• Joule's experiment

Joule's experiment.

Concepts demonstrated :

• Conversion of Mechanical Work
• Mechanical equivalent of heat

Equipment needed :

• Shot tube with lead inside
• Thermocouple
• CCD camera

• Measure the temperature of a known mass of lead shot and place this into a long tube.
• Invert the tube 50 to 100 times and then measure the temperature increase.

Physics involved :

Lead shot falling in a tube does mechanical work to produce heat. Measure the temperature of the lead before and after the experiment. Compare the heat to the work done.

How to use the Demonstration :

• Use in lectures when showing the conversion of mechanical energy into heat.
• Discuss this and show how this is the opposite to a heat engine (i.e. heat produces mechanical work).
• Use this as a POE demonstration.
• Start by asking the students what they think will happen to the shot if the tube is inverted 100 times.
• Once the students have worked out that there will be a temperature rise ask them to calculate the work done on the shot (i.e. find mgh x number of inversions) and compare this value to the heat developed.
  This is found by multiplying the rise in temperature by mass and specific heat of the shot.
• Measure the temperature of the shot after 100 inversions and compare to the theoreitcal value.
  Typical results are theory delta T = 5 deg C, experiment delta T = 3 deg C.

• Bend a coat hanger back and forth - thermocouple. Record the temperature (T) versus time. When does it increase temperature most ?
  Q Is the heating due to deformation.
  A (I think) the heating is due to friction on a microscopic scale.
• A copper (Cu) slug can be hit and the temperature (T) measured. The temperature increase for this experiment is greater than the above experiment (TH 05a).
  This is also a good qualitative experiment.
• Belt lead (Pb) with a steel hammer, with a thermocouple on the side or under the lead mass.

• Experiment with the thermocouple interface. (Can choice of materials be improved?)
• Compare the above demonstrations in "other (everyday) examples" to the compression of gas e.g. in a syringe.
• See also another experiment - Joule's original experiments. (See Pasco Catalogue for quantitative.)

• The experiment takes a few minutes to perform.
• The lecturer should ask a volunteer to perform the inversions so that he/she may walk around the theatre and observe what the students are doing.
• When this experiment was performed some students refused to believe that the temperature would change. Even after the experiment was performed they argued that the rise in temperature was caused by friction.
• Why not use copper balls rather than lead?