I keep a list in a notebook about possible things I could write a blog entry about. When I see something in the media, or something happens at work, anything to prompt me to think about a particular area of physics really, and I scribble it down and may choose to inflict it upon the world at a later date. For example, I thought I got a good one a couple of weeks ago when I learned about an impending mass-overdose on homeopathic medicine as a protest against it being stocked by chemists in the UK. Then I discovered it had been extensively covered on about six thousand science blogs already. I didn’t add to that list. (In case you are interested, and have somehow escaped reading about it, have a look at the campaign website on http://www.1023.org.uk/ )
On my list is ‘dynamic equilibrium’. I put in on there after writing my blog entry about feeling ‘cold’ radiating off something – it’s a very much related topic, and an important one in physics and chemistry (and I think probably biology too), but I haven’t got around to talking about it. I’ll give you a physics-coloured example.
We say things are in equilibrium when thing appear to stay the same. For example, the telephone in my office is in thermal equilibrium with the air in my office. That is, the temperature of the phone is the same as the temperature of the air (roughly speaking). But that’s not to say that nothing is happening. Two objects will always exchange heat with each other – if they touch then through conduction, if there is a fluid involved then through convection, and if there is a line-of-sight between them through radiation. So the air is transferring heat to the telephone. Likewise, the telephone is transferring heat back to the air. But the rates of transfer are equal. Consequently the telephone gains as much heat as it loses, and its temperature stays the same
Migration is another example. The population of a particular town might stay the same from one year to the next, but the people in it do not. People are born, people die, people move away, and people arrive, but the number being born and immigrating equals the number dying or emigrating. Overall, the town may not show any changes at all (the ‘equilibrium’ bit), but the components that make it up (the people) are constantly changing (the ‘dynamic’ bit).
Really, the important thing for physics is although macroscopically (on a large scale) things might appear to be constant, look on a small enough scale and they are not.