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THERMODYNAMICS 

Questions - Answers

 Thermodynamics deals with the macroscopic properties such as Pressure, Volume Temperature etc. It is an experimental science, used to study energy relations and the manner in which energy changes take place. Based on previous BIE, this chapter has a weightage of (2 + 2 + 4 M) or (4 + 8 M).

VERY SHORT ANSWER TYPE QUESTIONS (VSAQ)

1. Define thermal equilibrium. How does it lead to zeroth law of thermo dynamics?

A: If temperature of two systems are equal and there is no net flow of heat between them when they are brought in thermal contact, then two systems are said to be in thermal equilibrium.

Zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system seperately, then they must be in thermal equilibrium with each other.

2. Define calorie. What is the relation between calorie and mechanical equivalent of heat?
A: Calorie: The amount of heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C is called calorie. The amount of work needed to produce 1 calorie of heat is called mechanical equivalent of heat (J). 1 Calorie = 4.816 joule.

3. What thermodynamic variables can be defined by (a) Zeroth Law, (b) First Law?
A: Thermodynamic variable can be defined by Zeroth law is temperature and by first law is internal energy.

 

4. Define molar specific heat capacity.
A: The amount of heat required to raise the temperature of one mole of a substance by one degree is called molar specific heat capacity.


      
It depends on nature of the substance, its temperature and the conditions under which heat is supplied.

 

5. In summer, when the valve of bicycle tube is opened, the escaping air appears cold?
A: This happens due to adiabatic expansion of air of tube of the bicycle. This causes a fall in temperature hence escaping air from tube appears cold.

 

6. A thermos flask containing liquid is shaken vigorously. What happens to its temperature?
A: When liquid in a flask is shaken vigorously the internal work is done on the liquid and internal energy increases. So, the temperature of the system raises.

Q: Can a room be cooled by leaving the door of an electric refrigerator open?
A: No. A refrigerator extracts heat from cold reservoir and rejects heat to surrounding air (hot reservoir). Thus, if door is left open, more heat is rejected to the surroundings and room will get slightly heated.

SHORT ANSWER TYPE QUESTIONS (SAQ)

1. Derive a relation between the two specific heat capacities of gas on the basis of first law of thermodynamics.

A: Consider one mole of an ideal gas contained in a cylinder provided with a frictionless piston. When gas is heated, it expands and piston is pushed up through a distance of dx.

If dQ (or) ΔQ be the heat absorbed at constant volume dV = 0, then

       dQ = dU = Cv

 dT  (1)

If gas is heated at constant pressure, heat supplied is dQ = Cp dT (2)

Work done by the gas is dW = F.S = (PA) (dx) 
       dW = P dV
 (3)
From first law of thermodynamics dQ = dU + dW

From (1), (2) and (3)
     Cp dT = Cv dT + P dV
     [Cp − Cv] dT = P dV
From ideal gas equation PV = nRT
                                        PdV = nR (dT)
          [Cp − Cv] dT = nR (dT), but n = 1 mole
                    Cp − Cv = R

 

2. Write a short note on triple point of water.

A: The point at which all three phases of matter co-exist in equilibrum is called triple point. The graph between the pressure and temperature of a substance is used to study the behaviour of matter in different phases is called phase diagram. The temperature at which solid (ice) and liquid (water) phases are in equilibrium is called melting point. Along the curve 'PM' ice and water are said to be in equilibrium. It is called ice line.

The temperature at which vapour and liquid states are in equilibrium is called steam point. Along the curve PN, water and vapour are said to be in equilibrium. The temperature at which solid and vapour are said to be in equilibrium is called sublimation point. Along the curve PQ, solid and vapour are said to be in equilibrium.
All three curves PM, PN, & PQ meet at a single point 'P' called triple point of water.
Triple point of water = 273.16 K, 610.42 Pa.

3. Compare isothermal and adiabatic processes.

LONG ANSWER TYPE QUESTION (LAQ)
1. State second law of thermodynamics. How is heat engine different from refrigerator.
A: Second law of thermodynamics gives the direction of flow of heat. This law consists of two most significant statements.
1. Kelvin - Planck Statement: "No heat engine can convert whole of heat energy supplied to it into useful work".
                                                                  (or)
It is impossible by any thermodynamic process which results slowly in the removal of heat from a heat reservoir and convert it entirely into work.
2. Clausius Statement: It is impossible for a self acting machine unaided by any external agency to transfer heat from a body at low temperature to a body at high temperature.
It can also be stated as heat on its own cannot how from a cold body to a hot body.

Heat Engine: A device which converts heat energy into work is called heat engine.


          
It consists of

1. Source (or) Hot Reservoir: Heat (Q1) is extracted from this body and it is maintained at a very high temperature T1.
2. Working Substance: Working substance rejects heat energy (Q2) to sink. In a steam engine, steam is working substance.
3. Sink (or) Cold Reservoir: Sink is maintained at low temperature (T2). It absorbs heat energy (Q2) which is rejected by working substance.

Workdone: The workdone by the system in a cycle is equal to the differnce of heat absorbed from source and heat rejected to sink.
                                                W = Q1 − Q2
Efficiency: The ratio of workdone (W) by the engine to the amount of heat absorbed (Q1) by the engine.


                 
          η is less than one.
Refrigerator: It is a heat pump which is the reverse of a heat engine. In refrigerator, the working substance extracts heat energy (Q2) from sink of lower temperature (T2). The external work is done on the working substance and amount of heat Q1 is rejected to the source. The coefficient of performance


             
         α is greater than one.

         Therefore working of refrigerator is opposite to the working of heat engine.

Posted Date : 26-10-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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