Laws of Thermodynamics and Cyclic Processes
Zeroth Law, First Law of Thermodynamics ΔQ = ΔU + W, work done in gas expansion W = ∫p dV, isothermal, isobaric, isochoric, and adiabatic processes, and heat engine efficiency.
Marking a lesson complete represents finishing your study of the topic; it does not denote tested mastery.
Learning Objectives
- Apply the First Law of Thermodynamics ΔQ = ΔU + W with proper sign conventions across diverse thermodynamic processes.
- Calculate work done from p-V indicator diagrams and determine net work and thermal efficiency for closed cycles.
Core Theory & Concepts
Thermodynamics governs energy transformations involving thermal energy and mechanical work. The First Law expresses universal conservation of energy applied to thermodynamic systems.
First Law of Thermodynamics
When heat ΔQ is supplied to a system, it increases internal energy by ΔU and performs external work W: ΔQ = ΔU + W. For an ideal gas, U depends solely on absolute temperature T.
Thermal Efficiency of a Cycle
In a cyclic engine, the system returns to its initial state (ΔU_cycle = 0). Net work done equals net heat absorbed: W_net = Q_in - Q_out. Thermal efficiency is η = W_net / Q_in = 1 - (Q_out / Q_in).
Common Misconceptions
In an isothermal expansion, work done is zero because temperature is constant.
Syllabus Topic Outline
- 1Zeroth Law of Thermodynamics and concept of temperature
- 2First Law of Thermodynamics: ΔQ = ΔU + W sign convention
- 3Internal energy U = (3/2)nRT of an ideal gas
- 4Work done in isobaric (W = pΔV) and isothermal (W = nRT ln(V₂/V₁)) expansions
- 5Adiabatic processes: ΔQ = 0, pV^γ = constant, work done at expense of internal energy
- 6Cyclic processes: net work from enclosed area in p-V loop, heat engine efficiency η = W_net / Q_in