By PRAGYANSHARMA POLAVARAPU (Vrs.Somanchi)
Understanding nature of heat energy
was a puzzling problem even to scientists for two to three centuries. All they
understood was that heat is a mysterious thing and just a kind of energy. As
per old caloric theory heat was treated for a long period as an
invisible fluid called “caloric” which flowing from hot to cold bodies
and filling substances just as water fills the wet cloth and oozes from its
fibers.
To understand heat in a measurable
way, early scientists studied “level of heat” and “quantity of heat” in
analogy with “level of water” and “quantity of water” in a cubical water
tank. We can measure the level of water using a meter scale. We can
measure quantity of water as so many buckets of water, so many mugs of water,
so many liters of water etc. similar
Scientists continued with the idea
and devised methods to measure (1)“level of heat” (just like level of water in
a drum) and (2) quantity of heat (just like total quantity of water in a
drum). To understand the concepts let us consider the following
example.
(1)Heat a small needle to red
heat and drop it in a small cup of water. Water becomes only a little warm. The
“level of heat” in needle is very high but the “quantity of heat”
is very less.
(2)Now consider example of big drum
of water . We require a big fire made of a few logs of wood to heat
the water even to a lukewarm level. Here the “quantity of heat” produced by the
fire is of very large quantity but the “ level of heat” in the water(i.e.
temperature of water in the drum ) is very less . We
can put our hand in the lukewarm water in the drum.
MEASURING QUANTITY OF HEAT
(1)For measuring the “level of heat”
thermometers are available. In another essay the theory about construction of a
thermometer is explained. Just as a meter scale can directly read height ,the
thermometer can directly read temperature of water . We
simply immerse the thermometer up to a small level in the
water( till the mercury bulb is fully immersed) and then can directly
know the temperature from the readings .
(2)But measuring “quantity of heat”
is not so easy because it cannot be directly read from any scientific
instrument. we have to calculate it indirectly using formulas.
(3) Suppose water is being heated
in a metal vessel on a stove. First heat energy reaches from
the flame in to the metal body of the vessel. Then the heat
energy travels from the metal vessel into the water. (this is because
heat energy is communicated from one molecule to another among the billions
upon billions of molecules of water.
(4) The unit for measuring the
“quantity of heat ”is the “ calorie”. It is the “quantity of heat”
required to raise temperature of one gram of water just by one degree
centigrade.
(5)Scientists discovered after
several thousands of experiments that every substance like metal, wood, plastc,
rubber, stone etc requires a different quantity of heat energy to
raise the temperature of one gram of that substance by one degree centigrade.
(6)For example let us consider
individually 50 grams of all these substances—small blocks of metal,
wood, plastic, rubber and stone . Let us heat them in separate experiments in
the same range of tempeature—say from 20 degrees to 80 degrees centigrade in
every case. Scientists noticed that each substance requires a specific
different quantity of heat for raising the temperature in the same
range.This individual requirement of heat energy by one gram of each
substance to raise temperature by just one degree centigrade
is called the “specific heat” of that substance.
(7)The “specific heats”
of each metal is different . Scientists recorded
“specific heats” of several hundred substances in reference book called
“book of scientific constants”.
(8)The most important fact to
be noted in all these discussions is that “specific heat of water”
is one calorie.
Examples
(a)Suppose we heated 60
grams of water from 20 degrees to 30 degrees centigrade. The specific heat
of pure water is “1”.Then the quantity of heat that entered the water is
60x(30-20)x1=600 calories.
(b) Suppose we heated an iron block
of 50 grams weight from 40 degrees to 80 degree centigrade. Specific heat of
iron metal is 0.44. Then the quantity of heat that is required is 50x(80-40)x
0.44=880 calories
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