lecture23

# lecture23 - Phys 2101 Gabriela Gonzlez A force acting on a...

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1 Phys 2101 Gabriela González 2 A force acting on a system can heat it up (or cool it down), by working on it. A change in temperature produced in a system can be used to produce mechanical work. At any point in the process, the system (gas) will have temperature T, pressure p and volume V. dW = Fds = pAds = pdV W = pdV V i V f

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2 3 Changing the system’s pressure and volume from an initial to a final state can be done using different amounts of work (and heat transferred): Going back to the original state does not mean that no work was done! W = pdV V i V f 4 The work W done by the system during a transformation from an initial state to a final state depends on the path taken. The heat Q absorbed by the system during a transformation from an initial state to a final state depends on the path taken. However, the difference Q – W does not depend on the path taken! We define this quantity as the change in “internal energy”: Δ E int = Q – W The internal energy of a system increases if energy is added as heat, and decreases if energy is lost as work done by the system.
3 5 The figure shows four possible paths to take a system from an initial state to a final state. Rank the paths according to: the change in internal energy the work done by the system the magnitude of energy transferred as heat 6 First Law of thermodynamics: Δ E int = Q – W Special processes: 1. Adiabatic: Q=0 Δ E int = – W thermally insulated system, or very quick process 2. Constant Volume : W=0 Δ E int = – Q 3. Cyclical : Δ E int = 0 Q= W 4. Free expansion: Q = W = 0 Δ E int = 0 happens in isolated systems, it’s irreversible.

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4 7 Figure 19-39 a shows a cylinder containing gas and closed by a movable piston. The cylinder is kept submerged in an ice water mixture. The piston
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lecture23 - Phys 2101 Gabriela Gonzlez A force acting on a...

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