Unified Engineering Thermodynamics Bonus Quiz 1

Fall 2005

NAME SOLUTIONS

1) Explain in physical terms why c_v<c_p for a gas. In your explanation, please make reference to different forms of energy and heat and work.

The constant volume specific heat, c_v, is the heat that must be transferred to a kilogram of a substance to raise it’s temperature by 1K in a constant volume process. The constant pressure specific heat, c_p, is the heat that must be transferred to a kilogram of a substance to raise it’s temperature by 1K in a constant pressure process.

Gases are compressible (that is, by pushing on them it is possible to change their volume). Therefore, when heat is added to a gas at constant pressure (as opposed to constant volume) the gas expands. In doing so, it does work on the surroundings which tends to lower the energy of the system (per the First Law of Thermodynamics), thus it takes more heat to increase the internal energy of a gas for a constant pressure process than for a constant volume process.

2) Describe what enthalpy is in physical terms. When is it useful?

Enthalpy is the amount of energy that is transferred across a system boundary by a moving flow. This energy is composed of two parts: the internal energy of the fluid (u) and the flow work (pv) associated with pushing the mass of fluid across the system boundary. Because enthalpy is a measure of the energy that is transferred across a system boundary by a flow, it is most useful for flow processes involving open systems, e.g. applications of the steady flow energy equation