Question regarding temperature

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In summary, the conversation discusses the relationship between kinetic energy and temperature in gas and liquid states of element X. It is confirmed that the average molecular kinetic energy is proportional to absolute temperature, but there are factors such as pressure and humidity that can affect this relationship. The conversation also touches on the concept of water vapour being cooler than boiling water, which can be explained by the thermal energy required for vapourisation. The conversation ends with a question about the contradiction between the kinetic theory and the cooling process of gas in air conditioning systems.
  • #1
physics_dummy
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I have a question.

Is the gas state of element X having a higher kinetic energy than the liquid sate of element X? This is what my high school texts told me.

But kinetic theory says that "The average molecular kinetic energy is proportional to the absolute temperature"

If yes, then does that implies that gas is always having a higher temperature than liquid?
but then how can water vapour be cooler than boiling water?
 
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  • #2
the kinetic energy of each atom of element X will be greater in a gas than a liquid, but of course the liquid is far more dense and so per volume there is more thermal energy.

Your statement: "The average molecular kinetic energy is proportional to the absolute temperature" is correct.

Why do you think that water vapour is cooler than boiling water?

Water vapour can attain much higher temperatures than liquid water; at the point of evapouration it has the thermal energy of the boiling water plus the energy required for vapourisation.

Perhaps from personal experience steam may appear cooler than boiling water (although often not) but this is because the water molecules are very diffuse in air.

There's a whole host of additional caveats like pressure, humidity impureities etc that affect things, but generaly the higher T the higher KE.
 
  • #3
yes, you're right.

My question was arisen when I was reading about air conditioning system.

When the high pressure and temperature liquid refrigerant passes through a thing called "expansion valve", it expands and become gas and hence becoming cold. The I relate it with the kinetic theory, I see contradiction... how can gas be cooler than liquid? Maybe it has something to do with pressure...
 

1. What is temperature?

Temperature is a measure of the average kinetic energy of the molecules in a substance. In simpler terms, it is a measure of how hot or cold something is.

2. How is temperature measured?

Temperature is typically measured using a thermometer, which uses a liquid or gas that expands or contracts with changes in temperature to give a numerical reading.

3. What are the different temperature scales?

The most commonly used temperature scales are Celsius, Fahrenheit, and Kelvin. Celsius is based on the freezing and boiling points of water, Fahrenheit is based on a scale developed by German physicist Daniel Fahrenheit, and Kelvin is based on the absolute zero point of temperature.

4. How does temperature affect matter?

Temperature affects matter in various ways. It can cause changes in physical properties, such as expansion or contraction, and can also affect chemical reactions. Changes in temperature can also impact the state of matter, such as causing a solid to melt into a liquid.

5. What is the difference between heat and temperature?

Heat and temperature are often used interchangeably, but they are actually different concepts. Heat is a form of energy that is transferred from one object to another due to a difference in temperature. Temperature, on the other hand, is a measure of the average kinetic energy of the molecules in a substance.

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