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Thermal Properties - DOC

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					                                            Lab 1
                                   Thermal Properties
OBJECTIVES
 1. Observe phase transitions and the difference between heat of transformation and specific
    heat.
 2. Predict and measure the coefficient of thermal expansion.
 3. Predict and measure the latent heat of fusion and vaporization.

EQUIPMENT
 Thermal expansion apparatus, buckets of hot water and ice water, liquid nitrogen, Styrofoam
 cups, digital scale, DC power supply and power resistor, DataStudio and temperature probe,
 triple-beam balance.
THEORY
 All objects change size with changes in temperature. For a temperature change T, a change L
 in any linear dimension L is given by L = LT, where  is the coefficient of linear expansion.
 When heat is added to a substance, its temperature normally rises. This heat absorbed by an
 object is called the specific heat and is directly related to the object’s temperature change T.
 However, when a substance undergoes a change of phase, e.g., solid to liquid or liquid to gas, the
 heat energy goes into doing work against the intermolecular forces and is not reflected in a
 change in the temperature of the substance. This heat energy is called the latent heat of fusion
 Lf and the latent heat of vaporization LV for the phase changes that occur at the melting point
 temperature and boiling point temperature, respectively.

PROCEDURE
 Part 1: Thermal Expansion
 Study the apparatus in detail. When it is not being used, your group may use it.
 a. Your goal is to determine the coefficient of linear expansion of copper. Examine the working
     equation and determine what quantities you need to measure in order to solve for that
     coefficient. Set up your equation to solve for the coefficient in terms of your measured
     quantities.
 b. Use hot and cold water from the taps to adjust the temperature of the copper pipe mounted on
     clamps at the front of the classroom.
 c. There is a rotary dial attached to the rod that will measure length differences. If you are
     unsure of how to read it, ask John or Marcus.
 d. Compare your calculated value with the accepted value accepted = 16.6 x 10 -6/K using a
     percent difference. What factors contribute to uncertainty?

Part 2: Melting Ice and the Latent Heat of Fusion.
 a. Your goal is to determine the latent heat of fusion of ice. Again, find an expression that, when
    you plugged in your measured numerical values, would give you the latent heat. What values
    need to be measured before you can calculate? What assumptions did you make to get your
    expression?
 b. We will supply you with ice and liquid water, along with temperature probes for use in
    DataStudio.
 c. Hint: if you start with the water several degrees above room temperature and end with it
    several degrees below room temperature, the net heat loss to the room will be nearly zero.
    This improves your results – please explain why.



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 d. Compare your calculated value with the accepted value (Lf)accepted = 3.33  105J/kg using a
    percent difference. Were your assumption(s) valid?

Part 3: Boiling Nitrogen and the Latent Heat of Vaporization.
 a. Your goal here is to measure another latent heat, but now for vaporization of nitrogen rather
    than water.
 b. What you’ll do is supply heat (with a resistance heater) to a sample of liquid nitrogen at its
    boiling temperature and measure the amount of mass boiled off in order to calculate the latent
    heat. However, the temperature of the room will be adding heat also. You’ll want to find a
    way to compensate for the environment adding heat to your measurements of mass loss when
    the heater is switched on.
 c. Find an expression that will solve for the latent heat of vaporization of nitrogen in terms of
    quantities you will need to measure. Clearly state any assumptions.
 d. Connect the power resistor R (=10) to a DC power supply, adjust the voltage to Vsupply =
    10V, and turn off the supply.
 e. Some things to consider when capturing the effects of the environment:
              What insulation do you want the cup to have?
              Should the resistor be immersed or not?
              If so, how long should you wait before taking data?
              If the resistor is immersed, how else might heat be escaping?

 f. Take environmental data for about 3 minutes, turn on the power supply for exactly two
    minutes and continue recording the mass for an additional three minutes after turning it off.
 g. Plot your mass-time data and determine the mass of nitrogen boiled off by the resistor during
    the heating period
     Explain in detail what is physically happening in the graph and indicate on the graph the
        effects of the environment and the resistor on the liquid nitrogen..
 h. Determine the latent heat of vaporization of nitrogen Hint: Qresistor = Power  time =
    (Vsupply2/R)t.
 i. Compare your calculated value with the accepted value (Lv)accepted = 1.98  105J/kg using a
    percent difference. Were your assumption(s) valid?




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posted:8/31/2012
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