Heat and Expansion
Why does heat make particles expand? I
know that it increases the particles movement and
all that, but why?
Looking at your question, there may be some misconceptions. You asked
why heat makes particles expand. If, by "particles," you mean atoms or
molecules, the answer is that heat does NOT make atoms or molecules
expand. Heat simply causes atoms and molecules to vibrate more (this
is the increased movement you refer to); it does not cause the actual
atoms or molecules to expand.
Molecular movement or vibration is a fundamental result that occurs
when energy is absorbed by an atom. The energy that is absorbed cannot
just disappear; it causes the atom to increase its movement or
vibration, and we perceive this as increased heat.
Most substances do expand when heated, but not all do. This expansion
is caused (in simple terms) by the atoms or molecules needing more
"room" when they vibrate more at higher temperature. Not all
substances expand when heated, though. For example, if you heat water
at 0°C (32°F) it actually contracts until it reaches 4°C (39°F).
Heating further does cause expansion. Similarly, some metal alloys
contract when heated over a wide temperature range as a result of
complex interactions in the metal's crystals.
Heat is a transfer of energy. This energy when absorbed by objects tend to be
expressed or manifested as an increase in temperature. Temperature is a measure
of the average motion of particles within an object. So heat will often cause
particles to move faster.
In the case of solids, the particles in the solid will vibrate in place (they do
not move away from their original center, just vibrate back and forth around the
same center). So, imagine that a particle of a solid now vibrates faster and
takes up more space, the other particles do the same. As such, while the number
of particles remain the same, the size of the particle also remains the same,
the space taken up -due to the stronger vibration makes each particle take up
more space (without actually getting bigger). As a result the object expands in
In gases, heat allows the gases particles to move faster. They will hit the
walls of the container with more force. If the walls are allowed to move, then
the walls will actually move to a bigger volume because of the much stronger
striking of the gas particles against the walls. Again the container becomes
Liquids do the same.
Greg (Roberto Gregorius)
I am sure, by now, that some of the other scientists at this site have given
you good answers, and have distinguished between particles expanding
(a misconception) and the bulk material expanding.
In materials science, you will learn that much of matter is crystalline in
nature. As materials increase in temperature, the additional vibrational
energy of the particle can be great enough that it moves over one position
in the lattice structure. This creates a vacancy. If enough vacancies are
created on the atomic or molecular scale, a change in the geometry of the
bulk material can be measured.
---Nathan A. Unterman
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Update: June 2012