What's the difference between heat and agitation at the atomic scale?

[CuriousBanker]

hello. Simple question here for somebody just learning the basics. On the atomic scale what’s the difference between agitation and heat? For example, diluting salt in water. If you increase the heat of the water, more salt dissolves. Stirring it also makes the salt dissolve more too it seems. But what’s the difference between stirring (agitation I’m guessing is what it’s called) and heat? Both are increased movement of molecules. So how come stirring the glass really fast with a spoon doesn’t Heat it up a lot?

 

[sophiecentaur]

It's a matter of scale. Diffusion depends on the gradient of concentration and it is a random process and it will be faster as you increase the temperature but agitation will move regions of low concentration over the solute and that will increase the rate of diffusion near the salt. You have added a systematic element to the random movements in a stationary liquid.

 

[Dale]

So how come stirring the glass really fast with a spoon doesn’t Heat it up a lot?

It does. That is precisely the experiment that Joule did which showed the equivalence of work and heat. He showed that you could heat up a liquid by stirring it.

 

[sophiecentaur]

It does. That is precisely the experiment that Joule did which showed the equivalence of work and heat. He showed that you could heat up a liquid by stirring it.

It may be worth noting that the 'Mechanical Equivalent of Heat' that Joule found is that it takes 4.2 Joules of mechanical energy (work) to produce just one calorie of heat,
4.2 J corresponds to lifting 1kg by 0.42 m
1 calorie will raise the temperature of just 1gm of water by 1°C
A lot of Work for a very little Heat.