Week 2: Thermodynamics Introduction

Johnny Chen
3 min readJul 23, 2021

1st Law of Thermodynamics, Conservation of Energy

  1. 1st Law of Thermodynamics in closed system

It is also called Conservation of Energy.

  • Conservation of Mass (COM): The mass will never be destroyed.
  • Conservation of Energy (COE): The energy will never be destroyed.

Formula :

From this formula, we can see that the change of energy state is equal to heat transfer (Q) minus work transfer (W).

If a system is in adiabatic process, then its Q would be 0 in this case. There were no heat transfer occurs across a boundary. But if, let’s say there are two systems and one of them the temperature change is so small that it can be negligible. It would assumes to be zero in thermodynamics!

The energy here is the combination of kinetic energy, potential energy and internal energy, which denoted as U.

U: internal energy of the system, extensive property.

u: internal energy per mass, intensive property.

2. Heat Transfer (Q)

Heat transfer is not a property and it depends on the process path. If the heat is into the system, then Q > 0. Vice versa.

Also one thing to be noted that heat transfer is not system property, it cannot process heat or work. Thus, the integral of dQ doesn’t equal to Q2 minus Q1.

There are 3 types: conduction, convection and radiation.

3. Phase Diagrams: solid, liquid and gas. Often, the object would switch between liquid and gas.

  • 3D Phase Diagram

Saturation state: State which begin or end a phase change.

It shows the conditions where different phases occur and coexist at equilibrium.

3D phase diagram (source: https://www.pinterest.com/pin/80713018294076925/)
  • 2D phase diagram
PV diagram (source: https://physics.stackexchange.com/questions/491150/shape-of-isotherms-in-p-v-phase-diagram)
PT diagram (https://www.youtube.com/watch?v=aByuyfY5xcM)

In PT diagram, T is triple point where solid, liquid and gas coexist. C is called critical point since liquid-vapor coexist, once exceed this temperature, the gas cannot be liquified by pressure alone.

Some keywords:

Isobar: Process where the system remains at a constant pressure.

Isotherms: Constant temperature, since temperature is constant the internal energy would be 0.

Quality: Mass of vapor / mass of liquid + mass of vapor

4. Reference

Introduction to Thermodynamics: Transferring Energy from Here to There (https://www.coursera.org/learn/thermodynamics-intro/home/welcome)


Shape of isotherms in P-V phase diagram (https://physics.stackexchange.com/questions/491150/shape-of-isotherms-in-p-v-phase-diagram)

Phase Changes 1: P-T Diagrams (https://www.youtube.com/watch?v=aByuyfY5xcM)