What does it mean for an oven heating element to burn out? The heating element in my oven has just burned out and I needed to replace it with a new one, because it simply wouldn't heat. When I took it out it looked pretty much OK (at least no huge difference to the replacement item).
What does the "burning" mean from physics perspective? What happened? Was it a gradual process (over ~15 years)? Is the old element no longer conducting electricity? Or is it the opposite - it continues to be a conductor, but with significantly lower resistance?
 A: An oven element usually consists of a long, single-layer coil of wire packed in an electrically insulating powder inside a protective metal tube. At each end of the tube there are terminals making connections to the ends of the wire, so that a current can be passed through it. This makes the wire hot (probably to a dull red temperature). Heat from the wire escapes through the powder and the metal tube to heat the air in the oven.
Over the course of years of use, the red hot wire reacts with the oxygen in the air, forming an oxide layer. [Probably the electrically insulating powder helps keep air away from the surface of the wire, but it won't do so completely.] As the oxide layer gets thicker, the metal of the wire gets thinner. Inevitably it will not get thinner uniformly, and spots of particular thinness will develop. These will get hotter than the rest of the wire, and this will accelerate the local rate of oxidisation, so the thin spot 'quickly' gets even thinner, and hotter – until it melts, so the wire is interrupted and no current can flow through it. Your element is 'burnt out'. But you won't see this, because the protective metal tube will probably be unaffected. 
