Answer:
Out of the many specific heats of a gas, two are of special significance : one when the gas is heated at constant volume and another when the gas is heated at constant pressure. These are known as the two principal specific heats of the gas and may be defined as follows : (i) Molar specific heat at constant volume. It is defined as the amount of heat required to raise the temperature of 1 mole of a gas through \[1{}^\circ C\]at constant volume. It is denoted by \[{{C}_{V}}.\] (ii) Molar specific heat at constant pressure. It is defined as the amount of heat required to raise the temperature of 1 mole of a gas through\[1{}^\circ C\] at constant pressure. It is denoted by \[{{C}_{P}}.\] When a gas is heated at constant volume, all the heat supplied is used to increase the temperature or internal energy of the gas. When a gas is heated at constant pressure, the gas expands. It does work against the external pressure. The heat supplied is used in two ways : (i) partly to increase the internal energy and (ii) partly to do work against external pressure. So at constant pressure, an additional amount of heat equivalent to work done is utilized. Hence more amount of heat is required to increase the temperature of 1 mole of a gas at constant pressure than that at constant volume i.e., \[{{C}_{P}}>{{C}_{V}}\]. The difference between the two specific heats is equal to the amount of heat equivalent to the work performed by the gas during expansion at constant pressure.
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