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Unformatted text preview: Homework #1 Solutions Question 1a) The absolute temperature scale is based on the idea that classically, all molecular motion will cease at the absolute zero of temperature. The temperature is identified from plots of pressure vs. temperature of a gas, and extrapolating to zero pressure (i.e. no molecular motion) . . . . P T 1b) The first law of thermodynamics is a statement of conservation of energy for heat flow and work: Δ U = Q + W The change in internal energy of a system comes about because of heat added to the system Q , plus the work done on the system. Heat and work represent energy in transit and are not thermodynamic state variables. Energy is a thermodynamic state variable. In thermodynamics the only two ways to transfer energy to a system are by means of heat (the transfer of energy that occurs spontaneously due to a difference in temperature) and work (which is all other methods of transfering energy besides heat, and requires the action of an ’agent’). Common Errors: Some students did not mention ’work’. Work should also be included in the lst Law. Question 2a) C water (@0 o C ) = 4 . 22 kJ/kg- K 2b) C ice (@0 o C ) = 2 . 05 kJ/kg- K 2c) L ice =- 333 . 6 kJ/kg 2d) C iron (@25 o C ) = 0 . 450 kJ/kg- K 2e) κ glass = 0 . 8 W/m- K Question 3a) The heat capacity at constant volume is the rate of change of the internal energy with temperature: C V = dU dT ; [ C V ] = kJ K The specific heat is the heat capacity per unit mass [ C V ] = kJ kg × K 1 Assuming that the heat capacity doesn’t change significantly over the range of temperatures we are con- sidering, we can replace the infinitesimal quantities by finite changes. Then the amount of heat required tosidering, we can replace the infinitesimal quantities by finite changes....
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