Physics

Determining The Mechanical Equivalent Of Heat Using Electrical Methods

Objective:

The purpose of this lab is to conduct an experiment to determine the mechanical equivalent of heat using electrical methods. To do so, we need to find out how much work it takes in joules to produce a calorie of heat.

Materials:

• calorimeter (resistance heater, cover, jacket, etc)

• water

• scale

• calculator

• paper

• voltmeter

• ammeter

• pencil or pen

• thermometer

• power supply

• 20 Ω 15-watt rheostat

• stopwatch

Methods:

• 1. First, we will weigh the empty calorimeter cup and record this weight in the data table. Then, we will fill it to about 2/3 full of cool water from the tap. Finally, we weigh the cup with the water in it to determine the mass of the water. Calculate the mass of the water by subtracting the weight of the empty cup from the weight of the cup when it has water in it. Record this value in the data table.

  2. We then put the calorimeter in the jacket and put the cover with the immersion heater on it.

  3. After we have finished putting the calorimeter together, we need to wire the circuit as shown in Figure 1.  Connect the power supply (5VDC output), rheostat, ammeter, and calorimeter-heating coil as shown in Figure 1.

  4. Turn on the power supply and adjust the rheostat so that the ammeter reads approximately two amps.

  5. With the voltage properly adjusted, record the measured voltage level.  Turn the power supply off, place the thermometer in the water, and record the initial temperature of the water.

  6. Switch the power supply back on and start the stopwatch.  Leave the power supply on for about 15 minutes while gently stirring the water occasionally.  Record the measured current every minute.

  7. Remove the power supply, accurately recording the time.  Stir the water once more and then record the final temperature of the water.

Results:

Conclusion:   

    In our textbook, we find one of James Joule's experiments where he set up a descending weight on a string and pulley system attached to a paddle in a container of water.  When the weight descended, it turned the paddle, causing friction between the water and the paddle.  The friction caused the temperature of the water to rise by a small amount.  He was then able to correlate the mechanical potential energy that was lost by the falling weight (due to gravity) to the heat generated in the container of water.  Using this experiment, he came up with the formula for Joule's law, also known as the mechanical equivalent of heat, which is 4.186 Joules = 1 calorie of heat. 

     For our lab, we used electricity as our source of energy and converted it into heat using a resistance heater.  A resistance heater uses a coil of wire that is a resistor to heat things.  Resistors resist the flow of current by slowing down the electrons flowing through a conductor, which reduces the voltage or potential energy across the conductor.  In order to slow down the electrons, the element must be a material that does not conduct perfectly.  Since the voltage no longer will have the same potential because the resistor reduces it, the energy must go somewhere (it is not destroyed), so it is dissipates from the resistor as heat.  To acquire our experimental value of the mechanical equivalent of heat, we measured the temperature of a known mass of water.  We then put our heater in the water for a given period, while taking measurements of the current at set intervals to get an average of the current flowing though our heater.  After measuring the voltage across the heater, and the final temperature of the water, we were able to determine the work done in the given period.  From this, we derived our experimental value in Joules of work per calorie of heat.  In effect, this is what Joule did with his experiment; only he used mechanical energy instead of electrical energy as his source. 

    Some causes of our percent error could be lack of precision in the instruments used.  Our rheostat was very touchy, it was practically impossible for us to get an exact reading of two amps flowing through our heater.  This may have been because of the rheostat having a rough adjustment and not a precision adjustment screw; it also could be that the ammeter was not in perfect calibration; then again, other conditions such as poor connections of the cables or clips could have contributed to the imprecise readings.