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Generating heat from electricity from friction

  1. Nov 28, 2011 #1
    I am interested in generating heat manually by rubbing two materials together. I am trying to find the most efficient way to generate the most heat. I've read a little about thermoelectricity where if a current flows in a circuit consisting of two different conductors then one of the junctions is heated and the other is cooled.

    My question is, would it be possible to rub two conductors together to generate electricity from friction, and use that electricity to generate heat? I don't need to generate a huge amount of heat, just *more* heat than friction alone would generate.

  2. jcsd
  3. Nov 28, 2011 #2


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    Possible? I don't see why not. I just don't see any practical use for it. Other methods are far better for generating both heat and electricity.
  4. Nov 28, 2011 #3
    Sure there are better ways to generate heat. But it's the simplicity of rubbing two items together to generate more heat than expected that I'm trying to demonstrate.
  5. Nov 28, 2011 #4
  6. Nov 28, 2011 #5
    This works the other way round as well. Heating the junction will cause current to flow.


    There's no reason the heat couldn't be created by friction and the current generated used to operate a separate heater.

    I guess the trouble is that you'd be wearing away your junction in direct proportion to how much you used it to generate current.
  7. Nov 29, 2011 #6
    Thanks for the input. It got me thinking about some different methods, but I think the biggest hurdle is that I'm trying to do it without an additional power supply or battery.

    Solid state heat pumps are along the lines of what I'm looking for, but they seem to take large amounts of current (1+ Amps) to operate. Could these be useful only using the current generated by rubbing two objects together?

    It says that the voltage created by a thermopile is only in the range of tens or hundreds of milli-volts. Would additional heat generated by this small voltage be noticable?
  8. Nov 29, 2011 #7


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    Where does the input energy to do the rubbing come from?

    The problem I see with your idea is that heat is the final waste product of most processes and as such, direct heat production is nearly always about 100% efficient. So it really doesn't much matter how you generate it.
  9. Nov 29, 2011 #8
    The input energy is manual. Simply rubbing the objects together by hand.

    I agree that the waste heat is 100% efficient, but that's why I am considering a simple thermoelectric or heat pump device to give it a boost. The additional heat transferred by the device would make the heat output more than 100% of the input energy.
  10. Nov 29, 2011 #9
    That's referring to a thermopile as a commercially available electronic component. You can scale them up to any output. As I recall the specific metal combinations used, the area of contact of the dissimilar metals, and the temperature difference between the hot and cold ends are the important factors. Once those are optimized, then you just keep adding them in series like batteries. I once saw a photo of a guy hanging from an iron rod being held by an electromagnet being powered by a thermopile (which was being powered by a propane torch). You can get some current out of them.

    But, like Russ says, the original waste heat you have is already at maximum efficiency.
  11. Nov 29, 2011 #10
    Just converting work to heat 100% is not the maximum "efficiency" by his definition of efficiency.

    Using a heat pump gives more heat than work input, since you can also take thermal energy from the surroundings.

    Thermopile is a piece of trash. The classical fluid cycle heat pump is much better.
  12. Nov 30, 2011 #11
    A thermopile works on temperature difference. As I said earlier, the amount of electricity generated is dependent on the magnitude of the difference in temperature between the heated junctions and the cooled ones. If he had his "cold side" out in a Minnesota winter, for example he'd generate much more electricity with a given amount of heat friction than if the cold side were in the room with him. He could, in fact, passively generate current by sticking the cold junctions in a snowbank, and keeping the hot side in his armpit.

    Hometown, they also have these things now called Thermo-electric generators:


    You could buy two and rub them together.
  13. Nov 30, 2011 #12


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    Oh, I see: you want to use a thermoelectric device as a heat pump....

    Well, it could work, but I don't think you could get much from it. Peltier devices are pretty inefficient and using friction to generate electricity is even worse. Why does it have to be friction?
  14. Nov 30, 2011 #13
    And why does it have to be friction generated with his own muscle power? Clearly he has a specific and unusual application in mind.
  15. Nov 30, 2011 #14
    My thought is to generate heat in the middle of nowhere using the most basic components. Rubbing two things together seems like the simplest way to generate heat, so I was thinking about how to give it a boost. The theory of heat pumps is great, but you can't readily carry a fluid cycle heat pump around.

    Zoobyshoe, Thermo-electric generators may do the trick. Like you said, sticking one end in the snow and the other under your armpit alone gives you current. No rubbing required. I'll have to look into the size, cost, potential output, etc.
  16. Nov 30, 2011 #15


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    Making fire? Seen it done but there's no future in it I tell you!

    And what about the risks and the environmental damage?

  17. Nov 30, 2011 #16
    The snow/armpit thing would only work with thermopiles you engineered yourself, mind you, because you could make the physical separation between the hot and cold junctions as large as you wanted. The pre-made thermoelectric generators have the hot and cold side pretty close to each other. I have read there's a danger of the heat from the hot side overwhelming the cold side in some applications (i.e. where the cold side isn't actively cooled). That shouldn't be a problem in your scenario, but you'll have to do something like set the TEG on the cold ground and sit or lay on the hot side. Same principle, though: use the cold to your advantage.
  18. Nov 30, 2011 #17
    Those "fire" demonstrations are all hoaxes. Those are the same guys that hoax easy transportation with "wheels" and who claim you can move a big rock with a stick and a "fulcrum". ThunderSpirit Of The Clouds will strike them dead some day.
  19. Nov 30, 2011 #18
    With the TEGs, is the heat "consumed"? In the snow/armpit scenario, would your armpit eventually get cold? There has to be some cost to the current generated, right?

    I'll look into this "fire" thing as well.
  20. Nov 30, 2011 #19
    Yes, you'd be losing body heat to keep the system going but the return you get should prevent you from getting colder.

    If you want to dabble with such dangerous modern notions as "fire", I would recommend looking deeply into the "wood gas stove". This is a configuration of tin cans that is engineered to gasify wood and burn the generated gas for heat, cooking, etc. The great advantage of a "wood gas stove" is that it works well with twigs and sticks. You don't need to fell a tree or carry a hatchet with you. Properly made and operated, the result of a burning is charcoal, rather than ash, which can, itself be turned around and burned. (Alternately, you could draw on the walls of your cave with it.)

    Just go to YouTube and type "wood gas stove". About a hundred videos will come up.

    Once you get the principle down you could design a collapsable one from sheet metal for easy carrying in the back pack. People on YouTube have even made them from heavy duty aluminum foil. It is quite interesting to see someone get a few sticks to burn for a whole half hour with a large flame.
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