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Geothermal: Classroom Activities

Exploring the Transfer of Heat from Rocks


Goal

To explore the transfer of heat from rocks to fluids.

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Background Information

In geothermal energy systems heat from Earth's interior is transferred from rocks to water deep groundwater. Hot water rises to the surface because of hydrothermal convection. This water is tapped using wells that steer the flow to the surface. Upon reaching the surface, the pressure acting on the water decreases and steam is released. The steam is used to drive turbines and generate electricity.

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Activity Overview

Students observe the responses of various kinds of rocks to heat by heating them either in water or in an incubator. Each type of rock is then carefully transferred into a Styrofoam cup that contains a measured volume of water. Students will have to ensure that certain variables are the same so that they can compare their observations. A lid is placed on the cup and a thermometer is set in the lid to record any change in the temperature of the water. Students use their observations to rank the samples according to which might contain the most stored energy.

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Materials and Equipment

Styrofoam cups with lids, water, thermometers, tongs, watch, graduated cylinders, various types of rocks (non-fissile) including coal, mass balance, hot water bath or hot box to heat rocks.

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Activity

  1. Ask students to observe safety procedures concerning hot materials.
  2. Explain to students that they must design a way of scientifically exploring the relative amounts of energy stored in heated rocks. They must conduct their experiments so that they can compare the heat released from various kinds of rocks. Use this opportunity to discuss with them the concepts of independent and dependent variables.
  3. Before heating the rocks, tell students to record the mass of each one using a mass balance.
  4. Have students heat the rock samples using the available apparatus. You may prefer to do this and then give the hot rock samples to the students. Remember that as soon as the rocks are removed from the heat source they should be placed in the water. In advance, cut a small hole in the edge of the lid for the thermometer.
  5. Ask students to heat the rocks at a constant temperature.
  6. Have students fill each of their cups with the same volume of water using a graduated cylinder. Have students measure the temperature of the water with a thermometer.
  7. Instruct students to use tongs to place one rock sample in each cup.
  8. Have students place the lid on each cup and insert a thermometer so that it touches the water but not the rock.
  9. Tell students to record their observations over selected intervals of time (e.g. every 10 seconds).
  10. Have students discuss their results as a class. Ask them the following:
    1. Which rock released the greatest amount of heat? Explain.
      Answers will vary depending on the rocks used. However, most rocks have a specific heat value of around 0.2 kcal/kg° C. In general, the lighter the molecules in minerals and rocks the more energy they require to raise their temperature. For this reason, coal has a relatively higher specific heat capacity than basalt or a brick being closer to 0.3 kcal/kg° C .
    2. What were the sources of error in the experiment?
      Typical sources of error are delays between heating and placing the rocks into the water. Other sources of error can be caused by discrepancies in the volume of water used.

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