Purpose:

 

The purpose of this document is to provide some clarity about how I hope you will approach labs we do this term. The are some fundamental differences from the labs we did during the first term. In PH211 the labs are more directive in the sense that you were directed to make certain measurements and perform specific calculations. Part of the intent was to provide a model for you of how physics (and science in general) investigates phenomena. I expect that now you have a basic understanding of this investigative process and you are ready to step it up a couple of levels.

Procedures and Processes:

 

In the labs this term you will generally be asked to engage in a number of types of tasks. These can be generally broken down into:

 

Measurement and Plotting

 

Conceptual Coherence

 

Modeling and Calculation

 

Reverse Engineering/Evidence

 

In each of these contexts I have some thoughts and hopes to share about how you approach the activity.

Measurement and Plotting

An important feature of science is the effort to understand the universe around us through measurements and the illumination of relationships between various features of a situation with the use of plots. When I ask you to make a measurement or set of measurements it is usually so you can later calculate some relevant value from those measurements. This means that you will need to look at the calculation you will be doing and thoughtfully examine it to identify what symbol(s) represent the quantity you are calculating and what are the parameters that you will either have to know, look up, or actually measure. I expect that you will then examine the particular experiment or setting and determine what you are able to measure and how it can be accomplished. If there is some parameter that you need and it doesn't seem to be measureable is it something you could look up and would that make sense in the context of the experiment?

 

As an example, in one lab you are asked to determine the coefficient of linear expansion for an Al rod. From examining the relevant equation you hopefully notice that this magic number is labeled alpha. That the other parameters in the equation are the length of the object, the change in termperature of the object, and the change in length of the object. By examining the apparatus you might notice that there is another Al rod hanging on the outside that you could measure the length of, that there is a mysterious dial on the end of the apparatus which (with some help) can be used to determine the change in length, and there is no direct way to measure the change in temperature. Upon reflection you realize that the temperature of the water used to heat and cool the Al rod is related to the temperature of the rod. As a first attempt one could start by assuming that the temperature change of the rod is the same as the change in the temperature of the water. I imagine that you think to yourself -- "Hmm, what ever the temperature change of the Al rod really is it will not be more than the change in temp of the water." Now I can make the measurements.

 

When you make measurements for a plot I imagine that you first look at the variables you will be plotting to make sure that you are getting all the data you need. Is the plot a histogram of a single parameter or is it a relationship between 2 or more variables? Do I need to determine all the data values simultaneously or can I make each measurement independently? I hope that you have learned to make a rough plot of the data as you go along so that any weirdness in the data will become immediately apparent. If there is weird data you have an opportunity to check it on the spot in case it's something obvious and easily fixed.

Conceptual Coherence

This is the piece of the puzzle that instructors often imagine is "obvious" to you. In my experience the reality is that students are not so clear about how the concept applies to the situation that is placed in front of them. Conceptual coherence is essentially the question of whether the physics tool you are seeking to apply to the situation makes sense as a description of the system.

Imagine you are working with the concept of latent heats. This is a physics concept that only applies to changes in the physical state of some material. Is there some reason to believe that some substance is melting, boiling, or freezing in the system you are considering? If not then latent heat is unlikely to be relevant to the problem. If something condenses is there a way to figure out how much mass is involved and what the boiling point of the system is? This might mean that you will need to consider pressure effects on the boiling points. Is there a way to assess the system pressure in this situation? Do I have tools to measure the mass or the pressure or will I need to estimate them based on some other consideration.

Finally, having determined that a particular concept applies can you identify what feature of the system each term in the physics expression describes. Do you know which ones you can, or will need to, measure and which you can or should look up? What are we trying to determine and what underlying assumptions do I need to make? Only when you have all of this stuff straight in your head are you really ready to start making measurements

Modeling and Calculations

Modeling is the mental and mathematical picture that you have of how some process works. Newton's Laws are a model, special relativity is a model, quantum mechanics is a model. What we do with models is test them against the realities that we observe. If our model is clear we can determine (often by calculation) what we would expect to happen in some situation. The Sandbox Lab is a classic example of this and most folks experience the success of landing the ball on the line in the sand as confirmation that the kinematic model of projectile motion as constant acceleration in the vertical direction is valid.

 

In the labs this term you will be asked to apply models that you may be less familiar with to novel situations. The point of the activity is to practice exploring the numerical predictions of a particular model and seeing how well it does. This can be quite challenging when we don't understand the model very well but that's how science really is. If the model fails to correctly predict the observed behavior there are number of possible interpretations. One is that the model is wrong and its time to go home. Another is that the measurements were in error or incomplete and that is the source of the inconsistency. Another is that we are applying a good model to a situation to which it doesn't apply. Perhaps we made some error in our calculations which certainly possible. The deeper question is really how to figure out which of these explanations is most likely.

 

As I have mentioned before these sorts of labs require a different sort of approach if one is to avoid frustration. I am asking you to explore what you understand and wonder how your personal models could evolve so that you might be able to more successfully describe the world. Ultimately that is all science can do. It's models can describe the behavior of the world successfully or unsuccessfully but truth or correctness is out of the question.

Reverse Engineering/Evidence

In many of the labs this term I will be asking you to recognize and describe the evidence which supports your understanding of you something works. This is essentially the process of reverse engineering. It is no accident that when one car company creates a clever new process that very little time passes before all the competitors have a similar process in place. Car companies buy each others cars and reverse engineer them pretty regularly. People living in various places in the world don't have access to shop manuals for all sorts of electronic devices yet they figure out how to repair, recharge, and in some cases build the device. The observational and reasoning skills involved in reverse engineering are useful in all the sciences which is why I'm asking you to practice them.

Here are some links to various discussions of reverse engineering:

Andrew Huang does this for fun and posts these lovely challenges at bunnie:studios:

A fascinating set of videos starting with from Null Space Labs and Layerone: