Thermodynamics For Dummies Cheat Sheet

Astrophysics For Dummies

Thermodynamics sounds intimidating, and it can be. However, if you focus on the most important thermodynamic formulas and equations, get comfortable converting from one unit of physical measurement to another, and become familiar with the physical constants related to thermodynamics, you’ll be at the head of the class.

Important thermodynamic equations and formulas

Thermodynamics is filled with equations and formulas. Here’s a list of the most important ones you need to do the calculations necessary for solving thermodynamics problems.

Combustion equations:
- Air-fuel ratio:
- Hydrocarbon fuel combustion reaction:
Compressibility calculations:
- Compressibility factor Z: Pv = ZRT
- Reduced temperature:
- Reduced pressure:
- Pseudo-reduced specific volume:
Efficiency equations:
- Thermal efficiency:
- Coefficient of performance (refrigerator):
- Coefficient of performance (heat pump):
Energy equations:
Entropy equations:
- Entropy change for ideal gas, constant specific heat:
- Entropy change for ideal gas, variable specific heat:
- Irreversibility for a process:
Ideal-gas formulas:
- Ideal-gas law: Pv = RT
- Gas constant:
- Ratio of specific heats:
- Isentropic process for ideal gas:
Moist air properties:
- Relative humidity:
- Specific humidity:
Properties of mixtures:
- Quality liquid-vapor mixture:
- Saturated mixture property, y: y = y_f + x · y_fg
Work calculations:
- Isobaric process: W_b = P₀(V₂ – V₁)
- Polytropic process:
- Isothermal process of an ideal gas:
- Shaft power:
- Spring work:

Conversion factors for physical measurements

Sooner or later, you’re going to have to convert from one unit of physical measurement to another. This list gives you some of the most common conversion factors you need in thermodynamics.

Acceleration: 1 m/s² = 100 cm/s²
Area: 1 m² = 10⁴ cm² = 10⁶ mm²
Density: 1 g/cm³ = 1 kg/L = 1,000 kg/m³
Energy, heat, work, internal energy, enthalpy: 1 kJ = 1,000 J = 1,000 N·m = 1 kPa·m³

1 kJ/kg = 1,000 m²/s²
Force: 1 N = 1 kg·m/s²
Length: 1 m = 100 cm = 1,000 mm
Mass: 1 kg = 1,000 g
Power, heat transfer rate: 1 W = 1 J/s, 1 kW = 1,000 W
Pressure: 1 Pa = 1 N/m², 1 kPa = 1,000 kPa, 1 MPa = 1,000 kPa

1 atmosphere (atm) = 101.325 kPa
Specific heat, entropy: 1 kJ/kg · °C = 1 kJ/kg · K = 1 J/g·°C
Specific volume: 1 m³/kg = 1,000 L/kg = 1,000 cm³/kg
Temperature: T(K) = T(°C) + 273.15, T(°C) = (5/9) T(°F) – 32, T(°F) = (9/5) T(°C) + 32
Velocity: 1m/s = 3.6 km/hr
Volume: 1 m³ = 1,000 L = 10⁶ cm³

Physical constants in thermodynamics

You run across a few physical constants when working out thermodynamics problems. Following is a list of the constants you need when you’re working with potential energy, pressure, or the ideal gas law.

Acceleration of gravity: g = 9.81 m/s²
Standard atmospheric pressure: 1 atm = 101.3 kPa
Universal gas constant:

About This Article

About the book author:

Michael Pauken, PhD, is a senior mechanical engineer at NASA's Jet Propulsion Laboratory, an operating division of the California Institute of Technology, where he also teaches courses on thermodynamics and heat transfer.