CHAPTER 1Basic concepts of

thermodynamics

Missam Raza Mechanical Dept LJ Polytechnic

ThermodynamicsIntroduction In mechanics we deal with quantities such as

mass, position, velocity, acceleration, energy, momentum, etc.

Question: What happens to the energy of a ball when we drop it on the floor?

Answer: It goes into heat energy.

Question: What is heat energy

In Thermodynamics we deal with quantities which describe our system, usually (but not always) a gas.

Volume, Temperature, Pressure, Heat Energy, Work.Missam Raza Mechanical Dept LJ

Polytechnic

Temperature and HeatPractical ExampleEveryone has a qualitative understanding of

temperature, but it is not very exact.

Question: Why can you put your hand in a 400 F oven and not get instantly burned, but if you touch the metal rack, you do?

Answer: Even though the air and the rack are at the same temperature, they have very different energy contents.

Missam Raza Mechanical Dept LJ Polytechnic

Temperature and HeatFahrenheit and CelsiusNever use Fahrenheit, except for the weather.

You can always use Kelvin and you must use Kelvin when doing absolute temperature measurements.

You can use either Kelvin or Celsius when measuring differences in temperature

Missam Raza Mechanical Dept LJ Polytechnic

Thermodynamic Systems Definitions System

Isolated System: No matter or energy cross system boundaries. No work can be done on the system

Open System: Free exchange across system boundaries.

Closed System: Energy can be exchanged but matter cannot.

Missam Raza Mechanical Dept LJ Polytechnic

Idealized Thermodynamic ProcessesProcess Irreversible: Initial system state is unstable

or metastable and spontaneous change in the system yields a system with a lower-energy final state.

Reversible: Both initial and final states are stable equilibrium states and the path between them is a continuous sequence of equilibrium states.

Missam Raza Mechanical Dept LJ Polytechnic

Definition of Enthalpy

EnthalpyWe can define a new state variable (one

where the path to its current state does not affect its value) called enthalpy:

H = Ei + PV

Upon differentiation and combing with our earlier definitionfor internal energy:

dH = dEi + PdV + VdP

dEi = dq - PdV

Missam Raza Mechanical Dept LJ Polytechnic

Heat and WorkConceptWork (W) in mechanics is displacement (d)

against a resisting force (F). W = F d

Work has units of energy (Joule, J).

Work can be expansion work (PV), electrical work, magnetic work etc. (many sets of stimuli and their responses).

Heat as used in TD is a tricky term (yes, it is a very technical term as used in TD).

The transfer of energy as a result of a temperature difference is called heat.

“In TD heat is NOT an entity or even a form of energy; heat is a mode of transfer of energy”Missam Raza Mechanical Dept LJ

Polytechnic

Third law of thermodynamicsDefinationFor substances in internal equilibrium,

undergoing an isothermal process, the entropy change goes to zero as T (in K) goes to zero.

The law is valid for pure substances and mixtures.

Close to Zero Kelvin, the molecular motions have to be treated using quantum mechanics → still it is found that quantum ideal gases obey the third law.

0lim 0T

S

Missam Raza Mechanical Dept LJ Polytechnic

THANK YOU

Missam Raza Mechanical Dept LJ Polytechnic