Ask A Scientist Physics Archive

name Eva M.
status educator
age 50s
Question - I would like to know what heat and cold are. I have
read that cold is the absence of heat. Is this true?
---------------------------------
Heat is energy. Usually we think of heat as the fraction of the total
energy possessed by a particle that can easily be given to another
particle in a collision. Thus, if some molecule is whizzing across the
room and it whacks into another molecule that is moving more slowly, the
slow molecule will generally move faster after the collision, while the
fast molecule will generally move slower. Energy will have been
transferred from the fast molecule to the slow molecule, and that is the
sort of energy that is useful to think of as heat.

In another example, suppose there is a molecule that behaves like two
masses connected by a spring, and suppose that the molecule is not going
anywhere, but that its masses are spinning around and oscillating in and
out. Clearly, the molecule has energy. Now if some less energetic
molecule drifts by and gets whacked by one of the masses, the whackee
generally will gain some energy at the expense of the whacker. This is
also heat transfer.

Now that was a microscopic view of heat energy. How about two huge
collections of molecules: one whose molecules are, on average, zipping
around rapidly, spinning, and oscillating; and one whole molecules are, on
average, just slowly drifting around.
We would say that the zipping-around collection is hot, and the
slowly-drifting collection is cold. What would happen if we put the
zipping-around collection right next to the slowly-drifting
collection? Let us let them collide, but not intermingle. When zippers
collide with drifters, as we have seen, zippers generally lose energy
while drifters generally gain it. Sooner or later, both collections will
end up with the same average energy -- the same temperature. In this
macroscopic view, heat is what gets transferred from a hot thing to a cold
thing, when the two things touch.

Tim Mooney 
=====================================================
Eva -

There is no real scientific use (or definition of) cold. You are
right. It is often viewed as the absence of heat.

Heat is a measure of the motion of (i.e. the kinetic energy of) molecules.
When heat is added to a substance, molecules move more. Seeing that
kinetic energy is related to mass and velocity, the kinetic energy present
in a substance increases.

When heat is removed from a substance the molecules move more slowly.
Ultimately... and theoretically... the molecules would stop. This is
absolute zero. The kinetic energy in the molecule (because the velocity
is zero) would be zero. Anything times zero is zero.

Yet, to all of us a winter day is still cold!

Larry Krengel 
=====================================================
Eva,

Heat is just one of the ways we describe energy -- that which offers the capacity for change. 
Indeed, cold represents a relative absence of (heat) energy. The greater the dearth, the more 
intense is our description of "cold."

Regards,
ProfHoff 705
=====================================================
Heat and Cold are how we perceive the flow of energy, much like wind is how we perceive the 
flow of air.

If energy flows into us we perceive it as heat.  If energy flows out of us we perceive it as cold.  
In both cases it is the flow of energy from one object to another.  The object that loses energy is
 perceived as "hot" from the viewpoint of the object that gains energy and the object that gains 
 energy is perceived as cold from the perspective of the object that loses energy.

When energy is gained the temperature usually rises, when energy is lost the temperature usually 
drops

Greg Bradburn
=====================================================
Eva,
To understand heat and cold requires two concepts:  energy, temperature. Molecules vibrate 
around in all things.  Increasing the temperature of an object is making the molecules vibrate 
more quickly.  This vibration is a way to store energy for later use.

Heat is the transfer of energy that results from these vibrations.  If an object with very fast 
vibrations touches an object with very slow vibrations, the quickly vibrating molecules will 
bang against the slow molecules, making them vibrate faster.  This is what happens when your 
finger touches something warm.  It is also what happens when your finger touches something 
cool, but in reverse.  The energy travels from your finger to the cool object.

Something feels hot if it has a great deal of energy stored in vibrating molecules, ready to 
pass into other things.  Something feels cold if it does not have much of this stored energy 
and is ready to take in energy from things around it.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College
=====================================================