MadSci Network: Physics

Re: What is heat?

Date: Fri Apr 24 08:11:23 1998
Posted By: Malcolm Tobias, Post-Doc, Physics, Washington University
Area of science: Physics
ID: 892404263.Ph

What is heat?  Good question.  Heat is usually described as 'thermal
energy', or energy that an object has due to its temperature.  
To understand this, consider a large box filled with particles.  
Let's assume that the particles are initially
not moving, and sitting in a freezer at absolute zero (the coldest
temperature possible).  Now what would happen if we set this box on
a stove?  The stove would impart energy into the box, and this energy
has to go somewhere.  This energy would go into thermal energy of
the particles in the box.  Let me explain this in a little more detail.
The surface of the stove is hot, which means that the particles that
make up the stove are moving around rather quickly.  When these
stove particles encounter the particles that make up the box, they
will smack into them and set them in motion, just like the collision
of billiard balls.   Similarly, the particles that make up the box will
encounter the particles inside the box and collide with them.  Pretty
soon, all the particles (the stove, the box, and the particles in the
box) are moving at roughly the same speed.  This is what we mean when
we say that that they have reached 'thermal equilibrium', or more simply
that they are at the same temperature. This is what we usually think
of as heat energy, the kinetic energy of a bunch of particles.  

In real life, these particles are usually atoms or molecules.  The 
total amount of heat energy something has depends not only on how
fast the atoms or molecules are moving, but how many of them there
are.  That is why dense objects can hold heat in longer.  This is
why it takes longer to freeze a gallon of water than a cup.  The
gallon has a lot more heat energy that must be removed.  This is also
why aluminum foil coming out of the oven cools a lot faster than an
iron pan.

So what is the 'basic unit' of heat?  
This question is very difficult to answer
because thermodynamics is a macroscopic description of a system.
Ok, what does that mean?  It means that it describes the behavior
of a large number of particles.  One of the most fascinating aspects
of thermodynamics is that we can throw away all knowledge of individual
particles and instead discuss the average behavior of a group of particles.
If the number of particles is large (like the number of atoms in a 
cubic centimeter of normal stuff) this average description can be 
highly accurate and is easier to work with than trying to understand 
how every particle is behaving.  In the box example, we can associate 
the temperature with the average velocity of the particles in the box.

As far as black holes, it turns out that they aren't as black as once
thought.  When quantum mechanics is taken into account, it turns out
that a small number of particles can emerge from a black hole.  These
particles do not come from inside the black hole, rather, they're created
just at the surface of the black hole.  It turns out that it makes
good physical sense to associate the number of particles that escape
from the black hole with its temperature.  

Let me know if you'd like me to elaborate on any of this...

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