Units and dimensions in thermodynamics describe various physical quantities involved in the study of energy, heat, work, and their transformations. Understanding these helps in deriving equations, performing calculations, and ensuring consistency across systems.
Contents:
- Types of units.
- Fundamental units.
- Derived units.
- SI, cgs and other system of measurement.
- Common Thermodynamic units.
Why units are important in thermodynamics?
Understanding the unit system is more important in all forms of engineering.Units are not just a formality but a critical component of engineering practice. Their proper use ensures accuracy, efficiency, and effective communication in all engineering fields around the world.
Types of units:
Based on the quantity measured units are classified into two types:
- fundamental units and
- derived units .
Fundamental units :
Fundamental units are the basic units of measurement in a physical system. They are independent and cannot be derived from other units.
The Seven Fundamental Units in the SI System are:
- Length (meter, m).
- Mass (kilogram, kg).
- Time (second, s)
- Electric Current (ampere, A)
- Temperature (kelvin, K)
- Amount of Substance (mole, mol)
- Luminous Intensity (candela, cd).
Derived units:
Derived quantities are physical quantities that are obtained by combining the fundamental quantities through multiplication, division, or other mathematical operations. They are expressed in terms of the derived units, which are combinations of the fundamental units.
The following are the examples of derived units.
Area = length (metre) * length (metre) = m*m = m^2.
Velocity= distance (metre)/time(seconds) = m/s .
Based on system of measurement units are classified as follows:
SI Units (International System of Units):
The most widely used and standardized system globally.
Examples: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current.
CGS Units (Centimeter-Gram-Second):
Used in scientific work, particularly in physics.
Examples: centimeter (cm), gram (g), second (s).
Imperial/US Customary Units:
Used primarily in the United States and some other countries.
Examples: inch, foot, pound, gallon.
Units of Common Thermodynamic Quantities
| Physical Quantity | SI Unit | Symbol |
|---|---|---|
| Energy (Heat, Work) | Joule (J) | E,Q, W |
| Power | Watt (W) | P |
| Temperature | Kelvin (K) | T |
| Pressure | Pascal (Pa) | P |
| Volume | Cubic meter (m3) | V |
| Density | kg/m3 | ρ(rho) |
| Specific Volume | m3/kg | v |
| Entropy | Joule per Kelvin | S |
| Specific Heat Capacity | joules per kilogram per kelvin (J/kg.K) | c |
| Thermal Conductivity | Watts per metre -Kelvin(W/cm.K) | k |
| Internal Energy | Joule (J) | U |
| Enthalpy | Joule (J) | H |
| Gibbs Free Energy | Joule (J) | G |
| Universal Gas Constant (R) | joules per kelvin per mole(J/mol.K) | R |
| Specific Energy | joule per kilogram(J/kg) | e |
| Heat Flux | watts per square meter (W/m2) | q |
| Molar Mass | kg/mol | Mm |
| Diffusivity | m2/s | D |
| Dynamic Viscosity | pascal seconds{Pa·s} | μ(mu) |
Conclusion
These units form the basis for calculations in thermodynamics, allowing scientists and engineers to analyze energy systems and predict behavior.