If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:22

Thermal conductivity of metal and wood

Video transcript

so if you're in a room of some kind I encourage you to try a little experiment right now so look around the room and see if there's something in the room that's made out of wood or maybe paper or cloth and and it's been in the room for some time so hopefully it has the temperature of the room and then find something else that's made out of metal that's also been sitting in the room for a while and it doesn't have its own source of energy so don't use your computer should just be something this has been passively sitting in that room for a while it's not too hard to find and touch them both and what you will see is even though they've both been sitting in that room for a while the metal is going to feel a lot colder the metal is going to feel colder and this is a bit of a conundrum because they've both been sitting in this room for a while so they both should take on the ambient temperature so let's make this a little bit more concrete let's say that the temperature of the room let's say it's 70 degrees Fahrenheit and the key for this feeling colder is that the ambient temperature of the room is less than the then your body temperature your body temperature is going to be roughly 98.6 degrees 98.6 degrees Fahrenheit so let me write this this is body temperature and the temperature at the surface of your skin might be a little bit different than this but let's just assume that it's roughly that it's roughly 98.6 degrees Fahrenheit and so what's happening is is that this metal one this metal surface isn't actually colder doesn't actually have a lower average kinetic energy then though then the wood surface they're both if they've both been sitting in this room for a while they're both going to have the ambient temperature temperature of 70 degrees 70 degrees Fahrenheit so what just happened why to your to your skin to your brain does the metal actually feel colder and the simple answer is it's better at taking the heat away from you and so why is it better at taking the heat away from me well let's just imagine let's just imagine let's say these here are the atoms here are the atoms on the surface on the surface of my on my skin so these are the atoms on the surface of my skin on the bottom of my scale much to say in my hand is touching a surface like this that's my thumb right over there and I'm touching the surface and it's going to have some era it's going to have an average kinetic energy that would be in relation to a body temperature of 98.6 degrees Fahrenheit so these things are going to bounce around or vibrate around so they're going to have and maybe the covalent bonds between the the carbon molecules and the other MA the carbon atoms and the other atoms on my skin that keeps them from kind of kind of breaking free fully but they're going to be kind of oscillating around bouncing around a little bit and they'll they'll even kind of push on each other and this could be kind of the electrostatic forces doing it but they're going to have some average kinetic energy now let's say my hand is touching to both of these surfaces at the same time so I have I have the wood surface right let me do this in a different color so I have the wood surface I'll do that in yellow so I have the wood surface right over there that is wood and I have the metal surface I'll do that in white so I have the metal surface right over here and this metal surface we already talked about it's going to feel is going to feel colder let me draw the rest of my hand actually so the rest of that's not so the rest of my hand so there you go or my arm so you get the I you get the idea so what's going on here so let's just think about it at a more at a microscopic level so the wood first of all its surface is going to be uneven so you're going to have atoms up here but then you're going to have gaps there's going to be air here and let me actually scroll down a little bit so it's going to be like this so you're going to have gaps like like that and it also has internal gaps it'll also have internal internal gaps like that so this would be the wood while the metal the metal is much denser the metal is much denser and the surface is actually much smoother so the metal is let me do the metal in that white color so the metal the metal the atoms are much more closely packed it is much denser this top the surface is smoother it won't have any internal air pockets it's not going to have any internal air pockets in it and so what's going to happen well we've already always said you're going to have a transfer of heat from the higher temperature from the higher temperature system or the higher temperature thing to the lower temperature thing and so as I you know they're already going to have some kinetic energy they're going to be so these things are going to have a kinetic energy that's consistent or an average kinetic energy that's consistent with 70 degrees Fahrenheit so let me just draw a couple of these arrows same thing over here they're going to have the same average they're going to have the same average kinetic energy so these things are all jostling around bouncing around and pushing on each other with the electrostatic forces so hopefully this gives you an idea of things but my hand is warmer my hand has a higher average kinetic energy and so the molecule the the atoms and the molecules in my hand are going to bounce into the atom the atoms and molecules of the wood and they're going to make they're going to transfer the kinetic energy but what we realize in the wood is I'm making less contact because the surface first of all the surface of the wood isn't smooth so I'm making less contact so this one right over here might just bump into another air particle it actually won't bump into a wood particle but some of the wood particles will start to will start to take some of the kinetic energy away from me and I will sense that as being a little bit cool so maybe that takes a little kinetic energy that bumps into this guy and then so the kinetic energy does get transferred down it's going to be transferred down a lot slower than what would happen in the metal because one I don't have as much surface contact between my hand in the wood because of these gaps I also have air pockets I have air pockets in the wood like this and in general the wood is less dense so there's going to be there's going to be less collisions and it's going to take more time for that kinetic energy to be transferred away from my hand in the metal on the other hand as soon as something as soon as this this atom bumps into this one that's going to bump into that one that's going to bump into that one that's going to bump into that one and that kinetic energy is going to be very very quickly transferred is going to be very quickly transferred down the metal so it's going to be able to take more heat away from you because so this one's going to be this this molecule right over here it's going to be it's going to get some kinetic energy from a molecule in my hand but then it is going to bump into its neighbor and transfer that connecticut so it's going to lose its kinetic energy quite quickly and so it's ready to be bumped into again by another molecule for my hand and take on more kinetic energy so it's going to SAP the heat away from me faster so you have faster heat transfer heat heat transfer then you have with the wood and from your body's point of view this faster this heat being sapped away from you faster even though the two surfaces are actually the same temperature you perceive this you perceive this your body perceives this right over here as being your body perceives that as being as being colder