.
🎯 Try This Yourself
A brick wall is 0.2 m thick with k = 0.6 W/(m·K) and area 10 m². Inside is 20 °C, outside is 0 °C. Find the rate of heat loss.
You have finished Thermodynamics. You can now convert between temperature scales, predict thermal expansion, solve calorimetry and phase-change problems, apply the gas laws and the ideal gas equation, use the First Law across all four standard processes, read work off a PV diagram, calculate entropy changes, and judge whether an engine or refrigerator is physically possible.
The one habit that carries all of it: identify the process, convert to Kelvin and SI units, then check the answer against the Carnot limit and ΔS_universe ≥ 0.
Open and read all sections to complete this module