Why is Hot Air Lighter than Cold Air?

Click For Summary

Discussion Overview

The discussion centers around the question of why hot air is considered lighter than cold air, exploring the relationship between temperature, density, and mass in gases. Participants examine the implications of energy and mass as described by E=mc^2, and how these concepts apply to the behavior of air at different temperatures.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the relationship between energy and mass, suggesting that if E=mc^2 holds true, hot air, having more energy, should be heavier.
  • Another participant clarifies that at the same pressure, hot air is less dense, meaning there is less air per volume, which contributes to its buoyancy.
  • Some participants propose that the expansion of hot air allows it to rise above colder, denser air due to its lower density.
  • There is a suggestion that if two volumes of air at different temperatures were contained in separate vessels, the hotter air vessel would be infinitesimally heavier due to its higher energy content.
  • One participant agrees with the notion that the hotter gas would have a slightly greater mass than the cooler gas, despite the challenges in measuring this difference.

Areas of Agreement / Disagreement

Participants express differing views on the implications of E=mc^2 in relation to the mass of hot air versus cold air. While there is some agreement on the concept of density and buoyancy, the question of whether hot air is indeed heavier in terms of mass remains contested.

Contextual Notes

Participants acknowledge the complexity of measuring the mass difference between hot and cold air, as well as the dependence on definitions of density and volume. The discussion does not resolve the apparent contradiction between energy, mass, and buoyancy.

Who May Find This Useful

This discussion may be of interest to individuals exploring thermodynamics, gas laws, or the principles of buoyancy in fluids.

Skim Halo
Messages
3
Reaction score
0
If E=mc^2, why is hot air lighter than cold air? Hot air has more energy and should thus have greater mass and therefore be heavier as weight = mg
 
Science news on Phys.org
At the same pressure, hot air is less dense - there's less of it per volume.
Aside from that, a number of molecules on a hot gas would be heavier than the same number of the same cold gas. But it would be very hard to measure the difference. For E=mc^2, you have an awful lot of E equating to very little m.
 
  • Like
Likes   Reactions: Chestermiller
So it's the fact that the air's volume is not constrained which allows it to expand it's volume and being less dense it rises above the colder denser air.

I would assume then however that if two volumes of air were contact rained in equal sized separate vessels, the hotter air vessel would be infinitesimally heavier?
 
Skim Halo said:
I would assume then however that if two volumes of air were contact rained in equal sized separate vessels, the hotter air vessel would be infinitesimally heavier?

Exactly. Given two containers of gas, identical in all respects except for temperature, the hotter one would have a tiny bit more mass than the cooler one.
 

Similar threads

Undergrad Centrifugal separation into hot & cold air streams?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad How to use cold air mass flow to support supermarket cooling?
  • · Replies 21 ·
Replies
21
Views
3K
Undergrad Time for a cold volume of air to reach a higher air temperature
  • · Replies 11 ·
Replies
11
Views
3K
High School Why Hot Air Rises and Cold Air Sinks: Understanding Air Density
  • · Replies 6 ·
Replies
6
Views
3K
High School Why does hot water rise to the top of a water heater?
  • · Replies 66 ·
3
Replies
66
Views
5K
High School Why a mug handle gets very hot when microwaved
  • · Replies 25 ·
Replies
25
Views
17K
Undergrad Why does entropy increase when hot water is mixed with cold?
  • · Replies 12 ·
Replies
12
Views
5K
Undergrad Trying to wrap my head around gas turbine thermodynamics
  • · Replies 5 ·
Replies
5
Views
4K
Graduate Heat energy input from a hot source in a heat engine
  • · Replies 11 ·
Replies
11
Views
2K
Undergrad Why does hot air have low pressure? (wind formation)
  • · Replies 16 ·
Replies
16
Views
3K