answers-02-01-2011

answers-02-01-2011 - Answers to Physics 176 One-Minute...

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Unformatted text preview: Answers to Physics 176 One-Minute Questionnaires Lecture date: February 1, 2011 Why do scientists in different fields argue over the sign of work and heat? Argue is too strong a word. I dont know the history of why different conventions or choices of notation have been in thermodynamics or in other fields. Since the choices of sign for W and Q are really quite arbitrary, it is no surprise that different people have made different choices and then it is a political game of which researcher or journal or country has the most clout scientifically. And then there are communities like chemists, engineers, and physicists who largely dont coordinate their educational efforts and so differences are maintained. I am still confused about the distinction between heat and work. For example, if I light a candle below some container of gas, am I doing work on or adding heat to the system? Heat is a transfer of energy caused by a temperature difference between two objects. If you light a candle below a container of gas, you are transferring heat energy to the gas in two ways: by convection of hot air that rises above the candle flame and comes in contact with the gas container; and by radiation, the light from the hotter candle strikes the gas container and raises its temperature. (The gas container also radiates light in the form of blackbody radiation and so transfers some heat back to the candle via radiation.) The hot air rising from the candle can also do work, for example it can flow past a toy pinwheel and cause the pinwheel to spin. (This increases the kinetic energy of the pinwheel.) But if the hot rising air simply comes in contact with a container, it does no work on the container and so transfers energy only as heat. Im still confused about adiabatic processes, and how you begin with isothermal curves. Could you explain precisely why you have to begin with isothermal curves when explaining adiabatic pro- cesses? A related question was Would you explain again why the adiabatic curve has to lie above an isothermal curve? 1 Sorry if this was confusing. There was actually no need to relate the discussion of an adiabatic pro- cess to isothermal curves in the PV plane, all of the insights can be worked out directly from the key formulas that V P = constant and T f/ 2 V = constant for an adiabatic process. What I was describing in lecture was a pedagogical insight about why, during compression, an adiabatic curve must lie above an isothermal curve, when both processes start at the same initial point ( V ,P ) in the PV plane. Schroeder gives the same argument on page 25, see Figure 1.12 of the text. The idea is that you can think of the positive quadrant of the PV plane (where P > 0 and V > 0 as completely filled by adjacent constant-temperature hyperbola curves P = [ NkT ] /V . If you start at some initial point ( V ,P ) and follow a process for which the temperature in- creases (lets say over a tiny change V in volume), then the curve you...
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answers-02-01-2011 - Answers to Physics 176 One-Minute...

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