Tufts OpenCourseware
Author: Roger Tobin

Interview with Physicist Roger Tobin

Tracy Noble interviewed a Tufts University physicist, Roger Tobin, asking him some of the same questions about heat transfer she asked Alisha. Here are excepts from this interview.

T: Here are some metal and plastic spoons that have been sitting here for the last 20 minutes, at the same room temperature, and we agree that when we feel them with our hands, they don’t feel as though they are the same temperature.



R: As a physicist, the way we think about this is that that the temperatures of these objects are all lower than temp of my body. These are all maybe at, you know, 70 degrees, Fahrenheit, and my body, inside is 98, and even the surface of the skin is probably 90 or something. So all of these things are cold to me.

How cold they feel depends on how much the temperature of my skin drops, when I touch them. So, that has to do not only with how cold they are, but with how effective they are at drawing heat, allowing heat to flow away from my, hand into the, into the material.

So there are, well, two things that, that affect that. One is the, the characteristics of the surface: whether it’s very rough, if it’s very rough, like this [grasps wooden spoon], the, the wood is fairly rough, and this [wooden stirrer] is even rougher, so the points of contact between the actual material and my skin, on the sort of microscopic level, are very small, So that’s one, one factor, and then the other factor is, how well does the material itself carry heat?

So something like this [grasps metal spoon], a metal spoon, has very high thermal conductivity. It carries heat very well, and it’s also very smooth, so I get a lot of area of contact and so that feels cold because it’s very effective at lowering the temperature of my skin. To me, this [grasps plastic spoon] feels the next coolest. Even though the plastic is a pretty poor thermal conductor, it’s smooth, and so that carries heat a little bit better than this one [grasps wooden spoon]. The wooden spoon is probably, not a whole lot different thermal conductor than the plastic - that would be my guess - but it’s, it’s not as smooth, and then this [grasps wooden stirrer] is quite rough and so, barely carries the heat at all. That’s how I would think about it.

[Tracy puts a metal and a plastic spoon in a cup of ice water.]

T: What do you predict these spoons will feel like now that we’ve put them in a cup of ice water?


R: Well I’ll predict that after they’ve been there for a while, the metal spoon’s gonna feel cold. Ok. That’s easy.

R: [T: Yeah.] In fact, if you’d been doing this a couple of weeks ago, after a little while the metal one would probably start condensing water on it.

T: Right. When there was more humidity.

R: Today it’s- Today it’s probably not humid enough for that, but the metal spoon’s gonna get quite cold.



R: The others are a little tricky, to predict. Um, you know, the part of it that’s in the water is definitely gonna get cold. Here [in the air above the spoons] you’re gonna have heat transport going on. The top of it [plastic spoon] that’s in contact with the air, which wants to keep it warm. And then through its handle it’s in contact with the part of it that’s under water, which is cold and it’s a little hard to say where it’s gonna wind up [in temperature] in between, it depends on whether the thermal contact with the air, is more effective than the heat flow through the, the handle, and I don’t have a great feel for that.

I’m thinking about the shape of the spoon, particularly because, you’ve got a big surface area up here [top, or bowl of spoon] to be in contact with the air, and then a rather thin piece of material [handle of spoon] going down, so, um, my guess would be, but it’s a guess, that this [plastic spoon] will get a bit cool, and that it might be detectable, touching it, but I, I am not confident that it will really feel, that much colder than it would if it were just sitting, on the table.

T: How do you think about or envision this heat transfer issue? What goes up or down in this metal or this plastic spoon? What’s the story of what’s happening?


R: Well, the story is, the story’s about flow. The story is about energy. And so I mean, the first thing, that you think about is that, there’s no such thing as cold. All there is is heat, all there is is energy. And so, what I’m thinking about, when I think about a problem like this is, where is the energy coming from and where is it going, and is this a closed system that’s eventually going to reach equilibrium, or is it an open system, where I’m going to reach some kind of a steady state, but there’s always gonna be flow going on.


So I mean if I take, if I took one of these spoons, and I put it in a big vat of ice water, and it was completely immersed, and there was lots of ice in the water, so that it never all melted, you know that would be an equilibrium problem, and I would know that the answer has to be that in the end, the spoon, reaches the same temperature as the ice water. But in a case like this [spoons in mug of ice water], you’ve got sort of two, reservoirs of heat. You’ve got the room, and then you’ve got the ice water, and heat is flowing, all the time from one to the other, including flowing through the spoon. And so, you’ve got heat, energy up here [points to area around top of spoon], coming from the air into the spoon, flowing down the spoon, and then out into the ice water. In order for that to flow, there has to be some temperature difference, between the two. And so, the top has to be warmer than the bottom so that there can be this flow of heat.