Thermodynamics Test

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Thermodynamics Test - 1

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Question 1
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For a system to be in thermodynamic equilibrium_______

A
the temperature and the pressure have to be same everywhere

B
only temperature have to be same everywhere and the pressure can vary

C
the temperature and the pressure, both can vary from point to point

D
only pressure have to be same everywhere and the temperature can vary

Solution

The system is said to be in thermodynamic equilibrium if the conditions for the following three equilibrium is satisfied:

1) Mechanical equilibrium: When there are no unbalanced forces within the system and between the system and the surrounding, the system is said to be under mechanical equilibrium.

This means that the pressure of the system is constant everywhere.

2) Chemical equilibrium: The system is said to be in chemical equilibrium when there are no chemical reactions going on within the system or there is no transfer of matter from one part of the system to other due to diffusion.

3) Thermal equilibrium: When the temperature of the system is uniform and not changing throughout the system and also in the surroundings, the system is said to be in thermal equilibrium.

This means that the temperature of the system is constant everywhere.

A system is said to be in thermodynamic equilibrium if it is simultaneously in mechanical, thermal and chemical equilibrium.

Hence, for a system to be in thermodynamic equilibrium, the temperature and the pressure have to be same everywhere.

Question 2

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Which one of the following is the correct combination of type of thermometer and its thermometric property respectively?

Thermistor and Voltage

Thermocouple and Voltage

Mercury Thermometer and Resistance

None of the above

Solution

Concept:

In order to obtain a quantitative measure of temperature, a reference body is used, and a certain physical characteristic of this body which changes with temperature is selected.
The changes in the selected characteristics may be taken as an indication of the change in temperature.
The selected characteristic is called the thermometric property, and the reference body which is used in the determination of temperature is called thermometer.

Explanation:

A thermocouple is based on the heating effect.
When two metal having different work functions are placed together, a voltage is generated at the junction which is nearly proportional to the temperature.
The EMF produced is proportional to the temperature and hence to the RMS value of the current. Therefore a voltmeter using thermocouples measures RMS value.
A thermocouple works on the concept of see back effect in which temperature is measured using the voltage reading.
Hence voltage is the Thermometric property in Thermocouple.

Note:

There are five different types of thermometer each with its own thermometric properties as shown in the table below.

Sr. No	Thermometer	Thermometric property	Symbol
1.	Constant value gas thermometer	Pressure	P
2.	Constant pressure gas thermometer	Volume	V
3.	Electrical resistance thermometer	Resistance	R
4.	Thermocouple	Thermal e.m.f.	ϵ
5.	Mercury in gas thermo meter	Length	L

Question 3
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A can of soft drink at room temperature is put into the refrigerator so that it will cool. Would you model the can of soft drink as________

A
a closed system

B
an open system

C
an isolated system

D
Either (a) or (b)
Question 4
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The thermodynamic state of a closed system containing a pure fluid changes from (T₁, p₁) to (T₂, p₂), where T and p denote the temperature and pressure, respectively. Let Q denote the heat absorbed (> 0 if absorbed by the system) and W the work done (> 0 if done by the system). Neglect changes in kinetic and potential energies. Which one of the following is CORRECT?

A
Q is path-independent and W is path-dependent

B
Q is path-dependent and W is path-independent

C
(Q - W) is path-independent

D
(Q + W) is path-independent

Solution

here are two thermodynamics functions:

Path function: Heat and work are path functions. Their magnitude depends on the path followed during a process as well as end states.

Point function: Pressure, temperature, volume and internal energy etc. are point function. They depend on the end states only, not on the path followed.

According to first law of thermodynamics:

δQ = δW + ΔU

When a process is executed by a system, the change in stored energy of the system is numerically equal to the net heat interaction minus the net work interaction during the process:

ΔU = δQ – δW where U is the internal energy which is introduced by this law.

And change in internal energy is point function (path-independent) i.e. state function.

Important Point

Note that Q and W themselves depend on the path followed. But their difference does not.

This implies that the difference between the heat and work interactions during a process is a property of the system.
Question 5
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A reversible thermodynamic cycle containing only three processes and producing work is to be constructed. The constraints are: (a) there must be one isothermal process, (b) there must be one isentropic process, (c) the maximum and minimum cycle pressures and the clearance volumes are fixed, and (d) polytropic processes are not allowed. Then the numbers of possible cycles are

A
1

B
2

C
3

D
4