Purpose:

The purpose of this lab is to explore the mechanisms other than conduction by which heat is transported. These processes are called radiation and convection. All of these are important and relevant to our lives.

Procedure:

 

For this lab there are just a couple of tools. These are the Stefan Boltzmann Law and your previously developed understanding of thermal expansion.

 

Your first task will be to think about whether "radiant heaters" are aptly named. How do you verify that they actually work by using radiant transfer of heat energy? What are other possible explanations for how the heat gets to you from the heater and how do you their their veracity? Where is the convective heat transport part of the process and how does it illustrate the characteristics of convective heat transport. Where is the conductive heat transport part of the process and how does it illustrate the characteristics of conductive heat transport.

Your next task is to make the necessary physical measurements or estimates to determine the temperature of the filament of the radiant heater. You will find it helpful to use your personal experience of sunshine absorption to set limits on this process. Take some time to go outside and "know" what 800 W/m2 feels like on your hand. You will then use the Stefan-Boltzmann Law (in radiated power form linked above) to estimate the temperature of the filament. Be very thoughtful -- the elements are hot and I don't want you or the equipment damaged in any way.

Finally, I want you to estimate the temperature of the air rising above the heater. From this you can determine the density of the rising air relative to the room temperature air from the volumetric expansion of the air. Then, by looking up how to determine the bouyant force on an object in a fluid you can calculate the net force on a mass of hot air. If you really want to get deep into this you can consider how to determine the drag force on the rising hot air as it rises (how can you determine whether it is accelerating upwards or not?).

 

LAB DELIVERABLES:

 

I) How would you convince someone that the heat/energy from the "radiant" heaters is being carried by the light coming from the heater and not some other mechanism? What other mechanisms would people imagine play a role? Is there an example of convective heat transport embedded in the heater? Is there an example of conductive heat transport embedded in the heater?

II) Use the Stefan-Boltzmann Law to determine the temperature of the filament in the heater. This will take some effort and awareness of what you need to and can measure. High precision is not required. Be sure to compare your result to the known color temperature of heater wire and halogen light bulbs. Explain how your calculated results relate to the published color temperatures. Part of what I am expecting you to explore is how sensitively the behavior of the heater depends on the temperature of the filament. You will also need to think about issues associated with the behavior of your eye.

III) Calculate the upward acceleration of a mass of air which is warmer than the surrounding air. You will need to use calculational tools from thermal expansion a couple of weeks ago and look up how one calculates bouyant forces for any object embedded in a fluid. During this process you should compare data sources that give you the density of air at different temperatures with your own calculation of the density based on volumetric expansion from the first lab.

Thermal Transport II Rubric