Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Boiling water by high frequency AC current

  1. Jun 29, 2006 #1


    User Avatar

    I've heard that if you pass a high frequency AC (26kHz?) current through water it boils.
    Does anyone know if there is any truth to this?
    Has anyone ever done it?
  2. jcsd
  3. Jun 29, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The only way to make water boil is to heat it up. You can certainly heat it up by passing a current through it, but I doubt that the specific frequency is important. You will achieve the highest power transfer, of course, by matching your source's impedance with the impedance of the water.

    - Warren
  4. Jun 29, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Many steam humidifiers work by passing 60Hz AC through water and boiling it. Just do a Google search for "electrode humidifier".

    Note: They work rather poorly on de-ionized water (as expected), but for regular tap-water, they are pretty good.
  5. Jun 29, 2006 #4


    User Avatar

    au contraire, Warren. i'm sure you know that you can put an ice cube in a glass of some room temp water and put it in a bell jar, hook it up to a vacuum pump, suck as much air as you can out of it, and watch it boil (even as the ice cubes are cooling it down).

    the resistivity of the water is likely dependent on the frequency of the juice that's applied. you do get highest power transfer by matching impedances, but i wouldn't suggest that as the thing to do (unless you like tripping circuit breakers). you connect 120 volts AC (60 Hz) to a glass of tap water, and assuming the circuit breaker doesn't trip, i am not sure i would want to be in the same room (maybe if i had a plexiglass sheild)! a lot of energy will be quickly transferred to that little bit of water that the boiling will be more like an explosion.

    so, if the resistivity is higher, i might make sense to do it at a higher frequency.
  6. Jun 29, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If I'm not mistaken, the ac conductivity is quite flat over a large range of frequencies.

    The typical resistivity of tap water is 10-100 kOhm-cm. For a regular glass of water, with electrodes separated by an inch, the aspect ratio l/A is about 0.1 /cm. That makes the resistance of the water in the glass at least a kOhm (which makes the current about 100mA). And the joule heating power is about 10W. It'll take over a half-hour, just to get that water up to a boil and another half-hour to boil it all off.

    PS: (unrelated to above calculation, but) microwave absorption coefficients are tiny until you get to the GHz range. Peak absorption happens somewhere in the THz range.
    Last edited: Jun 29, 2006
  7. Jul 8, 2006 #6
    Excellent guesstimation work! :smile:

    To the original poster's question... there are people trying to build weapons using the idea you've described.

    It works using a high energy pulse from a big bank of capacitors. I think they used quite a high voltage, possibly a few kV or even tens of kV judging by the size of the arc, amount of noise and the flash.

    The discharge went off between a pair of electrodes and the water was either sitting between those at that point in time or injected between them, could have been as a mist or in droplet form - they didn't really go into the details of that. Might have had something mixed into it to improve conduction, maybe not. I would expect they've experimented a bit with different liquids.

    Anyway... big, big bang as the electricity boiled the water and turned it into a gas.

    Obviously, water expands a lot as it goes from a liquid to a gas so if the transitions occurs in a sealed tube that's only open at one end, anything loose placed in that opening will quickly vacate it.

    They didn't seem to have reached the tube stage and were just firing it in the open air with the discharge pointed at a plank of wood that was somewhere between 1/2 and 1" thick, which it was able to punch a hole through.

    Just yet another idea for how to kill other humans, and one that, like the hand-held gauss and rail guns, relies on a gigantic capacitor bank and so much juice per shot it's practically useless using currently available components. A lot of this is just the US millitary trying to reinvent the wheel in terms of human destroying technology in hopes of obtaining a more efficient splattering mode - or at least one that looks and sounds cooler, or is confusing enough for the general population that they can say it's okay to do it.

    And since RBJ mentioned it... :tongue:

    http://www.fluidedesign.com/images/vap press.wmv"
    Last edited by a moderator: Apr 22, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook