Heat transfer of hot metal immersed in water

  • Thread starter jangheej
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  • #1
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Main Question or Discussion Point

if you plunge a hot metal (like wire) into water, the water temperature obviously increases. I wonder what kind of heat transfer occurs between the metal and the water.
it clearly doesn't seem like convection or radiation. But im not so sure if conduction is the right one cos it is usually explained in the context of two solids in contact.

can anyone explain in detail how the heat is transferred from the hot metal to water?
and also whether the rate of transfer depends on the temperature difference.
 

Answers and Replies

  • #2
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It's conduction you're looking for.

I'd also point out that if the water temperature is above that of the metal then the water temperature will decrease.
 
  • #3
Conduction at first, but if the metal is sufficiently hot, then I believe a layer of water vapor can form between both. And it would seem to me that this layer could well allow convection and maybe also radiation.
 
  • #4
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Shouldn't there always some heat transfer in the form of radiation? Although I assume it's negligible in the context. I mean, don't we have from Stefan-Boltzmann's law that radiated power is something like P= k * sigma * T^4, where 0<k<1 is a constant describing how close to a perfect black body the rod is?

And also (which I think was in part what the original question was about), how is the conduction happening in detail? Is it something like that the outer atoms in the metal rod "give away" thermal energy to the surrounding water molecules until they are in equilibrium (also, thermal energy is transported from the inner parts of the rod to the surface)? Can one think of the transfer of thermal energy from the rod to the water as collisions between surface atoms in the rod and water molecules? (Note that all this is assuming that the water is at a lower temperature than the rod.)
 
  • #5
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At the wall there is always pure conduction due to the no slip boundary condition. The process can be described as kloptok put it.

Free convection can happen outside of that layer due to buoyancy forces. Radiation is always in play as well.
 
  • #6
Mapes
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Agree with kloptok and MrMatt2532. Where there is a fluid (gas or liquid), there will be some degree of convection. And radiation is always a factor, through the flux may be comparatively negligible unless the wire is red hot or hotter.
 
  • #7
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The atoms and electrons in the hot metal are moving with a higher energy than the water molecules. When a water molecule bounces off the metal, it will generally bounce off with a higher energy than it had before the collision, while the metal atom that it hit will be left with less energy. This is the way that energy is transferred from the metal to the water, raising the temperature of the water, and lowering the temperature of the metal. If there was no gravity, and the water was not raised to its boiling point, then there would just be a diffusion of heat as each heated water molecule then lost energy when it collides with another water molecule farther from the metal, heating the water further away from the metal. That will be conduction. If there is gravity downward, the heated water will also rise away from the metal (if the face of the metal is not vertical) and cooler water will rush in to cool the metal even more effectively. That will be convection. if the water is heated so quickly that it vaporizes, there will be bubbles of steam forming, which then will lose energy to the liquid water. The bubbles may be totally condensed before they reach the surface of the water, and all their heat goes back into the water. If the water is at its boiling point, the bubbles will not condense and steam will come off the surface of the water, carrying heat away from the water, and you will see the water boil.

There will be thermal (mostly infra-red) radiation from the metal that is absorbed by the water, and thermal radiation from the water that is absorbed by the metal. If the water is cooler, the metal will not absorb as much radiation as it emits and it will cool down. The water will absorb more than it emits, and it will heat up. But the amount of energy transferred this way will probably be much smaller than the mechanism mentioned above.
 

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