Thermodynamics and Energy
Thermodynamics and Energy tests heat, temperature, thermal energy transfer, gas behavior, the laws of thermodynamics, and efficiency. Expect calorimetry, PV reasoning, and energy-conservation calculations.
Core rules
Heat is energy transferred because of temperature difference; temperature measures average molecular kinetic energy. For a substance without phase change, Q = mcΔT. During a phase change, Q = mL and temperature stays constant. The ideal gas law is PV = nRT, with T in kelvin. The first law is ΔU = Q − W for work done by the gas; sign conventions must be stated. The second law limits natural processes and gives maximum efficiency for a heat engine: e_Carnot = 1 − Tc/Th, with absolute temperatures.
Worked example
A Carnot engine operates between 500 K and 300 K. Its maximum efficiency is 1 − 300/500 = 0.40, or 40%. If it absorbs 1000 J of heat from the hot reservoir, the maximum work output is 0.40 × 1000 = 400 J, and at least 600 J is rejected to the cold reservoir. Using Celsius values would give a meaningless efficiency, so the absolute-temperature requirement is central.
Traps
Do not use Celsius in gas-law or Carnot calculations. Heat and temperature are not interchangeable. During melting or boiling, added heat changes phase, not temperature. Efficiency cannot exceed the Carnot limit for the same reservoirs. Watch the sign of work and heat in the first law; define whether W is work done by or on the system.