Category Archives: Introductory physics

Introductory physics: the fourth state of matter

The relation between heat and temperature (last post) is not always straightforward: in some cases, large quantities of heat can be supplied to a substance without any observable change in its temperature at all!

When does this happen? It happens when matter changes state i.e. when a solid melts to liquid, or a liquid changes to gas. In fact, it takes a lot of energy to convert a solid to a liquid even if the solid is at the melting point, and it takes even more energy to convert a liquid to a gas even at the boiling point. Neither of these ‘phase changes’ can happen unless energy is continually supplied and there is no rise in temperature rise during the phase change; for this reason the energy consumed is known as latent heat (from the Latin for hidden). It’s interesting physics and quite fundamental at the same time; for example the liquid/gas phase change requires much more energy than the solid/liquid because the intramolecular forces must be completely broken down (as opposed to being weakened in the solid/liquid case). It’s also fascinating to observe that as a solid gradually melts into liquid, the resulting liquid stays stubbornly at the melting point temperature until all of the solid has undergone the change of state (ditto for gas).

Heating curve for ice: the temperature stops rising at 0 and 100 degrees Celcius during the phase changes

A good example of an application of the physics of phase change can be found in an electric kettle: a sensor detects when the top layer of water begins to bubble and quickly switches off the heating element – as opposed to boiling water in an open saucepan, a wasteful process where a lot of water is converted into useless steam, meanwhile consuming a large amount of energy.

Electric kettle : clever device

A change of state can also happen in the reverse direction: when a gas changes to liquid, or liquid to solid, energy is released. This process is exploited in the refridgerator, for example.

In a fridge, a gas-to-liquid phase change extracts heat, keeping the fridge cold inside, while a liquid-to-gas change outside the fridge dumps heat

The whole business of state change raises interesting questions about the nature of matter. Why do some substances exist as solids at room temperature and pressure, others as liquid or gas? To answer this requires a discussion of molecular bonding. Another common question is this: if supplying enough heat to a solid gives you a liquid, and eventually a gas, what happens if you keep supplying heat to the gas?

The answer is that if you supply enough energy, you eventually get another phase change as the atoms of the gas become ionized i.e. electrons are stripped off the atoms of the gas. In this case, the gas becomes a plasma, the fourth state of matter. Plasmas are plentiful in nature: a star exists as a plasma, as does lightning and even fire. Plasmas can also be produced in the lab under extreme conditions, for example by laser bombardment or by particle collisions in accelerators.

A supernova is not a liquid or a gas but a plasma

Update

Here is a super little YouTube video on plasmas by rock band They Might Be Giants, many thanks MattW

Filed under Introductory physics, Teaching

Introductory physics: heat and temperature

The teaching semester began again at 9.15 this morning. First day back, I’m always struck by how much I enjoy being in the classroom. I think it’s because lecturing is basically a performance, with never a dull moment; anything can go wrong and usually does! I also quite like big classes, it makes for a good atmosphere…

Then there’s the content. This morning 1st Science got to meet Heat and Temperature for the first time. This is my favourite kind of topic – quite simple but of fundamental importance. ‘Heat is a form of energy’, we tell our students, and ‘temperature is a measure of heat’. Actually, the discovery that heat is simply a form of energy was an enormous advance in science, possibly the greatest breakthough of 19th century physics.

And what sort of energy is it? Well, kinetic energy arises due to the motion of molecules (vibration in solids). But there is also potential energy;  since atoms in solids have more-or-less fixed positions in the lattice they have must possess an associated potential energy (so do atoms in liquids for a slightly more subtle reason). So heat is basically a type of internal energy. Except that it’s not always internal; there is also the whole business of heat transfer, a phenomenon that can occur by any or all of three very different mechanisms!

Then there is temperature; a philosopher would have a field day explaining the difference between a quantity that simply is (energy), it’s manifestation (temperature) and human temperature scales. Indeed, the relation between heat and temperature was only quantified with the intoduction of concepts such as specific heat capacity and specific latent heat. Temperature clearly has a fundamental aspect too; for example, what do really mean by absolute zero (or zero Kelvin)? ‘Absolute zero is the temperature at which all molecular motion ceases’, students are told. But what does this mean? Why can’t we reproduce this temperature in the lab? Is -30 Kelvin ( or -303 degrees Celcius) really a nonsense? Fundamental stuff indeed and a nice start to the term…

Achieving the impossible in the lab

Update:

Oops! I thought the dial read zero on the LHS but it doesn’t of course. Also, I’m not sure why it reads degrees Kelvin, there is no such thing.