What is heat? What is temperature?
OK, this is after all the physics forum. Let's talk about heat, specific heat, and temperature. We'll touch briefly on latent heat as well. And we'll make sure that we understand the difference between heat and internal kinetic energy- most people mean the second when they say the first.
Let's start with temperature. Temperature is the
average kinetic energy of all the molecules in a substance, region, or object. Gasses circulate pretty freely, so by and large the air is going to be pretty much all the same temperature inside your oven. It might be a little hotter at the top, a little cooler at the bottom. Not much, though- probably only five or ten degrees.
Now, what's heat? Well, actually, what most people call "heat" is really internal kinetic energy. It's the
total kinetic energy of all the molecules in a substance, region or object. So why is that not heat? Simple: because heat is the
difference in total internal kinetic energy between two objects, regions, or substances- not the total internal kinetic energy itself.
So what's the difference between the two? Simple: if you have, say, a liter of boiling water, it's at 100C. If you add more boiling water, say another liter, it's still at 100C. But the first liter contains about 1513 kJ of energy- and so does the second, so even though the water is still at 100C, because there's two liters now instead of one, you have about 3026 kJ of energy; twice as much.
So what's specific heat? Well, that's the amount of energy it takes to raise a certain amount of water by a certain temperature. In the SI system, it's measured in joules per kilogram-kelvin, or more conveniently kilojoules per kilogram-kelvin, abbreviated kJ/kg*K. Since I know that the specific heat of water at 100C is 4.216 kJ/kg*K, and I know that the temperature is 100C which is 373.15K, and water weighs 0.962kg/l at 100C, I just figure it all out and there you have it.
Now, it takes more than just specific heat to make water turn into steam. We know that, because even though the water is boiling, it doesn't all just suddenly turn into steam and go floating away- it takes a long time for boiling water on the stove to all boil away, which means even after we have our 1513 kJ/l in it, we still have to keep pumping heat into turn it to steam. Why is that? Well, even though the water is at 100C, to turn it to steam takes some additional heat so that the water molecules can "break away" from the liquid water. That heat is called "latent heat of vaporization." (Going from ice to water takes latent heat too- but a different amount. It's called "latent heat of liquifaction.") It's about 2270 kJ/kg. So the amount of latent heat for water is greater than its specific heat at the point of vaporization! Wow, no wonder it takes so long to boil it away!
So, we know just by thinking about it that the specific heat of air is pretty low- you wouldn't be able to open the oven if it weren't, the air would cook you alive. But of course the specific heat of the food and metal inside the oven is much higher- so you might be able to wave your hand around in there for a little bit, but don't touch anything! Now, what do you suppose the oven thermostat is measuring? The temperature, of course!
Cooking happens three ways: convection, which is the air moving around as it gets heated, and heating up whatever it touches; radiation, which (as someone else noted) is the burners putting out infrared energy that gets absorbed by whatever it hits (mostly the food, the cooking pan, and the sides of the oven; but of course it heats the air too); and conduction, where the air conducts heat into the food, and the heat spreads out inside the food.
Now, the burners inside the oven get really hot. Electric ones are made out of a special ceramic (ceramics can take incredibly high temperatures, the most temperature resistant things we know how to make are ceramics) with a high-resistance wire down the middle. The wire is very thin, because that way it makes more heat, so the ceramic protects it from breaking if it gets bumped, and also increases the surface area so that the heat can radiate more efficiently, thus keeping the wire from melting too.
The ceramic is essentially pretty close to a black body (it doesn't reflect much light, you'll notice- that helps it radiate the heat so the wire doesn't burn out) so we can guesstimate the temperature by the fairly red-orange color it is when the thermostat turns it on. Just by guess and by gosh, looking at the chart on the Wikipedia page for
black body radiation, I'd have to say it's probably somewhere around 1,500K. That's 1,200C, or 2,250F. I'm going to back off and say that the 1,900F that's been floating around here is probably as good an estimate as any. It might be a bit cooler or hotter than that; but not a lot. Now, that's well above the ignition temperature for just about anything you'd care to call "flammable." And of course, if it's a gas oven, you've got a flame right there; needs no more help than that. Last but not least, if grease collects, it can evaporate at high temperature, and catch fire. So there you have it.
), and the standard oven temp is 350. how do fires start in ovens? also where does the energy come for the first spark which starts the flame?
