# Chapter 12: Laws of Thermo

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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)