7 Thermo 4

7 Thermo 4 - Heat of vaporization of water is huge Lv =...

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kJ 336 ) C 20 C 100 ( C kg kJ 2 . 4 kg 1 = ° ° ° = Δ = T mc E h Heat of vaporization of water is huge, L v = 2257 kJ/kg , compared with its specific heat of c = 4.2 kJ/kg °C. It takes a kettle 5 minutes to heat 1 kg of water from the room temperature of 20 °C to the boiling temperature of 100 °C. How much time would it take the same kettle to evaporate the same amount (1 kg) of water at the same rate of energy supply (neglecting all energy losses)? Energy to heat to 100 °C is Energy to vaporize the water is kJ 2257 kg kJ 2257 kg 1 = = = v v mL E So the total evaporation will take min 33 kJ 336 kJ 2257 min 5 = = = h v h v E E t t

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First law of thermodynamics Conservation of energy in mechanics: mechanical energy of a closed system is conserved Kinetic energy: 2 2 mv K = Potential energy ( U ) : mgh E p = 0 = Δ + Δ p E K
External forces and work they do. work of the external forces ext p W E K = Δ + Δ The system is not totally on its own (not closed or isolated). External forces can do work on it and the system can do work on external objects. sys ext p W W E K = Δ + Δ W sys is the work done by the system (in this case – against friction forces). If W sys > 0 the energy decreases.

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1 st law of thermodynamics. In thermodynamics we usually assume that kinetic and potential energy of the system do not change, or their changes are negligible: sys ext p W W E K = Δ + Δ In thermodynamics, we introduce a new type of energy – internal energy of the system, U , which is a sum of energies of microscopic components of the system. sys ext p W W U E K = Δ + Δ + Δ 0 = Δ + Δ p E K sys ext W W U = Δ We unify W ext and W sys by introducing the work W , which is positive if done by the system and negative if done by external forces W U = Δ
1 st law of thermodynamics. We unify W ext and W sys by introducing the work W , which is positive if done by the system and negative if done by external forces W U = Δ How do we convert work into internal energy? This is easy!

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necessarily need to do some work on it. There is a simple and common alternative – transferring heat,
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This note was uploaded on 12/13/2011 for the course PHYS 2C PHYS 2C taught by Professor Groisman during the Spring '11 term at UCSD.

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7 Thermo 4 - Heat of vaporization of water is huge Lv =...

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