I'm having trouble with physics revision and heat transfer. There's a question in the book about whether a boiler should be dark and matt or light and shiny, and I just don't understnad! Can someone explain it to me?
Should be light shiny. A blackbody emits the most energy ata given temperature. Painting it black does not make it a perfect black body but it's better than white. cf. Stephan-Boltzmann law. The formula includes a coefficient expressing the degree to which a body is black. BTW 'black' in this case refers not only to visible light.
Is the boiler in your book at the focus of a solar collector? If so then dark and matt is good otherwise it really doesn't matter. If it's a wood, straw,dung or fossile fuel boiler then it is effectively surrounded by (or surrounds) a firebox ( if the outside of the firebox is light and shiny then that's great but it has nothing to do with the boiler) the one exception I can see is where you have a fan driven gas boiler where the main heat exchnger is actually inside the boiler and the outer surface of the boiler should be light and shiny to cut down on waste heat radiated from it. So if radiative transfer of heat out of your boiler is a problem then it should be light and shiny but if the boiler works by absorbing the maximum amount of heat by radiation from a source which is much hotter than the boiler surface then black and matt.
