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Convection, radiation, confusion

Here’s one in what I hope will not be an ongoing series of posts wondering what the hell is going on in my Perfect Non-Reader’s science homework. See for reference The Opposite of Read. So. What’s this a picture of?


Yes, it’s a teakettle, an image dear to my heart. But the context I am about to provide you with, Gentle Reader, is that it is an image from a worksheet on heat transfer, and the Perfect non-Reader of this Tohu Bohu was asked to indicate whether it was showing convection or radiation. Within that context, then, what’s the picture showing?

Is it a picture of steam? Because that would make sense, if you figure it backwards: we want a picture of steam, so we’ll show a teakettle. The steam, though, is only a few wavy lines; the center of the picture is the kettle itself. So it’s a picture of the kettle, right?

I was trying to count and assess the heat transfers in this picture. For one, of course, there’s the heating element not shown in the picture which is transferring heat to the ceramic (probably) stovetop we do see. Then the stovetop is transferring heat to the body of the kettle. The kettle is transferring heat to the water we don’t see inside the kettle, and the water is transferring heat within itself (by convection, remember?) (Actually, the kettle is transferring heat within itself, too, and also transferring heat to the lid, which we can see is a separate piece, and I must say I hope it’s not transferring too much heat to its handle because ouch) and presumably to whatever air is in the kettle. Then the water is becoming steam, which isn’t a heat transfer as such and then the steam is rising out of the spout and transferring heat to the air surrounding. And of course we can’t really see it, but all of those things that get hot are heating the air around them (by radiation, unless I am really not helping with the homework) even if they aren’t mixing with them the way the steam is. Am I missing any?

Really, though, in terms of getting points, it’s a question of what the picture is showing, isn’t it? If it’s a picture of a teakettle, then it’s radiation, and if it’s a picture of steam, then it’s convection, right? Or am I totally misunderstanding the science—which is completely plausible, of course, but even then, I think it’s a fair question: what is it a picture of?

Tolerabimus quod tolerare debemus,


I agree that the steam is supposed to be what the question is about, and that the (presumably stock clipart) they chose does a terrible job of making that clear. Even a circle or an arrow would have helped!

But I don't think radiation is what they'd be going for, even had the drawing shown no steam; at the level PN-R is being taught (forgive me if she's actually taking a high school or college course) I think radiation is only described as applying to objects that are hot enough to visibly glow; infrared (and other) wavelengths of radiation aren't yet introduced. So I agree about the kettle having convection inside it, and conduction within its walls and from the stovetop, but think the radiation isn't the answer they're looking for.

One more objection I have to that drawing: that's a *terrible* way to design a teakettle handle -- when you tip it to pour, the body of the kettle can just rotate around the axle and remain untipped, unless you push on its (hot) surface!

I think they're probably going for radiation; conduction from the stovetop to the teakettle seems like it would be fine, too. Personally, I think a much better question would be:

"All three types of heat transfer are happening in this picture. Identify where and how each one is happening."

But I teach high school.

I have nothing useful to say. I have, however, been irrevocably reminded of this Flanders & Swann song about the first and second laws of thermodynamics, and so I share it with you.

Wow, yeah, that kettle design is almost as bad as Donald Norman's "Masochist's Coffee Pot".

(Now for the "let's see if I can still do physics" part: hopefully YPN-R isn't reading this ;-)

Talking about the heat transfer from the kettle to the room, convection should really dominate.

The input from the stove is on the order of 2 KW. While the water is boiling, the temperature of the kettle doesn't change, so the total transfer from the kettle to the room should be 2 KW as well.

A 100°C object in a 20°C room radiates about 680W/m^2, and the surface area of a teakettle is on the order of 0.2 m^2, so the net radiative flux from the kettle to the room is ~135W.

The remaining ~1.865 KW has to be lost by either convection or conduction.

I don't have good estimates for the various chunks of Raleigh's equation, but that steam has a LOT of energy. And I'm betting it's an order of magnitude more than that lost by conduction, as well.

Two quick heuristic tests of that assertion:

1) Holding your hand above a teakettle doesn't hurt. Sticking your hand in the steam plume hurts quite a lot. If radiation -- or conduction into the air -- dominated, the steam wouldn't hurt much worse than just being near the kettle.

2) Once the kettle boils dry, the temperature of the kettle climbs quite rapidly, and then you burn the bottom out. If conduction dominated for heat transfer out from the kettle, burned-out bottoms wouldn't be a Thing.

Now depending on the stove or kettle, heat transfer IN could be conduction (electric coil), convection (internal electric coil), or radiation (induction), and I can't tell which from the clip art.

I would take it not as a picture of a teakettle, nor as a picture of steam, but as a picture of the action of steam coming out of a teakettle, in which case the answer would be convection.

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