Thermodynamics
Easy Overview
Think of a hot cup of chai cooling down on a winter morning. Why does it cool? Where does the heat go? Thermodynamics is the study of energy — how it flows, how it transforms, and why some things are just impossible (like a cold cup of chai spontaneously heating up). It's the physics of 'what goes where'.
Zeroth and First Laws
The zeroth law just says if A is at the same temperature as B, and B is at the same temperature as C, then A and C are at the same temperature. Obvious? Yeah, but you need to say it for everything else to make sense. The first law is more interesting: energy can't be created or destroyed, only converted. Mathematically, ΔU = Q - W. The change in internal energy equals heat added minus work done. If you add heat to a gas in a piston, the gas either gets hotter (ΔU increases) or expands (does work), or both.
Second Law and Entropy
The second law is the one that says some things just won't happen. Heat flows from hot to cold, never the other way around. You can't unscramble an egg. Entropy is a measure of disorder, and the second law says total entropy always increases. Think of your room — it naturally gets messier over time. You have to put in effort to clean it. The universe is the same way. Energy spreads out and things become more disordered.
Heat Engines and Refrigerators
A heat engine takes heat from a hot place, does some work (like turning a wheel), and dumps the leftover heat somewhere cooler. A car engine is a classic example — hot burning gases push pistons, and exhaust carries away waste heat. No engine can be 100% efficient because some heat always has to be dumped. A refrigerator is a heat engine running backwards — it uses work to pump heat from a cold place to a hot place. That's why the back of your fridge feels warm.
Carnot Cycle
The Carnot cycle is the theoretical best any heat engine can do. It's an ideal cycle with two isothermal (constant temperature) and two adiabatic (no heat exchange) processes. Think of it as the gold standard — no real engine can beat a Carnot engine. Its efficiency depends only on the temperatures of the hot and cold reservoirs. Hotter source and colder sink = better efficiency. Real engines try to get as close to Carnot efficiency as possible.
Key Points
- •First law: ΔU = Q - W. Energy is conserved.
- •Second law: Heat flows spontaneously from hot to cold. Entropy of the universe always increases.
- •Heat engine efficiency η = 1 - Q₂/Q₁. Can never be 100%.
- •Carnot efficiency η = 1 - T₂/T₁ (temperatures in Kelvin).
- •Refrigerators and heat pumps are heat engines in reverse.
- •Entropy is a measure of randomness/disorder. ΔS = Q/T for reversible processes.
Practice Questions
- State and explain the first law of thermodynamics. Apply it to an isothermal and adiabatic process.
- What is a Carnot engine? Derive the expression for its efficiency.
- Explain why 100% efficient heat engine is impossible using the second law.
- Calculate the change in entropy when 1 kg of ice at 0°C melts into water.