Science Content: What You Learn When You Measure
A Temperature
You would think we all knew what temperature is -- we have been
measuring it (and complaining about it) for years.
But what do we really learn from a thermometer?
It has these important aspects:
Temperature is a property of an object or substance that doesn't depend on how big
the object is, or what it is made of.
Temperature is not the same as thermal energy, but is an indicator of its presence.
Temperature influences the flow of thermal energy. If two objects are placed
in thermal contact (so that thermal energy can move from one to the other), energy
always moves from the higher temperature object to the lower; if no energy
moves, they are at the same temperature.
Understanding temperature may be easier using the following analogies with
water in lake:
What the level tells us, and what it doesn't tell us:
Knowing the level of water in a lake doesn't tell you the volume of water
that is in the lake, unless you know other things (how deep the lake is, and
how big in other dimensions) -- and yet if the lake level goes up, you
know that water was added.
Similarly, knowing the temperature doesn't tell you how much thermal energy is
in an object, unless you know other things (how big the object is, and
many details of how the object is put together) -- but if the temperature
of an object goes up, you know that thermal energy was added.
Consider the difference between warming a cup of water by one degree compared
with warming up Lake Michigan also by one degree. It takes enormously
more thermal energy to heat the lake than it does to heat the cupful, even though
they are both changing temperature by the same amount.
A difference in level influences the direction of flow:
When you connect two lakes together by a pipe, water will flow from
the higher lake to the lower lake. The flow will only stop when the
level of the lakes is exactly the same.
In the case of temperature, if you let two objects touch, energy will flow
from the higher temperature object to the lower temperature object.
Only when the two objects are at exactly the same temperature will
nothing happen.
There are many different ways to measure it:
You could measure the water level in a lake by setting up a tall post in the lake
and painting it with markings at even intervals. The amount of the post
that is visible would tell what the lake level is. Your neighbor
might choose to measure how far the shore line is from his back porch.
Figuring out how these two measurements are related is not simple, and yet
they agree in two important respects: when water is added to the lake, both
readings go up, and if your lake meter indicates the water level today
is exactly the same as it was two weeks ago, so will your neighbor's.
Similarly, there are many ways to measure temperature.
Almost every physical property of a material
changes in some way with the temperature, although
the change is usually small.
A thermometer is a device for which the effect is big enough to
see.
Some thermometers make use of thermal expansion -- the change in size of something
due to changes in temperature.
The red
liquid (alcohol) in an everyday thermometer expands slightly when heated,
taking up more space and so it rises in the thin glass tube. The
glass tube is designed to magnify this change enough to make it visible.