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Resistance Wire and Batteries

  1. Nov 25, 2005 #1
    hello, one of my projects is to develop some sort of self heating bottle. i must fabricate and build a working prototype. i have a couple of ideas. one is to use the same concept as a toaster. wrap the bottle in nichrome, a type of resistance wire. however, would the wire get hot enough to heat a beverage using maybe 3-4 AA batteries? is there another type of wire that you know of which will work? my other idea is to short 3-4 batteries attached to the bottle. i did some testing and found that this caused the battery to heat up very fast and the battery got very hot. i could be able to attach several double A batteries and short them...

    i would however would like to run with the wire idea, as it will probably work better, providing that it will work off of a couple of batteries.

    any input is appreciated, thanks guys.
     
  2. jcsd
  3. Nov 25, 2005 #2

    Gokul43201

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    If you have the patience, read through this thread : https://www.physicsforums.com/showthread.php?t=44551&highlight=butter

    Do not overdraw from the batteries...they won't last very long that way, and your device will have a very short shelf life.

    Do you want the bottle to heat up a beverage from room temperature, or just keep a hot beverage hot for a long time ?

    If it is the first, it's going to be tricky. The problem with using batteries (say 4 AAs), is that you get a maximum power output of 6V*1A = 6W. A typical soda-can will hold at least 250cc of water. To heat that up from room temperature to kinda hot (~70C or about 170F), takes 250g*4.2J/g-C*50C = 60,000J. At the rate of 6W, this will take 2.5 hours to heat up.

    You'd get there in about 5 minutes, if you could use a wall outlet instead.

    If you only have to keep something hot by gently reheating, you can easily make this work.
     
  4. Nov 26, 2005 #3
    "If you only have to keep something hot by gently reheating, you can easily make this work."

    could you elaborate a bit?

    hmmm...yes my first plan was to have a device that will heat a cold beverage up to something warmer...doesnt have to be hot necessarily, just warmer than its current state.
     
  5. Nov 26, 2005 #4

    Integral

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    You need to compute, as Gokul has done, the amount of energy you need to accomplish what you need. Given your source voltage and the fact that [itex] P = \frac {E^2} R [/itex] you can find the resistance required. Since Nichrome has a well know resistivity you now know how long to make your wire.
     
  6. Nov 26, 2005 #5
    could you help me out? im a little rusty in doing these calculations. i know what V, I, and A are. i beleive ohms law is V=IR? the amount of energy that i need is for the batteries or the wire? so then i find that resistance...then find a wire that will have that resistance? will i need to use Q=MC(T2-T1) ?
     
  7. Nov 26, 2005 #6

    Gokul43201

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    Just a bit...I'm outta here in a couple of minutes.

    If you had hot coffe in a thermos. It would stay hot enough for maybe 20-30 minutes. If you wanted it to stay hot for a couple of hours, you'd have to throw in just a tiny bit of heat to have it stay hot. That doesn't take much power.

    It all comes down to how much hotter you want to make the liquid. Inreasing the temperature by 10C (18F) from room temperature doesn't hardly make a tangible difference. You'll want at least a 30C increase in temperature to make the liquid noticeably warmer. And then you run into the problem I brought up in the previous post.

    Did you read the other thread ?

    PS : What grade are you in ? What class is this for ? And when is the deadline ?
     
  8. Nov 26, 2005 #7

    Gokul43201

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    Yes, that's what I used to calculate the 60,000J number in my first post.
     
  9. Nov 26, 2005 #8

    Gokul43201

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    On the other hand, if you want to warm up a refrigerated beverage to about room temperature (no idea why anyone would want to do that), that's only about a 15C increase in temperature.
     
  10. Nov 26, 2005 #9
    it wouldnt be a refrigerated beverage..it would be something that a consumer has left in his or her car in cold weather for instance. im in first year engineering and this project is for physics. we are going to be studying electricity in a week or so, so im just going off my highschool physics that i remember. this project isnt due for a month, as a matter of fact this project is sort of the culminating task for electricity. i just like to get a head start and start experimenting as soon as i can. sorry for the confusion. and thanks for your help, i will be starting those calculations and will post later, im going to bed right now. me tired.
     
  11. Nov 26, 2005 #10

    Gokul43201

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    1. Give yourself a deadline one week before the real deadline. So you have three weeks. This is the only way to do something that's better than just enough.

    2. Do you have a description of the project ? If you have a written description (as given by your prof), quote it exactly. If not, it is important to have a precise definition of the problem, so ask your prof for one (asking will be seen as a positive sign). You need to know (i) the typical volume of liquid that needs to be heated, (ii) the initial and final temperatures, and (iii) how much time is acceptable.

    3. Goodnight.

    PS : Incidentally there's another member here that goes to Uni at London, Ontario (UWO, is it?)...can't recall who.
     
    Last edited: Nov 26, 2005
  12. Nov 27, 2005 #11

    Ouabache

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    To give you some food for thought, here is a homebrew design describing construction of a heater using nichrome wire. He also describes an easier design using 330ohm 1/2W resistors, in a parallel arrangement. His design was intended for keeping the dew off the optical surfaces of a telescope and uses 12Vdc source.
     
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