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➔ In order to maintain a body in an isothermal condition, heat has to be either supplied or withdrawn.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

Heat Transfer due to conduction

In this mechanism, heat transfer is due to vibration amplitudes of molecules & atoms present in solids.

Consider a cubicle of solid. Let us maintain one face of cube at high temperature (TH) and other opposite face at low temperature(Tc).

Due to temperature difference an amount of heat energy (Q) passes from  hot face to cold face in time 't'.

Conduction rate  Pcond (amount of energy transferred for uni time) is

Pcond = Q/t = K*A*(TH-Tc)/d;

where 'K' is coefficient of thermal conductivity, a constant for given material.
           'd' is thickness of slab
           'A' Area of slab
            't' is time of conduction

Therefore, Q= K*A(TH-Tc)*t/d

Note: i) 'K' depend on nature of material of which slab is made
         ii) A good thermal conductor has 'K' greater value.

Thermal Resistance to conduction(R-value):

This explains resisting of thermal conductivity. The R-value(thermal resistance) of a slab of thickness 'd' is defined as R=d/k. Thus material having less value of 'K' will have higher R-value and thus acts as a good thermal insulator.

Note:

i) 'R' is properly assigned to specified thickness of slab but not to material of slab.
ii) In steady state, conducting rates thru any no. of materials must be equal.
     Therefore, Pcond = A*(TH-Tc) / Σ(d/K)
iii) Heat is transferred from molecule to molecule by conduction. In this case molecules do not bodily move but simply vibrate.