The work never ends

By Wednesday, July 26, 2017 No tags 0

Just as the IB exams finally end, the GCSE and A-Level ones begin. Now it is all finished, I have one week before the IB results come out – there seems to be no end to the workload for a teacher – it is a hell of a life and I would not change 1 minute of it.

Rather than make this a blog discussing bland stuff like ‘housekeeping’ for teachers, I thought this might be the first of a number of blogs to consider the content of the topic areas and how we actually present them, what are the important parts, etc. And for this first one, I would like to consider…

Thermal Physics

I love thermal physics although it seems to be the topic most teachers hate. I am not sure why although I would take argue that in part, this is because of the following:

  1. It is one of the areas that has ideas that do not instantly appear to be ‘clear and visible’ i.e., you can’t actually see the particles as they move and collide.
  2. It is possibly the single topic where exact words used are really important when we define what we are discussing.
  3. When the material is considered on the web and in text books, there are contradictions and physicists argue over what some of the material means.

Sections of the work

There are three basic areas for thermal physics: (a) basic ideas, (b) Phase & States and (c) Ideal gases. I will take these areas in turn.

Basic ideas

When I start teaching this topic, I usually start by taking scrap paper from the bin, tearing A4 sheets in half and giving them to the students. I then ask everyone to write down what they feel the following terms mean – Work, heat, internal energy and temperature.

We then discuss each one of these and talk through what the term means and what the terms are often misunderstood. I don’t wish to waste time here, so I will keep it brief (if you want more detail, contact me) by pointing out a couple of points:

Work – not a type of energy. This is the amount of energy converted from one form to another. Often the work converted, is replaced with transferred but I don’t like this – transferred means to be moved from one location to another. Converted, means changed in some way. The other problem with the term work is that there is a second definition (force x distance moved in the direction of the force). Students need to know both, but in thermal physics, converted energy is the way to go.

Heat. The amount of energy transferred from one place to another due to a temperature difference. We now have a discussion about the importance of the work ‘transferred’.

Internal energy. Emphasise that this is not a simple as the total energy of the system but rather, that the IB like to emphasise that there are two types (KE and PE) and we need to total amount of both.

Temperature. I tend to now state what temperature means in terms of the energy of the system and I put the equation for the average KE a lot. I also usually discuss the idea of why it took so long to realise that temperature was an important property – try listening to In Our Time – The second law of thermodynamics (BBC) – it has some great insights and if you want students to listen to it, give it as homework and get them to make a spider diagram – it is an excellent radio programme which can be streamed from the BBC website.

Phase and states

These two terms are often exchanged in sentences and used to mean the same thing. For most areas of physics I guess, this is not a big idea, but in thermal physics is it. I emphasise to the students that they mean completely different things, then consider them in detail:

Phase: I usually say that this is a reference to a material being a solid, liquid or gas. If the groups of students are strong, I might then discuss the fact that the term is actually associated with the atomic or molecular arrangement and point out that there are many solids where the atomic arrangement changes below a certain temperature and this is also called a phase change.

State: The collection of values for the quantities that define the system. This description I find, leave the students looking blank. I then give the example for an ideal gas and we write down PV=nRT and when I point out that the exact values for P, V, n, T define the state, they understand.

The final part of this work is to discuss phase and state changes, where the equations are introduced. The IB seems to like questions that consider the PE changes for a phase change so I usually emphasise this.

Ideal gases

This is the final part and most of the work is pretty standard stuff. The students do find that the definition of an ideal gas (i.e., a gas that obeys the ideal gas equation) is a bit strange – it is something that obeys a formula! I find this a good moment to remind them about what we are trying to do in physics and that we use maths for most of our descriptions. We then discuss the assumptions of an ideal gas, the different changes (including use of the terms isobaric, isochoric (I do not like ‘isovolumetric) and isothermal) and the importance of the P-V graph.

The last parts of the topic area discussion of what causes pressure microscopically. I usually emphasise the process of particle collisions leading to a change of velocity, acceleration, force, force on the wall, etc and this is linked to a discussion of the three changes we have discussed e.g., why does the pressure change for an isothermal change? I will usually end with a derivation of the pressure equation using kinetic theory.

The Horizon program “What is one degree” is also good to see. If anyone would like a pdf of a set of student notes for this topic then let me know and I will email them to you.

I hope the above is a useful view of this topic and allows you to reflect on how you present it. Other thoughts and views would be good to hear.

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