A reservoir is a supply or source of something
(think like reservoir lakes that have water for us
to drink). Water is often used as a heat
reservoir because it can hold a lot of heat.
This is hard to grasp, but one way to think of it
is that water can take in or gave out a lot of
heat without changing its temperature a lot--it
takes a lot of energy to heat up or cool down
water. For example, if you put a sealed bucket of
hot water and a sealed bucket of hot air in a
room, the water will stay warm much longer than
air. That is because it contains a lot more heat
that it has to give off to lower the temperature.
So this "heat reservoir" gives off a lot of
heat for a long time. The opposite is also
true: if you heat water up to a boil it takes a
long time, you have to put in a lot of heat from
the stove. On the other hand, the air around the
burner on the stove immediately gets really hot
because the air "holds" less heat as it
increases in temperature. Air is not a good
reservoir of heat.
This is related to a property called "specific
heat."Specific heat is the amount of energy you
need to raise the temperature of one gram of a
substance 1 degree Celsius. For example, it takes
about 4 joules of energy to raise water 1 degree
Celsius. This is pretty high. Also this amount of
heat energy is called one calorie (like in
You'll learn about this in high school if you take
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