# What is heat?

**URL:** <https://boards.straightdope.com/t/what-is-heat/187671>\
**Category:** Factual Questions\
**Created:** [July 11, 2003, 7:45am UTC](https://boards.straightdope.com/t/what-is-heat/187671 "2003-07-11T07:45:20Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Major\_Kong](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@Major\_Kong](https://boards.straightdope.com/u/Major_Kong)\
**Post date:** [July 11, 2003, 7:45am UTC](https://boards.straightdope.com/t/what-is-heat/187671/1 "2003-07-11T07:45:20Z")

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Is it some form of radiated energy?

I looked at:

[http://scienceworld.wolfram.com/physics/Heat.html](http://scienceworld.wolfram.com/physics/Heat.html)

but it didn’t really answer my question?

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**Author:** ![Popup](https://avatars.discourse-cdn.com/v4/letter/p/94ad74/32.png) [@Popup](https://boards.straightdope.com/u/Popup)\
**Post date:** [July 11, 2003, 8:21am UTC](https://boards.straightdope.com/t/what-is-heat/187671/2 "2003-07-11T08:21:17Z")

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Well, that page, while summing up the first law of thermodynamics rather nicelly, tells you absolutely nothing about what heat **is**.

To put it simply, heat is thermal energy. It is important to realise that while _temperature_ is a rather complex concept, _heat_ is just the thermal energy of particles.

In order to get to the bottom of this, we have to look at a what experts call a _very, very small scale_. i.e. look at individual molecules. To make it easier, let us restrict ourselves to ideal monoatomic gasses. An ideal gas is a collection of monoatomic molecules, moving around at random in three dimensions. You may think of the molecules as small billiard balls, flying around at random, and colliding with each other.

You may remember from physics class, that the kinetic energy of a moving object is _mv[sup]2[/sup]/2_, but even if you don’t, it’s fairly obvious that the balls moving faster have higher energy.  
As long as all this motion is random (i.e. there are as many molecules moving in all directions), this energy is called _heat_.

Basically that’s all there is to it. (As long as you restrict yourself to monoatomic ideal gasses.) By counting vibrational and rotational modes in the molecules, it is possible to look at polyatomic molecules, but in taking the step to looking at solids, we need to take into account the gitter structure of te solid, as well as freely moving electrons. That’s beyond the scope of this post.

The _radiated energy_ you mention in the OP, is something almost unrelated. It is a property of every moving body with a temperature above absolute zero to radiate energy in the electromagnetical spectrum, but that is quite hard to understand from basic principles, and is often taken for given.

(If you want to know more about _temperature_, and how that relates to _heat_, I suggest you do a search here, or look at [this](http://unidata.ucar.edu/staff/blynds/tmp.html), a very readable guide that purports to explain the difference between heat and temperature, in very simple language.)

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**Author:** ![xash](https://avatars.discourse-cdn.com/v4/letter/x/c6cbf5/32.png) [@xash](https://boards.straightdope.com/u/xash)\
**Post date:** [July 11, 2003, 10:34am UTC](https://boards.straightdope.com/t/what-is-heat/187671/3 "2003-07-11T10:34:23Z")

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I’m sorry, my **Popup** blocker blocked your response… could you repeat that ? 😃