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Thermodynamics: Energy Transfer (page 2)

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Author: Janice VanCleave

Why?

According to thermodynamics (the study of energy movement), when two materials at different temperatures are placed together, such as the hot nails and the cold water, heat is transferred from the hotter to the colder material. This exchange of heat continues until both materials finally reach the same temperature. If you assume that no heat is lost to the surrounding area, then the heat gained by the water is lost by the nails. The mixture is an example of the first law of thermodynamics, which states that the total energy of the isolated system is constant. The energy remains constant because as the energy of one part of the mixture increases, the energy of another part decreases.

Try New Approaches

  1. Does the type of metal affect the results? Repeat the experiment replacing the iron nails with nails of equal size but that are galvanized or made from other metals such as aluminum.
  2. Does the quantity of the substances affect the temperature change?
    1. Repeat the original experiment two times, first decreasing the amount of cold water in the bowl by half, and then using twice as much water.
    2. Repeat the original experiment two more times, first using one nail, and then using nine nails.
  3. Science Fair Hint: Display photographs with temperature labels to represent the procedure and results.

Design Your Own Experiment

  1. The reason for the difference in the change of temperature of the water and the metals is due to their specific heats (energy required to raise the temperature of one gram of matter one degree Celsius). The value of the specific heat varies for each substance, and water has one of the highest specific heats. Thus, more heat has to be gained or lost to change the temperature of water.
  2. Demonstrate this by filling three Styrofoam® cups each three-fourths full, one with water, one with sodium chloride (table salt), and one with sucrose (table sugar). Allow the cups to stand at room temperature until the temperatures of the contents of the cups are equal. Place the cups in a freezer for two hours. Remove them from the freezer and insert a thermometer in each. Record the temperature immediately and every 15 minutes until one of the materials reaches room temperature.

  3. Land and sea breezes are caused by the difference in the specific heats of water and soil. During the day, the land heats more quickly than the sea. Hot air above the land rises, and cooler air above the sea rushes in to take the place of the rising warm air. This air movement produces a sea breeze. At night, the land cools faster than the sea. The hotter air above the sea rises, and the cooler air above the land rushes toward the sea to create a land breeze.
  4. Demonstrate this by filling two Styrofoam® cups half full, one with water and the other with soil. Place a thermometer in each cup and set the cups together on a table until their temperatures are equal. Position a desk lamp so that the light evenly hits both cups (see Figure 28.3). Record the temperature on each thermometer after two hours. Turn the light off and record the temperature on each thermometer every 15 minutes for one hour. Display photographs of this experiment along with diagrams showing the movement of air during sea and land breezes.

  5. The heat of materials can be calculated with the following equation:
  6. heat (Q) = mass × specific heat × temperature change

    The specific heat value for water is one calorie per one gram per one degree Celsius. Thus, it takes one calorie of heat to change one gram of water one degree Celsius. With the specific heat of water and the fact that in a mixture the heat lost equals the heat gained, Q (gained) = Q (lost), the specific heat of a substance can be calculated.

    Demonstrate by cooling or heating a substance with water. Weigh 50 ml of corn syrup and pour it into a Styrofoam cup. Place the cup of syrup in a freezer for one hour and then determine its temperature. Pour 50 ml of hot tap water into a measuring cup and determine its temperature. Add the hot water to the cold syrup. Stir. Then cover with a second Styrofoam cup. Tape the cups together and insert a thermometer through a hole in the top cup. Stir the solution with the thermometer until the temperature remains constant Record the final temperature.

Calculate the specific heat of the syrup, assuming the Styrofoam cup does not change temperature. See Appendix 12 for an example calculation.

Thermodynamics: Energy Transfer

Get the Facts

  1. The heat required to raise the temperature of a substance varies depending on its phase (solid, liquid, or gas). Use a chemistry text to find out more about specific heats. You could display a chart comparing the values of common materials.
  2. Heat is not always where you want it or when you want it Solar heating is a prime example. The windows on the south side of a solar home receive more direct sunlight than those on the other sides, and the sun does not shine at night Solar homes need a way to store solar energy. Find out more about materials used for heat storage. What materials make good "heat sponges" for absorbing heat during the day but cooling down slowly at night? What are the advantages and disadvantages of some of the materials used to store heat?
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