Define Thermal equilibrium. How does it lead to Zeroth Law of Thermodyna... | Physics

THERMO DYNAMICS

Very Short Question Answers

Q1. Define Thermal equilibrium. How does it lead to Zeroth Law of Thermodynamics?
Ans.

Thermal equilibrium: If there is no transfer of heat energy in between two bodies which are in contact with each other, then they are said to be thermal equilibrium.

zeroth law of thermodynamics: If two systems are in thermal equilibrium with third system (separately) then they must be in thermal equilibrium with each other. (or) Out of three bodies A, B, C. If two bodies A and B are separately in thermal equilibrium with third body C, then A and B will also be in thermal equilibrium with each other.

Q2. Define Calorie. What is the relation between calorie and mechanical equivalent of heat?
Ans.

Calorie: The amount of heat required to raise the temperature of 1g of water by 1°c called calorie.
Relation between calorie and mechanical equivalent of heat is 1 𝑐𝑎𝑙𝑜𝑟𝑖𝑒 = 4.2𝐽

Q3. What thermodynamic variables can be defined by a) Zeroth Law b) First Law?
Ans.

a) Temperature b) Internal energy

Q4. Define specific heat capacity of the substance. On factors does it depend?
Ans.

Specific heat capacity: The quantity of heat required to rise the temperature of unit mass of the substance through 1℃ (or) 1K is called the specific heat capacity of the substance.

$\mathbf{\therefore \:\;\:S= \frac{1}{m}\frac{\triangle Q }{\triangle T}}$

i) Temperature ii) Nature of the substance.

Q5. Define molar specific heat capacity.
Ans.

$\\ \mathbf{\frac{V_{rms_1}}{V_{rms_2}}= \sqrt{\frac{T_1}{T_2}}} \Rightarrow \mathbf{V_{rms_2}= V_{rms_1}\sqrt{3}} \\ \therefore increase\:in\: the\:rms\: velocity \:\: \mathbf{V_{rms_2}-V_{rms_1}=\sqrt{3}-1=0.732}$

Q6. For a solid, what is the total energy of an oscillator?
Ans.

Average energy of an oscillator in three dimension = 3K_BT
For a mole of solid, the total energy is U = 3K_BT × N_A= 3𝑅𝑇

Q7. Indicate the graph showing the variation of specific heat of water with temperature. What does it signify?
Ans.

The specific heat of water decreases with increase in temperature from 0°C to 30°C and increases from 30°C to 100 °C

Q8. Define state variables and equation of state.
Ans.

State variables: The variables which determine the thermodynamic behavior of a system are called sate variables.
Example: pressure (p), volume(v), temperature(T), mass (M)
Equation of state: The ideal gas equation PV = μRT

Q9. Why a heat engine with 100 % efficiency can never be realised in practice?
Ans.

The efficiency will be 100% or 1, if T₂= 0 𝐾 𝑜𝑟 T₁= ∞ .Since, both these conditions cannot be attained practically, a heat engine cannot have 100% efficiency.

Q10. In summer, when the valve of a bicycle tube is opened, the escaping air appears cold. Why?
Ans.

This happens due to adiabatic expansion of the air in the tube of bicycle. Hence the air cools.

Q11. Why does the brake drum of an automobile get heated up while moving down at constant speed?
Ans.

When an auto mobile moving down with constant speed its potential energy decreases. This decrease in potential energy is converted in the form of heat energy. As a result, the break drum of an auto mobile get heated.

Q12. Can a room be cooled by leaving the door of an electric refrigerator open?
Ans.

No, when the refrigerator is working in a closed room with its door closed, it is rejecting heat form inside to the air in the room, so, temperature of room increases gradually. When the door of the refrigerator is kept open, then heat rejected by the refrigerator will be more. Therefore, the temperature of the room will slightly be increased.

Q13. Which of the two will increase the pressure more, an adiabatic or an isothermal process, in reducing the volume to 50 %?
Ans.

In isothermal 𝑃𝑉 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛t	In adiabatic 𝑃𝑉^γ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛t
P₂ = 2P	P₂ = 2^γP
γ>1 hence,in adiabatic process increases in pressure is more

Q14. A thermos flask containing a liquid is shaken vigorously. What happens to its temperature?
Ans.

Temperature of the liquid increases.

Q15. A sound wave is sent into a gas pipe. Does its internal energy change?
Ans.

Yes, the internal energy change when a sound wave is sent into a gas pipe. Because, the sum of all the energies contained in the system in equilibrium is called its internal energy.

Q16. How much will be the internal energy change in i) isothermal process ii) adiabatic process
Ans.

i) in an isothermal process, T = constant i.e, dT = 0  du=0 so, in isothermal process, there is no change in internal energy. ii) In an adiabatic process, Q = constant i.e, dQ = 0 du= -dw  0 So, in an adiabatic process, the change in internal energy is equal to the amount of work done

Q17. The coolant in a chemical or a nuclear plant should have high specific heat. Why?
Ans.

In nuclear plants a lot of heat energy is generated to protect the plant from this heat energy, a coolant with more specific heat will be used to absorb this heat energy from the plant. Hence the coolant should have high specific heat.

Q18. Explain the following processes i) Isochoric process ii) Isobaric process
Ans.

i) isochoric process: The process, in which the volume remains constant, is known as
isochoric process. dv=0, v= constant
ii) Isobaric process: The process, in which the pressure remains constant, is known as
isobaric process.dp = 0, p = constant

Short Question Answers

Q1. State and explain first law of thermodynamics.

Ans. THERMODYNAMICS

Q2. Define two principal specific heats of a gas. Which is greater and why?

Ans. THERMODYNAMICS

Q3. Derive a relation between the two specific heat capacities of gas on the basis of first law of thermodynamics

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Q4. Obtain an expression for the work done by an ideal gas during isothermal change.

Ans. THERMODYNAMICS

Q5. Obtain an expression for the work done by an ideal gas during adiabatic change and explain.

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Q6. Compare Isothermal and an adiabatic process.

Ans. THERMODYNAMICS

Q7. Explain the following processes i)Cyclic process with example ii) on cyclic process with example

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Q8. Write a short note on Quasi-static process

Ans. THERMODYNAMICS

Q9. Explain qualitatively the working of a heat engine.

Ans. THERMODYNAMICS

Essay Question Answers

Q1. Explain reversible and irreversible processes. Describe the working of Carnot engine. Obtain an expression for the efficiency

Ans. THERMODYNAMICS

Q2. State second law of thermodynamics. How is heat engine different from a refrigerator

Ans. THERMODYNAMICS

Problems Question Answers

Q1. If a monoatomic ideal gas of volume i litre at N.T.P. is compressed (i) adiabatically to half of its volume, find the work done on the gas. Also find (ii) the work done if the compression is isothermal.

Ans. THERMODYNAMICS

Q2. Five moles of hydrogen when heated through 20 K expand by an amount of 8.3 x 10^-3 m² under a constant pressure of 10 N/m. If C=20 J/mole K, find C

Ans. THERMODYNAMICS