Thermal conductivity of metal and wood
Why metal at room temperature feels cooler than wood at room temperature.
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- So different materials have different Thermal Conductivity watt/(meter*kelvin). Now if both items are at 70 degrees they will feel different to the touch, metal will feel colder. How could I or calculate to make the two objects feel the same. Like if the wood was 70 degrees and the metal was 80 degrees they would feel the same when touched.(6 votes)
- Actually it's rather easy if you know a little bit of calculus... If you don't know, don't panic, it's also easy... I'm assuming you don't know calculus. So the thing you feel when you touch is actually the rate of heat transfer with respect to time. This means the heat flowing to your body per unit time, say 1 second. So if you can find equal values for rate of heat transfer your question is done.
And to give you a formula I can say that the original formula is like that:
Rate of heat transfer = (Thermal Conductivity*Surface Area*Temperature Difference)/Length
This is called Fourier's LAW or something like that
So back to your question you need to get an equal Rate of Heat Transfer you need to use thermal conductivity of metal and wood and you need to write this equation for both iron and wood and equate them. You will get a ratio of Temperatures.
Sorry for long and ambigious explanation if anything is not clear let me know(15 votes)
- If I understand this well, if the metal and the wood were at body temperature, our brain should feel the same for both ?? (in term of temperature)(4 votes)
- its an interesting question...
At room temperature, if you feel the metal of a door handle and the wood of the door, then I would say that the metal would feel colder. (Try it...)
But actually, they must be at the same temperature...why? because they are in thermal contact and therefore same temperature...
The sensor in your body (in your finger tips for example) are measuring the temperature of your skin. why?...because thats where they are: they are in thermal contact with your skin...So: as the heat leaves your skin, the sensor will tell your brain the skin is (getting) colder.
The point here, is that the metal sucks the heat out of your skin faster than the wood does and so the sensors will tell your brain...."metal = cold....wood = warm". But actually, they are the same temperature. Just that metal can draw the heat out of your body faster, so it feels colder to touch.
sooooo, if you, the metal and the wood are all at the same (body) temperature, then, I agree with you... there should be zero heat transfer and, therefore metal and wood would both feel the same temperature. It would make for a very interesting experiment....how might we set it up? :)
- What material has the highest heat conductivity?(6 votes)
- maybe carbon related materials...i think diamond has the highest thermal conductivity.(5 votes)
- I'm from China, hope you can understand what I mean. To extend this lecture, I was wondering if it's that objects with different materials have different amount of time to reach equilibrium? And is there a formula to calculate the time elapsed? And is there a coefficient number of different materials of their conductivity?(3 votes)
- Hello Noah,
You have asked a wonderful question.
You could search for “thermal conductivity.” You will find tables listing the conductivity of various materials. For example copper is a excellent conductor of heat. Just for fun examine the construction of fancy cooking pans. You will often find a layer of copper to help distribute the heat allowing a more uniform cooking surface.
You asked about the “temperature vs time.” There is a field of study called “Fourier Series” that describes the heat transfer. The math is very advanced – 2nd or 3rd year college. Know that this Fourier Series is applicable to a wide variety of engineering fields. What started out as a description of heat transfer is also applicable to music and digital signal processing of radio signals.
Please leave a comment below if you have any questions.
- But what if I keep my hand touched on the wood surface for a long time? Shouldn't it gradually feel as cold as the metal surface after some time?(4 votes)
- No. Why would you expect that? Does the wood's conductivity change?(0 votes)
- Why does metal at0:22feel colder?(1 vote)
- Marking something as hot or cold is just a matter of transfer of energy..If energy influx takes place in the system then it might feel hot and vice versa...What happens in this case is metal and wood at microscopic level are quite different...Metals atoms are very compactly arranged and are connected by metallic bonds..while particles in wood are diffused..SO what basically happens is when we place our hand on Metal the energy is radiated by the contact particle in all directions ..and there are molecules in every direction to catch its vibrations..while in wood the contact particle vibrates a bit less efficiently(due to restrictive covalent bond) and plus the particles may no be present all around it(as they are diffused) so transfer is not that efficient..Hence due to better transfer of energy in Metal we perceive it as colder.!!(3 votes)
- i thought that this was the different c effect as metal need more quantity of heat to increase his temperature than wood
or the c value is based on the number of molecules ?
so same result different way?(2 votes)
- specific heat is dependent primarily upon particle density, temperature, and particle degrees of freedom. This last factor is the most important. Basically degrees of freedom are simply how many ways each particle can move, either along each axis or twisting about each axis, and how difficult it is to move them in each direction. This determines how much kinetic energy a particle can store as compared to how readily it transmits that energy to an adjacent particle (how much energy it takes to get it hot).(1 vote)
- Stupid qurstion, but the atoms/molecules of the wood and the metal move all the time or only when I touch the material with my hand?(1 vote)
- Unless the temperature is absolute zero, all atoms are constant motion.(3 votes)
- I was told that conduction was only possible in solids, not in gases or liquids ...
is this the wrong concept ? can conduction occur in gases and liquids as well ?(1 vote)
electricity will flow gas and liquid too.
For example the gas in a fluorescent tube and salt solution such as copper sulphate
mercury is a liquid at room temperature :)(3 votes)
So the denser the object the better it conducts heat?(1 vote)
- There is a correlation between the density and the specific heat, though it is not so simple, but in many cases the higher the density the higher the specific heat. So it conducts heat more(2 votes)
- [Voiceover] 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 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 awhile and it doesn't have its own source of energy, so don't use your computer. It should just be something that's been passively sitting in that room for awhile, it's not too hard to find, and touch them both. And what you will see, is that even though they've both been sitting in that room for awhile, 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 is 70 degrees fahrenheit. And the key for this feeling colder, is that the ambient temperature of the room is less than your body temperature. Your body temperature is going to be roughly 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 98.6 degrees fahrenheit. And so, what's happening is, this metal one, this metal surface, isn't actually colder. It doesn't actually have a lower average kinetic energy than the wood surface. They've both been sitting in this room for a while, they're both going to have the ambient temperature of 70 degrees fahrenheit. So what just happened? Why, to your skin, and to your brain, does the metal actually feel colder? And the simple answer is, it's better at taking the heat away from you. So why is it better at taking the heat away from me? Well, let's just imagine, let's say that these here are the atoms on the surface of my skin. So these are the atoms on the surface of my skin, the bottom of my skin, let's just say 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 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. And maybe the covalent bonds between the carbon atoms and the other atoms on my skin that keeps them from breaking free fully, but they're going to be kind of oscillating around, bouncing around a little bit. And 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. And let's say my hand is touching both of these surfaces at the same time. 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, is going to feel colder. Let me draw the rest of my hand, actually. So the rest of my arm, you get the idea. So what's going on here. So let's just think about it 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. Let me actually scroll down a little bit. So it's going to be like this, so you're going to have gaps like that. And it also has internal gaps, like that. So this would be the wood, while the metal is much denser. And the surface is actually much smoother. So the metal, let me do the metal in that white color, the metal atoms are much more closely packed. It is much denser, 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 always said, you're going to have a transfer of heat from the higher temperature system, or the higher temperature thing, to the lower temperature thing. And so, they're already going to have some kinetic energy, these things are going to have 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 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 atoms and molecules of my hand are going to bounce into the atoms and molecules of the wood, and they're going to transfer the kinetic energy. But we realize in the wood is, I'm making less contact. Because, first of all, the surface of the wood isn't smooth, so I'm making less contact. So this one 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 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. So the kinetic energy does get transferred down. But 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 and the wood, because of these gaps. I also have air pockets in the wood, like this. And in general, the wood is less dense. So, 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. And the metal, on the other hand, as soon as 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 quickly transferred down the metal. So it's going to be able to take more heat away from me. So this molecule over here is going to get some kinetic energy from a molecule in my hand. But then, it's going to bump into it's neighbor and transfer that kinetic energy. So it's going to lose it's kinetic energy quite quickly, and so it's ready to be bumped into again by another molecule from 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, than you have with the wood. And, from your body's point of view, this heat being sapped away from you faster, even though the two surfaces are actually the same temperature, you perceive this, your body perceives this right over here as being colder.