Overdoing a mince pie in the microwave resulted in a cracked dish and a big burnt smell! It got me thinking about the treatment of the 1st Law of Thermodynamics at GCSE these days. Instead of just saying that work and heat can cross a system boundary, we now say it is mechanical work, electrical work, heating by particles and heating by radiation. If this pie had been in the oven at 180oC, it would not have been able for it to get higher than that temperature. The heat would be crossing the system boundary by conduction, ie heating by particle vibration. If the outside is at a higher temperature than the inside, then there is more energy transfer into the pie, but there will also be some coming out. At 180oC, the same amount would be coming in and coming out. Thermal equilibrium is reached. In the microwave, the it is heating by radiation that crosses the system boundary. As the pie heats up, there will then be some conduction (heating by particles) back out but this will be slower. So if a thermal equilibrium is reached it will be at a higher temperature. I suppose this neglects the idea that the hot pie itself is a black body emitting infra-red, but this clearly doesn't bring it to a sensible thermal equilibrium in the face of constant extra radiation heading inwards.
