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6m_lab2

# 6m_lab2 - HEAT AND THERMODYNAMICS 1 The Absolute...

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1 HEAT AND THERMODYNAMICS 1. The Absolute Temperature Scale (Knight §16.3) In the textbook you have been introduced to the concept of temperature, and to the fact that there is a natural zero of temperature, the temperature at which the motion of mole- cules ceases. This temperature is called absolute zero . With a simple experiment we can determine the location of absolute zero on the Celsius scale, by making use of the ideal gas law , PV = Nk B T , where P is the pressure in N/m 2 , N is the number of atoms or mole- cules in the gas, k B is the Boltzmann’s constant ( k B = 1.38 x 10 -23 Joule/K), and T is the temperature in Kelvins. For our purposes the formula can be re-written p = Nk B V T If we change the temperature of a fixed volume of gas, then everything inside the paren- theses in the above equation will remain constant; the pressure should therefore be di- rectly proportional to the absolute temperature. We can use this fact to determine where absolute zero temperature must lie on the Celsius scale. (You likely already know, of course, that absolute zero = -273.15 Celsius, but sus- pend that knowledge for now -- this lab is where you get to prove it.) Simply take a fixed volume of gas, measure its pressure at two or more different temperatures, and extrapo- late backwards to find the temperature at which P would fall to zero. Experiment: You will find two sealed, stainless-steel bulbs, one containing helium gas and one con- taining neon gas. Each bulb has an attached pressure gauge which measures the pressure of the gas inside the bulb. Note that these gauges read absolute pressure, not “gauge pressure”. Read the pressure of each bulb at room temperature. (It won’t necessarily be one atmos- phere, since the bulbs are sealed and are not in contact with the air outside.) Immerse each bulb in a bath of ice water. Give them time to come to equilibrium, so that the gas inside the bulb is the same temperature as the ice water (0 degrees Celsius, of course.) How do you predict the pressure in the bulb will change? (You don’t have to make a quantitative prediction, just a general trend.) Once the bulbs have come to equi- librium at their new temperature, record the pressure that you actually measure for each

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6m_lab2 - HEAT AND THERMODYNAMICS 1 The Absolute...

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