Experiment 3: 1st Law of Thermodynamics - Closed Systems

Objective: In this experiment, the students will gain an understanding of the application of the first law of thermodynamics to a simple closed system.

Goals: After completing this experiments, the students will

• Calculate the total electrical work added to the system, and the total heat transfer for the processes.

• Plot the temperature of the object vs. time (and energy input through work).

• Compare the rate of cooling through the uncooled object and the cooled object.

• Prepare a report containing their calculations, comparing the experimental measurements to the predictions from theory.

Preliminary Experimental Apparatus:

Equipment needed for each setup

(1) Wok/popper

(2) 6 thermistors

(3) Wattmeter

(4) Small fan

(5) Beaker / Water

(6) Insulated base

Basic Experimental Procedure:

(1) Students turn on wok/popper and heat up surface to a high temperature.

(2) Electricity is turned off, and wok/popper is allowed to cool, with a fan blowing air over the surface to speed cooling if necessary.

(3) During steps 1 and 2, the students measure the electrical work used, and the temperature of the wok/popper to determine how much heat is lost (or missing) from the first law. Then, as the wok/popper cools, the students continue to monitor the temperature, until it returns to room temperature. A total amount of heat transfer can then be calculated, and compared to the total work input.

(4) Step 1 is repeated.

(5) Step 2 is repeated, but to cool the wok/popper, a beaker of added is placed on top. The temperature of the wok/popper and the water is then monitored, and a comparison of the heat added to the water and the heat lost from the wok/popper is made. From this, the students can calculate how much heat is lost to the environment.

(6) The students should also try to rationalize about heat conduction in the wok/popper and the water to explain why the temperature may vary over the surface or volume, or why there may be a delay between turning on/off the wok/popper and seeing the surface heated/cooled.

So, the students are using the 1st law for a closed system to determine how heat and electrical work affect the temperature of a closed system.