Constructing Physics

Elementary Physics Course Notes

Adam Johnston
Department of Physics
Weber State University

 

[Note: These notes are based on a collection of different classes and lectures, not all of which you may have been in.]

The class resisted the temptation to take the rubber bands that Johnston had just passed out and turn them against the course instructor.  They knew that, collectively, they could have easily taken over the helm of this ship in a mutinous onslaught of rubber band firing.  Sure, many of the government issue rubber bands would have missed the target, yet just as surely enough of them could have hit the target and stunned the instructor for a long enough time that they could have taken control . . . if only they would have organized the revolt.
But they didn't.  Instead, they were stretching and relaxing the rubber bands and holding them up to their lips to note the change in temperature that was taking place.  Similarly, many thought about rubbing hands together and how that friction caused hands to warm slightly.
This led to a discussion of heat.  Heat is a means of transferring energy due to temperature difference.  We didn't yet understand temperature, besides the fact that it's what a thermometer measures, but that was okay.  Remember that "heat" is what you actually feel when you're thinking of temperature.  This will make more sense once we've described temperature itself more fully.
We were thinking about "work" as well, and how that was also a transfer of energy, but via mechanical means.  "Heat" just gives us another way to think about how energy gets from one place to another; how it is conserved.
There are several means of heat transfer:

Conduction
Convection
Radiation


Adam described and showed each of these.
Now what's temperature?  Imagine two glasses of water, one with "hot" water and one with "cool" water.  In each a drop of food coloring is placed, and we notice that in the hot water the color disperses much more quickly than in the cool water.  This is telling us what temperature physically means.  It is the measure of the random motion of the molecules that make up some substance.  In other words, it is the measure of the random kinetic energy in some substance.  You don't feel temperature directly, because you don't feel individual molecules directly.  What you do feel is the heat transferred due to temperature differences.
* * *
"Where is the semester going?!"  Johnston was obviously agitated.  He began to mumble to himself, rolling up his sleeves absentmindedly and fumbling with equipment.  Where had the time gone?  Students, upon realizing that the course was more than half over, also began to worry amongst themselves.  Where would they find the time to study?  Where would they find inspiration for projects?  And what would they do with their existences once PHSX 1010 had ceased?  The room grew silent, and a general angst settled in like a thick fog throughout the lecture hall.
To soothe this displeasure, we started doing some physics.  Today's class was filled with demonstrations that took place simultaneously, making the front of the lecture hall seem even more like a three-ring circus than ever before, all directed by a monkey in a button up shirt.
Last we were together, the idea of temperature was discussed.  It was important to emphasize that temperature is not a measure of "heat," but rather a measure of the activity of molecules within matter.  You don't "feel" temperature (because in order to do so you would have to feel the physical random bouncing around of molecules); but, you do feel "heat."  Heat is the transfer of thermal energy from one object to another.  The reason a low temperature object feels cold is because heat is transferred from your body to the low temperature object.  The reason a high temperature object feels hot is because heat is transferred from that object to your body.  Two objects can have the same temperature but transfer different amounts of heat and thus feel different (e.g., your bare feet on the carpet versus your bare feet on a tile floor).
Heat transfer happens in a variety of ways.  Direct contact of bodies transfers heat via "conduction."  The transfer of heat through space is known as "radiation."  The transfer of heat by physically transporting matter is known as "convection."  Each of these three were described and connected to examples. 
Then, some research was conducted.  A beaker of water was heated from below and its temperature was recorded as time passed.  What we witnessed was that the temperature increased regularly for some time, but after a while the temperature remained constant (at 95 degrees Celsius), even though heat was continually given to the beaker.  An astute learner pointed out that the water was bubbling, boiling, or changing phase at this point.  It seems that heat transfer can do a couple of different things to a material: Either increasing the temperature or changing the phase.  However, conservation of energy only allows one of these two things to happen, otherwise you'd be getting more energy out of the system than what was being put into the system!  A debriefing of this would be featured in the following class lecture.  It would be shown later how the type of substance, the mass of the substance, and the amount of heat given to the substance all affect its temperature change or its phase change.

Why did the water boil at 95 degrees Celsius?

Under the thermodynamics demonstrations is a virtual experiment in heat, showing a temperature versus "heat added" graph to demonstrate how heat produces both temperature and phase change in a material.
At the same time that water was boiling, some other experiments were being conducted.  (The "conducted" experiments had to do with "conducted" heat!  A pun!)  These experiments basically demonstrated that different materials are affected differently by heat and transport heat differently.  This feature would also be used in a quick debriefing next class, showing how the different materials transport heat differently and show different temperature changes.

A demonstration of what "temperature" means is found under the thermodynamics demonstrations.
Like the water molecules that were boiling, members of the class began to escape the confines of the lecture hall, off to pursue other academic interests: history, mathematics, lunch, and probably even some physics research.