Purpose:

The purpose of this lab is to continue our consideration of heat transport as it is illustrated in the refrigerator. In the process we will use the concept of specific heat capacity in our calculations. The basic task is to come up with a plausible explanation of why there are all those little wires all over the coil on the back of the refrigerator. ( it can't be just to make it hard to clean!?)

In essense this is a lab about experimentally defining what happens in a specific heat transport situations with a focus on being able to predict behavior rather than having a theory driven explanation.

At the same time we are asking a question about experimental design. How do we craft an experiment to answer the question "Does the thickness or material of a cup shaped container matter to the rate at which heat leaves?". It seems like an obvious question but it turns out to be pretty rich..

Procedure:

 

In this lab you will need to design your own experiments (equipment will be limited to help keep us on a reasonable track) to answer the two questions above. This experiment is a follow on to the experiments we did in Thermal Transport 1. Be sure you have understood our preparatory discussion about specific heat capacity.

 

1) Explain your planned experiment(s) to you lab instructor and get their approval for your plan. Hot water, containers, and temperature measuring devices will no doubt figure prominently in your plan. Be prepared to articulate what results would indicate that the thickness or material is or is not relevant in the thermal transport process.

2) Using the concept of specific heat capacity, calculate how much heat flows into the air from the warm/hot fluid in your "can" (k_eff ) during your experiment(s). This should be expressed in units of J/m2.s.C . Don't forget to determine the mass of water in your "can". The specific heat capacity of water is 4.18 kJ/kg-C (4186 J/kg-C). There will be a number of different "cans" in the classroom.

3) We will determine in class what a reasonable task for a mini-refrigerator looks like. It could be cooling a few liters of soda in a couple of hours or something more. Once we know what the task is determine how much heat must be extracted from the beverage to cool it to fridge temperature.

4) The heat calculated in part 2 must pass through 2 liquid-wall-air interfaces on it's pathway to the room. The first is through the wall of the soda bottle and the second is through the coils at the back of the refrigerator. Given our experiments with effective thermal conductivity (k_eff ) how much area does there need to be in order to accomplish this in each case in the given time? Why are each of these processes examples of our calculated k_eff?

4) Determine the area of the main coil and the little wires. Does this explain why the little wires were added?

Note: steps 3 and 4 can be done in different orders depending on how you think about the situation. Do what seems to make sense to you as a process.

LAB REPORT:

I) Present and explain your calculations for part 2). Be sure and include any measurements and assumptions you may have made. Why is this not the same as the standard coefficient of thermal conductivity of the metal can?

II) Present the data from all the various experiments in class and discuss what they mean about the probable heat transport from the coils on the back of the fridge or through the walls of the soda bottle/can. There will undoubtedly be lots to talk about so don't skip over the surface too much. Describe trends in the data and which k_eff you will choose for your calculations and why.

III) Show your calculation for part 3). Discuss (briefly!) why this or isn't a good estimate of the task required of a refrigerator.

IV) Describe how you determined the area of the heat exchange coil on the back of the refrigerator and whether the heat can get out of the soda bottle(s) in time. Include all measurements, assumptions, and calculations. Explain, from your calculations, why the little wires are there.

Thermal Transport III Rubric