Heat Conduction

  1. Introduction

    The only remaining heat transport mechanism that we will study in this section is heat conduction. We will give the students an opportunity to see head conduction in action and to make measurements of this heat transport property.

  2. Objectives

    By watching cold water become warm and hot water become cool via heat conduction, the student will see that heat can be transported by a thermal conductor. The students will hopefully see that the rate of heat transport can be altered by the difference in temperature of the thermal sources, the cross-sectional area of the thermal conductor and the length of the thermal conductor.

  3. Materials
  4. Vocabulary
  5. Presentation

    When the students arrive, the instructor should have a beaker of boiling water on a ring stand and a beaker of ice water on a ring stand with several heavy copper wires between them.

    Invite the students to touch the copper wire. Start and the end that is submerged in the ice water and carefully move down the wire. BE CAREFUL. The rod can be very hot. Ask them to notice what is happening to the ice. It should be melting.

    When learning new concepts, we are often held back by old concepts. In this case, the students may struggle with the idea that we are not looking at an equilibrium case but a dynamic system. We are not interested in the end result but the rate at which the systems change.

    Review the steps in the scientific method. Ask the students to devise a hypothesis or hypotheses about the way the heat moves through a thermal conductor. They may have trouble getting to the point where they realize that they have to measure the temperature of the water as a function of time. Encourage them to devise and perform experiments to test their hypothesis. Some may choose to use a Bunsen burner to keep one end of the copper hot. Other may choose to start with cold water on one and hot (but unheated) water at the other. Don't let them waste too much time trying to find an equilibrium situation. Help them to understand this is a dynamic system and we are interested in the rate of change of temperature.


    Here is a quick math review for the instructor. The students should not see this because it involves calculus notation and they are to devise equations similar to these.

    The rate of heat transfer should be proportional to the difference in temperature between the two ends of the copper wire.

    Rate of heat transfer proportional 
to difference in temp.

    The rate of heat transfer should be proportional to the cross sectional area of the wire.

    Rate of heat transfer proportional 
to cross sectional conductor area.

    The rate of heat transfer should be inversely proportional to the length of the wire.

    Rate of heat transfer inversely 
proportional to length of conductor.
  6. Evaluation

    In class today, we discovered that the transfer of heat was faster when the temperature between the ends of the copper wire was larger. We also discovered that when we used more wires or a thicker wire, the transfer of heat was faster. We might have discovered that when the copper wires were longer the transfer was slower. Let's think about the implications of these relationships in our houses. (If you did not devise the mathematical expressions for these ideas, make some reasonable guesses as to what they should be. Write them down and use them to answer the questions below.)