Chapter 12: Laws of Thermo. Law of Conservation of energy Change in U (internal energy) = Q – W. Q...

Chapter 12: Laws of Thermo

Law of Conservation of energy

Change in U (internal energy) = Q – W. Q = amount of heat added so this is positive when

heat added and negative when heat leaves the gas. W = Work done by the gas. If the gas expands, work

done by the gas is positive. If work is done on the gas (the gas contracts), then W is negative (the internal energy U goes up).

Work is done only when gas expands or contracts.

Transfer of energy into a gas

Energy can be transferred in 2 ways. Work can be done on a gas to compress the

gas and raise the temperature. Heat can be added to the gas to raise the

temperature. If the internal energy of the gas changes, the

temperature must also change (U = 3/2nRT

Work done BY a gas

Work done when a gas expands Work = force x distance , Force = P x area, and

area x distance = volume, so Work = P x change in Volume.

W = P (Vf –Vi) W = area under the PV diagram. Work is positive if the gas expands, negative if the

gas contracts. The amount of work depends on how you get from

the initial to the final states.

Isothermal process= constant temp 15-2

If the temperature does not change, then internal energy also does not change, so Q = W

Work done by the gas = heat added as the gas expands and pushes the piston up.

Types of processes

Cyclic process: process returns to its original state after the cycle. Therefore, the internal energy U doesn’t change. Then Q = W.

Adiabatic: No heat is transferred so Q = 0. This yields change in U = -W.

Isobaric = process at constant pressure where the work done is then W = P(Vf-Vi)