A Devil of a Misconception

Heat Doesn't Rise

Okay, ha ha.  This is kind of funny, but it merits discussion because of the Stupid phrase “heat rises”.  That’s utter nonsense, and we’re going to talk about why.

People often use heat as a synonym for hotness or temperature, but it’s not the same thing.  From a physics standpoint, temperature is a measure of the average kinetic energy of the molecules in a body.  When an object is hot, its molecules move around faster than they do when the object is cold.  Heat is the spontaneous transfer of thermal energy from a region with high temperature to a region with low temperature.  If you have a block of, say, iron, with a temperature of 400º Celsius, and you place it right next to an iron block with a temperature of 20º Celsius, heat will flow from the 400º block to the 20º block until their temperatures are the same.  (This example ignores heat lost to or gained from the surroundings, which will complicate the issue somewhat.)

Why does heat flow from the hot block to the cold block when they touch?  The hot block’s atoms are vibrating faster, with more energy.  When the atoms of the hot block vibrate against the atoms of the cold block, vibrational energy gets transferred primarily in one direction; from hot to cold.  The hot block loses energy while the cold block gains it.  The hot block’s atoms decrease their wiggling, while the cold block’s atoms increase.  The situation reaches equilibrium when both blocks’ atoms have the same average kinetic energy; i.e. when they have the same temperature.

So heat itself – as a transfer of thermal energy – does not have a natural tendency to move upward; it follows the temperature gradient, whatever direction that may be.

But perhaps you know from experience that the attic of a house is warmer than the lower levels, or that steam rises from the surface of boiling water to form droplets on the lid of a pot.  And of course hot air balloons soar above the countryside.  Aren’t these textbook examples of heat rising?

Well, they may be textbook examples; however, any textbook that parrots the phrase “heat rises” is contributing to a widespread misconception.  But now we’re no closer to understanding the problem:  If heat has no natural tendency to rise, why is it that, especially in gases, hot regions tend to rise up while cold regions tend to sink?

Slightly more sophisticated textbooks use density and buoyancy as a tool to explain why, say, a hot air balloon is capable of soaring above the countryside.  As a parcel of air is heated, its molecules move faster, they correctly point out.  As the molecules move faster, they tend to expand a bit, which lowers the overall density of the parcel.  The surrounding air, which is cooler and therefore more dense, buoys the hot air parcel skyward, in much the same way that dense seawater buoys up an ocean liner.

This explanation is better than the perfunctory “heat rises” account, and it would probably suffice for an introductory physical science class, but the sharp student will see through it.  Sure, it makes sense to claim that the air in a hot air balloon rises because it is more-or-less thermally isolated from the surrounding air, but that doesn’t explain why an unconstrained warm air mass tends to rise or why a similarly unconstrained cold air mass tends to sink, as in weather systems.  After all, there is no envelope or barrier between these two air masses.  Shouldn’t they interact and exchange heat, thereby equalizing their temperatures and preventing any net movement of the air?

In the absence of gravity, they might.  We have to dig deeper if we want to really understand why a warm air mass would rise above a cool air mass (or why the first level of Hell would be the hottest).  We have to get all the way down to the molecular level.

All molecules are subject to gravity, and they experience a downward force that is proportional to their mass.  But molecules – particularly gas molecules – also bounce around randomly and collide with other molecules.  Now this might sound bizarre, but in a body of colliding molecules, there is a net tendency to transfer momentum upwards – in other words, the bazillions of collisions that happen with a typical gas parcel tend to work against the pull of gravity.

When a parcel of air gets heated, its molecules speed up (on average) which means there are more frequent collisions within that parcel; ergo, there is an increase in the net upward momentum of the parcel’s molecules.  Heating a gas actually increases its parcels’ ability to bounce themselves away from the ground.  The molecules in a cold air parcel collide less frequently, ergo, the molecules tend to bounce around less vigorously.  When a cold air mass meets a warm air mass, the molecules in the warm air mass naturally tend to bounce off of and above the molecules in the cold air mass.

(On a related note, when powerful solar flares strike Earth, they dump lots of energy into Earth’s atmosphere, causing it to expand outward.  This can increase the drag on Earth-orbiting satellites and shorten their useful lives.)

So to sum up:

  • Heat has no inherent tendency to rise, even in Hell.
  • Heat is the transfer of thermal energy from regions of high temperature to regions of low temperature.
  • The molecules in a hot air mass move around faster, which means they collide more frequently than the molecules in an adjacent cold air mass.
  • The increased frequency of collisions in the hot air mass enhances the molecules’ upward momentum compared to molecules in a colder air mass, therefore:
  • A hot air mass tends to rise above a cold air mass.

I find it inexcusable that the devil is lecturing Joe about his lack of science knowledge, especially when he doesn’t quite get it himself.  According to some theologies, Old Scratch was present almost from the beginning of Creation (in fact, some sects hold that Satan is the author of the physical world.)  You’d expect him to be more well-versed in the physics of the atmosphere.  Then again, Satan has been called the Father of Lies; maybe he delights in spreading scientific misconceptions?

One more thing before I sign off…and this is really just a nitpick:  How is it that Satan did not know Joe’s name, but was able to determine that this was the same Joe that cheated on a test in 3rd grade?


2 thoughts on “A Devil of a Misconception

  1. If Joe has the worst level of Hell, he didn’t go there because he cheated on his test, he must have committed the worst sin imaginable. He used the last of the toilet paper and didn’t replace the roll.

    • Who occupies the lower circle of Hell…the guy who doesn’t replace the toilet paper, or the guy who DOES replace it but sets it up so that the hanging end is BEHIND the roll?

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