Heating plates in an oven.

[peter]

Here's a phenomenon I have observed over the years and have a small theory about that I want to bounce of you guys. When you place a plate in the oven to warm up and remove it some short while later, if it hasn't been in for quite long enough you will notice that while the central flat area of the plate is still relatively cool (ie cool enough for you to easily place the flat of your hand against) the curved up rim of the plate is often much hotter, to the point where you would have trouble picking it up. Why this difference? Why is the plate not uniformly hot all over?

Clearly the heat of a given part of the plate will be a reflection of the relative motion of the molecules of the plates material in this place, so we can conclude without much fear of contradiction that the molecules at the rim are in greater motion than those on the 'plane'. Now why would this be? I can't really say for sure - but we do in physics see a somewhat similar effect in the way that static electricity will leak from a charged body in a way that is related to the curvature of the surface of the charged body. Hence, to build up static in a body, a soft curve will function best (think the surface of the dome on a van-der-graff generator), where a sharp point will serve to 'spray' charge away ala the pointed tip of a lightening conductor. It thus seems to me that in some way the curved rim of the plate is serving to 'collect' molecular motion at the expense of the flat expanse of the base (by what means I know not how) and thus delivering the highly irritating (and totally opposite effect of what is required) phenomena of being too hot to handle but too cold to keep the frikkin food warm!

[wayfriend]

I would guess it's due to the fact that heat transfers most rapidly where there is more surface area relative to the volume. The edges of the plate are thin, so that's lots of surface area, while the center is thick, so less surface area.

Also, your oven's heating element is a round circle. The edges of the plate align with the circle. QED.

The real question is why does coffee, left to cool on your office desk, become so much colder than room temperature? Physicists have been stumped for centuries.

[Vraith]

Yea, gotta be the surface-to-volume.
If you hadn't noticed, peter, the reverse also happens.
If/when the whole plate is warmed, the edges will cool faster...so when you touch it to see if it is cool enough to move to the table, hold by the edge is fine...but touch the bottom/middle to stabilize it, or show off your waiter carrying skills, you burn your palm on the bottom.

[Cord Hurn]

Yea, gotta be the surface-to-volume.
If you hadn't noticed, peter, the reverse also happens.
If/when the whole plate is warmed, the edges will cool faster...so when you touch it to see if it is cool enough to move to the table, hold by the edge is fine...but touch the bottom/middle to stabilize it, or show off your waiter carrying skills, you burn your palm on the bottom.

True. Working as a waiter delivering hot plates from the kitchen, I've discovered this to be the case over and over again.