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conantee
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« on: December 31, 2009, 07:09:27 AM »

14. For an ideal gas, the specific heat at constant pressure C_p is greater than the specific heat at constant volume C_v because the
(A) gas does work on its environment when its pressure remains constant while its temperature is increased
(B) heat input per degree increase in temperature is the same in processes for which either the pressure of the volume is kept constant
(C) pressure of the gas remains constant when its temperature remains constant
(D) increase in the gas's internal energy is greater when the pressure remains constant than when the volume remains constant
(E) heat needed is greater when the volume remains constant than when the pressure remains constant
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conantee
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« Reply #1 on: December 31, 2009, 07:16:53 AM »

Let's remind ourselves what the specific heat is. The specific heat is the amount of heat needed in order to increase temperature of a system by a certain degree (unit is J/K). The amount of heat is related to work done by a system and its internal energy via the 1st law of thermodynamics (conservation of work-energy).

At constant volume, the work done by a system is absent. However, at constant pressure, as the temperature increases, the volume also increases according to ideal gas law. That is, the system or gas does work at constant pressure while its temperature is increased. By 1st law of thermodynamics, the total amount of energy needed for constant pressure becomes higher since it is required for work done by gas.

The correct answer is (A), and 42 of 100 people got the right answer.
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