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S4-“LEVEL OF HEAT ” AND “QUANTITY OF HEAT “

 

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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