# Thermal expansion again

Our new house (new to us – it’s twelve years old) can make some ferocious noises sometimes. It has some huge steel beams supporting the roof – these being held up as far as I can tell partly by steel supports and partly by concrete walls. I’ve been trying to work out just what is supporting what in the house – for example there appears to be one beam that is held up only at one end. That can’t be right – there must be some kind of structure buried away that I can’t see – but it does look a bit odd.

With all that steel, and warm days and cool nights, there’s a lot of thermal expansion and contraction going on. At about the time you want to start sleeping, the house can start banging as  the steel moves slightly to relieve stresses that are beginning to build up. I’m sure the architect knew what he was doing, but it it’s a bit disconcerting sometimes.

Thermal expansion can be used to good effect however. The classic example is the bimetallic strip used as a thermostat. Here, strips of two dissimilar metals are placed together. The two have different coefficients of expansion – so as the temperature changes there is stress built up within the strip. To relieve this stress,  the combined material bends.  Physicists talk about the strain energy contained within the strip – in parts of the strip that are stretched, for example, the atoms are further apart than they would normally be leading to extra energy – in parts that are squashed the atoms are closer together, again leading to extra energy. Overall, the strip moves to minimize that strain energy, which means changing the amount it bends. This bending can be used to make and break an electrical contact, and, hey presto, you have a thermostat – a switch that is switched on and off by temperature. Wikipedia has a nice video of a spring uncurling when it is heated.

A similar effect is seen in our cat Mizuna. In this case, rather than acting to minimize the strain energy, he acts to maximize his heat energy. On a cold morning, he’s curled up; but as the temperature increases, he slowly unfurls. Plonk him on a hot deck on a summer afternoon and he’ll be stretched out, maximizing his total body area exposed to the sun. We could use him as a thermostat too, though I don’t think he’d take too kindly to being used as part of an electrical circuit.