W12≠W12 II Volume Work depends on the path Work is area under curve of P(v) a)Work is path dependent; b) Properties only depend on states: c) Work is not a property, not a state variable d) when we say W1-2, the work between states I and 2, we need to specify the path e)For irreversible(non-reversible)processes, we cannot use PdV; either the work must be given or it must be found by another method Muddy points How do we know when work is done?(MP 0.3 10) Heat Heat is energy transferred due to temperature differences a) Heat transfer can alter system states b)Bodies dont"contain"heat; heat is identified as it comes across system boundaries c) The amount of heat needed to go from one state to another is path dependent d) Heat and work are different modes of energy transfer ferred e) Adiabatic processes are ones in which no heat is transfer I1) First Law of Thermodynamics For a system AE=Q-w E is the energy of the system Q is the heat input to the system, and w is the work done by the system E=U(thermal energy)+ Ekinetic Potential+ If changes in kinetic and potential energy are not important a)U arises from molecular motion b)U is a function of state, and thus au is a function of state(as is 4E c)Q and w are not functions Comparing(b)and(c) we have the striking result that d )40 is independent of path even though Q and w are not! 0-50-5 P Volume Work is area under curve of P(V) V1 V2 W1-2 ≠ P Work depends on the path V I I II W1-2 II a) Work is path dependent; b) Properties only depend on states; c) Work is not a property, not a state variable; d) When we say W1-2, the work between states 1 and 2, we need to specify the path; e) For irreversible (non-reversible) processes, we cannot use ∫PdV; either the work must be given or it must be found by another method. Muddy points How do we know when work is done? (MP 0.3) 10) Heat Heat is energy transferred due to temperature differences. a) Heat transfer can alter system states; b) Bodies don't "contain" heat; heat is identified as it comes across system boundaries; c) The amount of heat needed to go from one state to another is path dependent; d) Heat and work are different modes of energy transfer; e) Adiabatic processes are ones in which no heat is transferred. 11) First Law of Thermodynamics For a system, ∆E QW = − E is the energy of the system, Q is the heat input to the system, and W is the work done by the system. E = U (thermal energy) + Ekinetic + Epotential + .... If changes in kinetic and potential energy are not important, ∆U = Q −W . a) U arises from molecular motion. b) U is a function of state, and thus ∆U is a function of state (as is ∆E). c) Q and W are not functions of state. Comparing (b) and (c) we have the striking result that: d) ∆U is independent of path even though Q and W are not!