Gas Processes

Gas Process The thermodynamic state of a gas is defined by pressure, volume, and temperature. A “gas process” describes how gas gets from one state to another state. Processes depend on the behavior of the boundary and the environment more than they depend on the behavior of the gas.

T1T1 T2T2 T3T3 Gas “isotherms” Isothermal Process (constant temperature) P V PV = nRT  T = 0 (constant T) Initial State of Gas Final State of Gas Isothermal Process

Isobaric Process (constant pressure) P V T1T1 T2T2 T3T3 PV = nRT  P = 0 (constant P) Isobaric Expansion Isobaric Contraction

Isometric Process (constant volume) P V T1T1 T2T2 T3T3  V = 0 (constant V) PV = nRT

Adiabatic process (insulated) P V T isotherm adiabat Q = 0 (no heat enters or leaves) Temperature, pressure, and volume all change in an adiabatic process. PV = nRT

Work Calculation of work done on a system (or by a system) is an important part of thermodynamic calculations. Work depends upon volume change. Work also depends upon the pressure at which the volume change occurs.

p Work done BY gas VV p W gas = p  V W env = -p  V Positive work Negative work

p Work done ON gas VV p W gas = p  V W ext = -p  V Negative since  V is negative Positive since  V is negative

Sample problem Calculate the work done by a gas that expands from m 3 to 0.80 m 3 at constant atmospheric pressure. How much work is done by the environment when the gas expands this much?

Sample problem What is the change in volume of a cylinder operating at atmospheric pressure if its internal energy decreases by 230 J when 120 J of heat are removed from it?

W CD = p 1  V Work (isobaric) P V P2P2 V1V1 A V2V2 B P1P1 C D W AB > W CD Where we are considering work done BY the gas W AB = p 2  V

W ACD Work is path dependent P V P2P2 V1V1 A V2V2 B P1P1 C D W ABD > W ACD Where we are considering work done BY the gas W ABD

Problem One mole of a gas goes from state A (200 kPa and 0.5 m 3 ) to state B (150 kPa and 1.5 m 3 ). – A) What is the change in temperature of the gas during this process?

Problem One mole of a gas goes from state A (200 kPa and 0.5 m 3 ) to state B (150 kPa and 1.5 m 3 ). – B) Draw this process, assuming the smoothest possible transition (straight line) for the process. – C) Estimate the work done by the gas during the process. – D) Estimate the work done by the environment during the process.

Sample Problem Draw the gas process from state A (200 kPa and 0.5 m3) to state B (150 kPa and 1.5 m3).