Figure 32 - One British thermal unit is the

quantity of heat required to raise the

temperature of 1 Ib of water 1°F

Specific Heat.- So far we have been dealing with a single substance, viz., water. The amount of heat required to raise the temperature of 1 g of another substance 1 °C may be compared with the amount of heat required to raise the temperature of 1 g of water from 15 °C to 16 °C. Such a comparison gives a definition of what is called the specific heat of the substance. The specific heat of a substance is numerically equal to the number of calories required to raise the temperature of 1 g of the substance 1 °C, or, what amounts to the same thing, the number of British thermal units required to raise the temperature of 1 lb of the substance 1 °F.

Let Q denote the quantity of heat added to a mass of M g, let t and t’ be the initial and final temperatures, and let S be the specific heat of the body. Then

Q = SM(t — t’)

Heat of Combustion. - The heat of combustion is theheat liberated by burning unit mass or unit volume of a fuel, such as coal or gas. To find it, the fuel is placed in a crucible С inside a bell jar, which is closed so that the products of combustion cannot escape except through the openings at the base of the jar. The bell jar is placed inside a vessel, the mass of which is known, and this vessel is then filled with a known weight of water. The temperature of the water is determined, and then a supply of oxygen is admitted through the opening at the top of the bell jar until all the fuel has been burned. The products of combustion bubble up through the water. When the combustion is complete, the temperature of the water is again observed. From these data the heat of combustion of the fuel can be found.

mass of water x temperature change x sp ht

Heat of combustion = –––––––––––––––––––––––––––––––––––

mass of fuel

where sp ht = specific heat

In this expression for the heat of combustion, it is assumed that the water equivalent of the calorimeter is so small that it can be neglected in comparison with that of the water in the calorimeter. In case this is not true, the water equivalent of the calorimeter must be added to that of the water in the calorimeter [2, С. 84 - 88].

Questions to the text “Heat Is a Form of Energy”:

1. What type of energy is heat: kinetic or potential?

2. Why do gases play an important part in the study of heat?

3. Who was the author of mechanical theory of heat?

4. Regarding what standard is temperature usually estimated?

5. What is the principle of mercury thermometer operation?

6. What is the difference between Centigrade and Fahrenheit Thermometers?

7. What unit is more convenient to measure heat?

8. What is calorie?

9. What is the British thermal unit?

10. What is the specific heat of a substance?

11. How can we find the heat of combustion?

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