2. Q. Define heat.
Heat is a form of energy which produces the sensation of hotness or coldness
of a body.
Heat is a form of energy which flows from higher temperature to lower
temperature by conduction, convection and radiation processes.
Q. Define Temperature.
The temperature of a body is the property that determines whether the body is
in thermal equilibrium with another body or not.
3. Q. Distinguish between heat and temperature.
( Read it from pdf file)
Q. What is Isothermal Process?
When a gas undergoes a change in pressure and volume at constant temperature, the gas is said to
undergo an isothermal change, and the process in which the change takes place is called isothermal
process.
In isothermal process, the relation between pressure and volume of a gas follows the Boyle's law.
5. Q. Write down the conditions for isothermal process?
The following conditions are to be satisfied for isothermal process
(i) The container of the gas must be good conductor of heat.
(ii)The thermal capacity of the surroundings should be high.
(iii)The change of pressure should be made very slowly.
(iv)By absorption or rejection of necessary heat, the temperature is to
be kept constant.
6. Q. What is Adiabatic process?
The process in which a system neither receives heat nor rejects heat is called adiabatic process. In
this process, a gas undergoes changes in pressure, volume and temperature under thermal isolation.
The relation between pressure and volume in adiabatic process is PVγ=constant. The pressure versus
volume curve is called the adiabatic curve [Fig. 2]. The adiabatic curve is steeper than the isothermal
curve.
7.
8. Q. What are the Conditions for adiabatic change?
The following conditions are to be satisfied for adiabatic change
(i) The container of the gas should be non-conducting.
(ii)The thermal capacity of the surrounding should be low.
(iii)The change of pressure of the gas must be very rapid so that
(iv)the heat exchange with the surrounding does not take place.
9. # Platinum Resistance Thermometer
• The platinum resistance thermometer is based on the principle that the electrical resistance of metallic wire is
found to increase uniformly with temperature.
• The device in which temperature can be found out by measuring the resistance is called a resistance thermometer.
Platinum is extensively used as resistance thermometer.
10. 𝑅 𝜃 = 𝑅0 1 + 𝛼𝜃 𝑝
or, 𝑅 𝜃 = 𝑅0 + 𝑅0 𝛼𝜃 𝑝
or, 𝑅 𝜃 − 𝑅0 = 𝜃 𝑝 𝑅0 𝛼
∴ 𝜃 𝑝 =
𝑅 𝜃−𝑅0
𝛼𝑅0
……………… (2)
and 𝑅100= 𝑅0 1 + 𝛼. 100
∴ 𝛼 =
𝑅100−𝑅0
100𝑅0
……………… (3)
Q. For platinum resistance thermometer, show that the unknown temperature 𝜽 𝒑 =
𝑹 𝜽−𝑹 𝟎
𝑹 𝟏𝟎𝟎−𝑹 𝟎
× 𝟏𝟎𝟎 where, the symbols
have their usual meanings.
OR, Derive the temperature measuring equation for platinum resistance thermometer.
The electrical resistance of a conducting wire increase with increase in temperature over a wide range. Depending on this
property, platinum resistance thermometer is constructed.
The scientist Claudius was first who develop a relation as,
11. 𝜃 𝑝 =
𝑅 𝜃 − 𝑅0
𝑅100 − 𝑅0
100𝑅0
. 𝑅0
=
𝑅 𝜃 − 𝑅0
𝑅0
×
100𝑅0
𝑅100 − 𝑅0
∴ 𝜽 𝒑=
𝑹 𝜽−𝑹 𝟎
𝑹 𝟏𝟎𝟎−𝑹 𝟎
× 𝟏𝟎𝟎…………………. (4)
Putting the value of 𝛼 from (3) in (2) we get,
By knowing the values of 𝑅0, 𝑅100 and 𝑅 𝜃, we can determine the value of 𝜃 𝑝
0 𝐶 from equation (4).
12. Q. What is the general form of first law of thermodynamics?
Clausius expressed the first law of thermodynamics in a general form. He expressed the law in the following
way.
“If some heat is supplied to a system which can do work, then the quantity of heat absorbed by the
system is equal to the sum of the increase in internal energy of the system and the external work done
by the system”.
If dQ be the amount of heat absorbed by a system and dU be the change in the internal energy and dW be the
work done by the system due to absorption of heat, according to the first law of thermodynamics,
dQ = dU + dW
Or, dQ = dU + PdV
13. # What is Entropy?
It is a measure of disorder or randomness of molecular motion of the system.
Entropy is measured by the rate of change of heat absorbed or rejected with respect to temperature of the
system.
The change in entropy (dS) of a system is defined as
dS = dQ / T
Where dQ is transferred amount of heat and T is the absolute temperature of the system.
14. Q. Define Molar Specific Heat of Gas at Constant Volume
The quantity of heat required to raise the temperature of one mole of a gas through 1 K (10 C) when the
volume is kept constant is called molar specific heat of gas at constant volume. It is denoted by Cv. Its SI
unit is J mol -1 K-1.
15. Q. Show that the change in entropy of n mole of a perfect gas is
𝑺 𝟐 − 𝑺 𝟏 = 𝒏 𝑪 𝒗 𝒍𝒏
𝑻 𝟐
𝑻 𝟏
+ 𝑹 𝒍𝒏
𝑽 𝟐
𝑽 𝟏
where, the symbols have their usual meanings.
Let us consider a system containing n mole of a perfect gas. Entropy can be measured by making it a reversible
process. The energy transferred as heat to or from the gas is dQ, the work done by the gas is dW, and the change in
internal energy is 𝑑𝑈.
Now, according to the 1st law of thermodynamics we can write,
𝑑𝑄 = 𝑑𝑈 + 𝑑𝑊 … … … … (𝑖)
As it is reversible process, we can write,
𝑑𝑊 = 𝑃𝑑𝑉
And, 𝑑𝑈 = 𝑛𝐶𝑣 𝑑𝑇
Putting these two values in Eqn. (i) we get,
𝑑𝑄 = 𝑛𝐶 𝑣 𝑑𝑇 + 𝑃𝑑𝑉 … … … … … . . . (𝑖𝑖)
16. From ideal gas law we know,
𝑃𝑉 = 𝑛𝑅𝑇
𝑜𝑟, 𝑃 =
𝑛𝑅𝑇
𝑉
Inserting this value in Eqn. (ii) we get,
𝑑𝑄 = 𝑛𝐶𝑣 𝑑𝑇 +
𝑛𝑅𝑇
𝑉
𝑑𝑉
=>
𝑑𝑄
𝑇
= 𝑛𝐶𝑣
𝑑𝑇
𝑇
+
𝑛𝑅𝑇
𝑇𝑉
𝑑𝑉
[dividing by 𝑇]
=>
𝑄1
𝑄2 𝑑𝑄
𝑇
=
𝑇1
𝑇2
𝑛𝐶𝑣
𝑑𝑇
𝑇
+
𝑉1
𝑉2 𝑛𝑅
𝑉
𝑑𝑉
∆𝑆 = 𝑆2 − 𝑆1 = 𝑛𝐶𝑣 ln
T2
T1
+ 𝑛𝑅 ln
V2
𝑉1
∆𝑆 = 𝑆2 − 𝑆1 = 𝑛 𝐶𝑣 ln
T2
T1
+ 𝑅 ln
V2
𝑉1
This the required expression for change in entropy.
17. Q. Define Molar Specific Heat of Gas at Constant Pressure
The quantity of heat required to raise the temperature of one mole of a gas through 1 K (10 C) when the
pressure is kept constant is called molar specific heat of gas at constant pressure. It is denoted by Cp. Its SI
unit is J mol -1 K-1.
18. Q. Show that for an ideal gas, 𝑪 𝒑 > 𝑪 𝒗 where, the symbols have their usual
meanings.
Or, Cp-Cv= R, where the symbols have their usual meanings.
In order to find the difference between Cp and Cp, let us take a cylinder C with a frictionless movable piston
D. Both the cylinder and the piston are bad conductors of heat. Let us consider an amount of n moles of an
ideal gas is kept in the cylinder at volume V and pressure P. Keeping the volume fixed, the temperature of
the gas is raised by dT, by supplying heat.
So the amount of heat required, dQ = nCvdT
19. According to the first law of thermodynamics,
dQ = dU + dW
dQ = dU + PdV ………………… (1)
Since volume is constant [ dV = 0, So PdV=0 ]
From eqn (1), dQ = dU
or, nCvdT = dU …………………………….. (2)
Now keeping the pressure constant, the temperature of the gas is raised by the same amount dT by supplying heat.
The amount of heat supplied to the gas,
dQ = nCPdT
Inserting the values of dW and dQ in equation (1) we get,
nCpdT = dU + PdV ……………………………(3)
From equation (2) and (3) we get,
n CP dT = n CvdT + PdV ……………(4)
20. But for n mole of ideal gas.
PdV = nRdT ………………………………………….. (5)
From equation (4), we get
n CP dT = n CvdT + n R dT
or, nCp= n Cv+ n R
or, n(Cp – Cv) = n R
or, Cp – Cv= R
Therefore, the difference between the two specific heats of an ideal gas is
equal to the molar gas constant,𝑅.
21. Q. Define Brownian motion.
Brownian motion is the random motion of particles suspended in a fluid (a liquid or a gas)
resulting from their collision with the fast-moving molecules in the fluid. The direction of this
motion changes time to time.
This motion is named after the botanist Robert Brown
