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Building a water heater- using battery and wire

  1. Feb 15, 2009 #1
    1. The problem statement, all variables and given/known data
    Need to heat 30mL of water from 20C to 40C in 1 min(60s).
    Using 6V battery and Nickel Chromium alloy Nichrome wire that has a resistivityabout 10^-6 ohms/meter. So you use the battery and the wire to build a water heater.
    Batter has no internal resistance.


    2. Relevant equations
    1How much heat energy in joules needed?
    2How much power do you need to do it in the time indicated?
    3What resistance should your nichrome coil have in order to produce this much power in heat?
    4Can you create a coil from the wire having these perperties? (length needed)
    5If the internal resistance of the battery were1/3 ohms, how would it effect your calculation?
    (only explain what you have to do, dont recalculate the size of your coil)


    3. The attempt at a solution
    1:mc(delta)=2508J
    2:total joules/time(60s) =41.8W
    3:R=v/I = 0.8587 ohms
    4: R needed divide by the resistance of the wire, so 857142 m
    5: I dont get this part but I think that a longer time will be required?

    I think the length of the wire I calculated is probably wrong, but I dont know which step I did wrong, so please help, thanks =p
     
  2. jcsd
  3. Feb 15, 2009 #2

    Delphi51

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    Your calcs look good to me, but this "resistivity about 10^-6 ohms/meter" is unreasonable - better check that number! Looks like gold wire rather than nichrome.
     
  4. Feb 15, 2009 #3
    What units have you used in the first step? I got smaller amount of energy needed. And my guess would also be, that the unit of resistivity is wrong. Usually it's ohm*meter, not ohm/meter.
     
  5. Feb 15, 2009 #4

    mgb_phys

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    Bulk resistivity is ohm-meter, but for wire of a given CSA you can quote Ohm/meter
     
  6. Feb 15, 2009 #5
    Okay. Thanks for correcting.
     
  7. Feb 15, 2009 #6
    thanks everyone
    I was just unsure about the length of the wire I got
    but I get it now =P
     
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