The point was made early in this set of pages that
thermal energy is hard to notice. It takes 50,000 Joules to
get a cup of coffee hot, which seems like a much less
exciting event than shooting the coffee 50 Km into the air
(This also requires 50,000 Joules)! However, with the aid
of the thermal sensing sheet we have been able to detect thermal energy:
A cup of hot coffee on a table leaves some of its
energy on the table top, and we leave warm fingerprints on
everything we touch
It takes energy to erase a pencil mark, which we supply
scrubbing the eraser back and forth; the energy remains on
the paper in the form of a warm mark which we can detect.
Likewise, the energy of a hammer is converted into thermal
energy when it hits a board, leaving a hot spot.
Wires carrying a current get warm, due to electrical resistance.
In our example, we didn't give the electrical circuit anything else to
do, and so all the energy was converted into heating of the wire.
This is how an electric stove or a toaster makes the thermal energy it uses to cook our food.
Electrical circuits are designed to deliver most of the energy
to do something useful; however, a significant fraction of all of
the energy leaving a power plant is converted into thermal energy in the wires.
Light delivers energy. Incandescent light bulbs (the
kind with a hot glowing filament as the source of light) are
actually much better heaters than illuminators -- most of the
energy is in the form of invisible infrared light. However,
the thermal sensing sheet sees energy, not light, and can
detect this energy easily.
The idea that energy is conserved is relatively new to
science. Optics, the laws of motion, electricity, and
magnetism were all well on their way to being understood
before this idea was discovered, even though it is now
regarded as fundamental. Part of the reason must have
been that thermal energy was so hard to detect, that when
other kinds of energy turned into thermal energy it just
seemed to disappear. We hope that the thermal sensing
strip and the activities of this section have made thermal
energy seem more real, and made it easier to believe in the
conservation of energy.