13.3 Mechanical Equivalent of Heat

Historical development of Mechanism and Heat lead to different units of measurements for mechanical energy and heat energy. In the last century it was recognized that heat is also a form of energy and it can be converted into mechanical energy and vice versa. From Joule's experiments the correlation between the units of 1 Joule and 1 Calorie was found to be

1 Calorie = 4.18 Joule

Therefore, the numerical relation between the amount of work or mechanical energy W (Joule) and amount of heat energy Q (Calorie) can be expressed as where J = 4.18 Joule / Calorie. The constant J is called Mechanical Equivalent of heat. It is also known as Joule's constant.

Solved Problem

1. How much heat is released when steam of mass 1 gm at 100⁰ C is converted to ice at 0⁰C? `L' of condensation of steam is 540 calorie / gm, freezing of water is 80 calorie / gm and specific heat of water is 1 calorie / gm ´ ⁰C.

Solution :

Q =  Heat Released by steam at 100⁰ C to become ice at 0⁰C.

    = Heat Released from steam to water at 100⁰C + Heat        Released from water to cool from 100⁰C to 0⁰C + Heat        Released from water to freeze into ice at 0⁰C.

    = m ´L_vaporisation + m ´ c ´ (100 - 0) + m ´ L_fusion

    = 1 ´ 540 + 1 ´ 1 ´ 100 + 1 ´ 80

    = 720 calories

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Index

13.1 Heat Capacity, Water Equivalent & Specific Heat
13.2 Latent Heat of Fusion, Freezing, Vaporization and Condensation
13.3 Mechanical Equivalent of Heat

Chapter 14