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373 Hw4 crib

373 Hw4 crib - HOMEWORK SET 4 CRIB Fall 2010 1 The system...

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HOMEWORK SET 4 CRIB Fall 2010 1) The system does work by lifting a 1lb weight 1ft using an internal battery with a power output of 1W (which produces both work and heat). a) = -(0.454 kg)(9.8 m/s 2 )(0.305 m) W= -1.36 J The sign is negative in this case because we define work being done by the system on the surroundings as negative. b) Δ U = Q + W , in part a) we obtained W= -1.36 J, and the total energy change is Δ U = 1 J/s x 2 s = -2 J so, -2 J = Q +(-1.36 J) and thus, Q= -0.64 J Again the sign is negative because heat is dissipated from the system to the surroundings.

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c) = (-0.64 J)+(-1.36 J) Δ U= -2 J Again, sign is negative because system loses energy to the surroundings (in the form of both work and heat exchanges). d) We would expect the temperature to increase if the system were not connected to the heat sink. The heat capacity of any material tells you how much heat it would take to raise its temperature by one degree. So, it makes sense that any material would have a positive heat capacity, and so we expect the temperature of the system to increase if it were not able to exchange heat with the surrounding isothermal bath.
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