Thermodynamics - important points to be noted for competitive exams

➔ Tephigram is the name of temperature entropy diagram

➔ PV graph in a adiabatic change is called Isentropic.

➔ Entropy of a system is an index of “unavailable energy”.

➔ When gas is expanded, work is done by the gas on surroundings.

➔ The size of “Kelvin degree” is equal to “Centigrade”.

➔ The efficiency of a carnot engine increases by raising the temperature of the source.

➔ Work done per cycle is given by the area enclosed in the indicator diagram.

➔ Conversion of heat energy into electrical energy can be made by “Thermocouple”.

➔ f(P,V,T) =0 exists for an equilibrium state and is called equation of state.

➔ The area of cycle of T-S diagram gives the “available thermal energy for useful work” in a reversible process.

➔ Uses of TS diagram: 

a) used in meteorology b) check efficiency of heat engine c) useful in predicting defects of performance of engine d) to obtain work value of fuel used.

➔ Change in entropy of universe due to free expansion is 
∆S = nR log e(Vf/Vi)

➔ Loss of available of energy = To.dS where ‘To’ is lowest available temperature in system.

➔ In order to maintain a body in an isothermal condition, heat has to be either supplied or withdrawn.

➔ When a gas expands adiabatically, the temperature decreases.

➔ When a gas is compressed, the temperature increases because work is done on the gas.

➔ The work done in an adiabatic change in a particular gas depends upon only change in temperature.

➔ In an adiabatic compression, the decrease in volume is associated with increase in temperature & increase in pressure.

➔ For an isothermal expansion of a perfect gas, the value of dP/P is equal to -dV/V. 

 

➔For an adiabatic expansion of a perfect gas, the value dP/P is equal to -𝛾dV/V.  

 

➔ A reversible process is always “quasi-static”, but every quasi-static process need not be a reversible process.

➔ For reversible cycle: ∆P = ∆V = ∆T = ∆U = ∆H

➔ dW = PdV is only applicable to reversible process. 

➔ In case of “irreversible processes”, dW is not equal to PdV; 

 

➔For free expansion, dW=0

➔For free expansion, dV=0, the work may be zero (in case of PV work)

 

➔ Work and heat are path functions.

➔ Work is not a thermodynamic property as it is not a state function and it is not a exact differential.

➔ Both thermodynamic and temperature scales use a single reference temperature i.e triple point of water.

➔ dW = PdV is only applicable to reversible process. 

➔ In case of “irreversible processes”, dW is not equal to PdV;

WHAT IS ABSOLUTE ZERO?

Absolute Zero could be defined in different ways as follows:

Absolute Zero is a state of minimum molecular movement

Absolute zero is that temperature at which disorder of a system reaches its minimum value.

Absolute Zero is a state of zero Entropy.

Absolute Zero is the temperature at which a “Heat Engine” can operate at 100 percent efficiency.

Absolute zero is the point where no more heat can be removed from a system, according to the absolute or thermodynamic temperature scale. This corresponds to 0 K or -273.15°C. In classical kinetic theory, there should be no movement of individual molecules at absolute zero.

A system at absolute zero will not have enough energy for transfer to other systems. It is therefore correct to say that molecular kinetic energy is minimal at absolute zero i.e; the motion of molecules in a system is least at absolute zero

Absolute zero is defined as 0 (zero) K on the Kelvin scale and as −273.15°C on the Celsius scale. This equates to −459.67°F on the Fahrenheit scale.

The third law of thermodynamics says absolute zero is not obtainable in a finite number of steps (and it is impossible to practically have an infinite number of steps.

You couldn't send a current through a wire at absolute zero because the electrons would not move.
The critical point when there is more resistance is absolute zero itself.

The ideal gas law says that PV=nRT. This means that in order for the temperature to be 0 (absolute 0), P or V would have to be zero. An actual gas cannot have a Pressure or Volume of zero and still have mass. This is a reason while absolute zero is unobtainable.