Thermodynamics Questions and Answers – Properties of Atmospheric Air

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This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Properties of Atmospheric Air”.

1. Dry air consists of
a) oxygen, nitrogen
b) carbon dioxide
c) hydrogen, argon
d) all of the mentioned
View Answer

Answer: d
Explanation: Dry air is a mixture of all these gases.
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2. Complete dry air exists in nature.
a) true
b) false
View Answer

Answer: b
Explanation: Complete dry air does not exist in nature.

3. Which of the following is true?(here pa=partial pressure of dry air, pw=partial pressure of water vapour, p=atmospheric pressure)
a) p=pw
b) p=pa
c) p=pw + pa
d) all of the mentioned
View Answer

Answer: c
Explanation: This comes by Dalton’s law of partial pressures.
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4. In a mixture of dry air and water vapour,
a) mole fraction of dry air = pa/p
b) mole fraction of water vapour = pw/p
c) both of the mentioned
d) none of the mentioned
View Answer

Answer: c
Explanation: This comes from the concepts of Dalton’s law of partial pressures.

5. When pw is very small,
a) saturation temperature of water vapour at pw is less than atmospheric temperature
b) water vapour in air exists in superheated state
c) air is said to be in unsaturated state
d) all of the mentioned
View Answer

Answer: d
Explanation: Mostly partial pressure of water vapour is very small.
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6. Relative humidity is defined as
a) (saturation pressure of pure water) / pw
b) pw / (saturation pressure of pure water)
c) (saturation pressure of pure water) / p
d) p / (saturation pressure of pure water)
View Answer

Answer: b
Explanation: Here pw is the partial pressure of water vapour and ps is the saturation pressure of pure water at same temperature of mixture.

7. For saturated air, relative humidity is 0%.
a) true
b) false
View Answer

Answer: b
Explanation: For saturated air, relative humidity is 100%.
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8. If water is injected into a container with has unsaturated air,
a) water will evaporate
b) moisture content of air will decrease
c) pw will decrease
d) all of the mentioned
View Answer

Answer: a
Explanation: The moisture content of air will increase and pw will increase.

9. Humidity ratio is given by the ratio of
a) (mass of dry air per unit mass of water vapour)^2
b) 1/(mass of dry air * mass of water vapour)
c) water vapour mass per unit mass of dry air
d) mass of dry air per unit mass of water vapour
View Answer

Answer: c
Explanation: Humidity ratio is also called specific humidity.
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10. The degree of saturation is the ratio of
a) (saturated specific humidity / actual specific humidity)^2
b) 1/(saturated specific humidity * actual specific humidity)
c) saturated specific humidity / actual specific humidity
d) actual specific humidity / saturated specific humidity
View Answer

Answer: d
Explanation: Here both saturated specific humidity and specific humidity are at same temperature.

11. The degree of saturation varies between -1 and 0.
a) true
b) false
View Answer

Answer: b
Explanation: The degree of saturation varies between 0 and 1.

12. Which of the following statement is true?
a) dew point temperature is the temperature at which water vapour starts condensing
b) dry bulb temperature is recorded by thermometer with dry bulb
c) wet bulb temperature is recorded by thermometer when bulb is covered with a cotton wick which is saturated with water
d) all of the mentioned
View Answer

Answer: d
Explanation: These are the definitions of dew point temperature, dry bulb temperature and wet bulb temperature.

13. The wet bulb temperature is the ____ temperature recorded by moistened bulb.
a) lowest
b) highest
c) atmospheric
d) none of the mentioned
View Answer

Answer: a
Explanation: This is a property of wet bulb temperature.

14. At any dbt, the ____ the difference of wbt reading below below dbt, ____ is the amount of water vapour held in mixture.
a) smaller, smaller
b) greater, greater
c) greater, smaller
d) smaller, greater
View Answer

Answer: c
Explanation: This is an important property of dbt and wbt.

15. When unsaturated air flows over a sheet of water in an insulated chamber
a) specific humidity of air decreases
b) the water evaporates
c) both air and water are cooled during evaporation
d) all of the mentioned
View Answer

Answer: a
Explanation: The specific humidity of air increases during this process.

Sanfoundry Global Education & Learning Series – Thermodynamics.
To practice all areas of Thermodynamics, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter