Just about everyone has used rubber bands, but few people
have taken the time to observe the less obvious properties of these everyday
objects. In this activity you will examine the thermal properties of rubber,
that is, the behavior of rubber as it relates to heat, a form of energy.
In the first experiment you will attempt to detect heat
flow into or out of a rubber band. To do this, you need a rather sensitive heat
detector. Fortunately, you have such a detector with you at all times. Surely,
you've felt the heat of a flame or the cold of an ice cube. Therefore, you know
that your skin is sensitive to heat flow. In this experiment, you will detect
heat flow using some of your most sensitive skin, that on your forehead or on
your lips.
- Place your thumbs
through the heavy rubber band, one on each end. Without stretching the
band, hold it to your forehead or lip. Does the band feel cool or warm or
about the same as your skin? Repeat the test several times until you are
sure of the result.
- Move the rubber band
slightly away from your face, so it is not touching your skin. Quickly
stretch the band about as far as you can and, holding it in the stretched
position, touch it again to your forehead or lip. Does it feel warmer or
cooler or about the same as it did when it was relaxed?
- Move the stretched
rubber band away from you face. Quickly let it relax to its original size
and again hold it to your skin. Does it feel warm or cool?
- Repeat the stretching
and testing, and relaxing and testing several times until you are sure of
the results.
An object feels cool or cold to you when heat flows from
your skin to the object. Conversely, an object feels warm or hot when heat
flows from the object into your skin. If the stretched rubber band feels cool,
then it absorbs heat from your skin. If it feels warm, then it gives off heat
to your skin. If the band feels neither warm nor cool, then there is no
detectable heat flow. These three cases can be represented as follows:
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Case 1.
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Relaxed Band + Heat  Stretched Band
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Case 2.
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Relaxed Band Stretched Band + Heat
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Case 3.
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Relaxed Band Stretched Band (No Heat)
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Which of these three cases best
describes what you observed?
There is another way to test which of the three statements
is correct. We can see what happens to the length of a rubber band if we heat
or cool it.
- Hang one end of the
rubber band from the wall or ceiling and suspend a weight from the other
end of the rubber band. (What you use for a weight will depend on what is
available. The weight should be heavy enough to stretch the rubber band,
but not so heavy that it is likely to break the band. For example, hang the
band over a door knob and suspend a hammer from the band.)
- Heat the rubber band
with a hair dryer. Start the dryer and, when it has warmed up, turn its
heat on the stretched rubber band. Does the stretched rubber band become
longer or shorter when it is heated?
Does this observation agree with what you found in the
first part of the experiment? Doing an experiment several ways and checking for
agreement in the results is an important strategy in science.
When rubber is heated it behaves differently than most
familiar materials. Most materials expand when they are heated. Consider the
liquid in a thermometer. The thermometer works because the liquid expands when
its temperature increases. Similarly, a wire made of metal, such as copper,
becomes longer as it gets hotter. The expansion of metals with increasing
temperature is the principle behind the functioning of home thermostats and of
jumping discs.
Whether a material expands or contracts when it is heated
can be ascribed to a property of the material called its entropy. The entropy
of a material is a measure of the orderliness of the molecules that make up the
material. When the molecules are arranged in an ordered fashion, the entropy of
the material is low. When the molecules are in a disordered arrangement, the
entropy is high. (An ordered arrangement can be thought of as coins in a
wrapper, while a disordered one as coins in a tray.) When a material is heated,
its entropy increases because the orderliness of its molecules decreases. This
occurs because as a material is heated, its molecules move about more
energetically. In materials made up of small, compact molecules, e.g., the
liquid in a thermometer, as the molecules move about more, they push their
neighboring molecules away. Rubber, on the other hand, contains very large,
threadlike molecules. When rubber is heated, the sections of the molecules move
about more vigorously. In order for one part of the molecule to move more
vigorously as it is heated, it must pull its neighboring parts closer. To
visualize this, think of a molecule of the stretched rubber band as a piece of
string laid out straight on a table. Heating the stretched rubber band causes
segments of the molecules to move more vigorously, which can be represented by
wiggling the middle of the string back and forth. As the middle of the string
moves, the ends of the string get closer together. In a similar fashion, the
molecules of rubber become shorter as the rubber is heated, causing the
stretched rubber band to contract
