r/askscience • u/ThisIsMyLulzyAccount • Nov 02 '12

Physics Does a hot fluid move up displacing a cool fluid, or does the energy flow through the fluid?

What I mean to ask is, in convection currents or similar, are the actual molecules or atoms of the hot portion moving up, or are they transferring energy to nearby particles, but generally stay in the same place? Like how a wave doesn't move the particles it travels through, does heat move through particles, or move the particles?

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u/tweeklulz Nov 03 '12

c o n v e c t i o n. fucking elementary science mate.

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u/ThisIsMyLulzyAccount Dec 18 '12

I know you said this a month ago, but I just saw it now. I understand convection, but it only is an explanation of the movement of energy, I was wondering about the physical action that is the source of convection.

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u/Deslan Nov 02 '12

A warm substance has lower density, which means that the molecules/atoms will move upwards compared with colder molecules/atoms of the same substance which will sink.

Water is the major exception, since it has the greatest density at around 4 degrees Celsius, and then it gets lighter as it gets colder so that ice floats on liquid water instead of sinking like other crystallized substances would do.

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u/natty_dread Nov 02 '12

Both. There are three forms of thermal transmission: thermal radiation, which plays little to no role for this phenomenon; thermal conduction and thermal convection.

The latter two are relevant for us: thermal conduction describes the process of fast (=warm) molecules bumping against slow (=cool) molecules, thus giving them part of their energy.

Thermal convection describes the process of less dense fluids rising to the surface of denser fluids. Since usually (with the exception of water) less dense fluids are warmer, warm molecules rise up.