Bill asked in Science & MathematicsPhysics · 3 weeks ago

How long should a kettle (water jug) take to boil on a different supply?

If a kettle takes 1 minute to boil on a 240 V supply, how long would you expect it to take on a 120 V supply.

Also, how long would it take when connected to a 12 V auto-mobile battery.


Sorry, I should have said and electric kettle that is powered directly from the supply.

5 Answers

  • 1 week ago
    Best Answer

    Assuming the resistance of the element remains fairly constant, since power is given by

    Power = V²/R

    Then if V is halved, the power will be reduced to (½)² = ¼. In a perfect system it would take four times as long for the water to boil. However, kettles are not perfect and lose heat through the kettle's body, so it will probably take perhaps 5 minutes to boil on a 120 V supply.

    Since kettles are normally purely resistive, they should work equally well on AC or DC supplies. On a 12 volt battery a perfect kettle with no heat losses should take

    (240 / 12)² or 400 times longer to boil.

    In a practical system, the losses from the kettle to the atmosphere will probably be far too large for the kettle ever to be able to boil the water. A boiled kettle will normally go cold in a couple of hours, so unless the kettle had perfect thermal insulation in 6 hours and 40 minutes the water will only be partially heated.

    I hope this helps.

  • oubaas
    Lv 7
    3 weeks ago

    P = V^2/R ; t = 1 min

    P' = (V/2)^2/R = V^2/(4R) = P/4 ; t' = 4t = 4.0 min

    P'' = (V/20)^2/R = V^2/(400R) = P /400 ; t'' = t *400 = 400 min

    • Bill1 week agoReport

      Only answered half the question but a reasonable calculation.

  • 3 weeks ago

    Assuming you mean boil the water, not boil the kettle itself. And bring the water to a boil, not boil it all away.

    half the voltage, with a fixed resistance, is 1/4 the power. Typical kettle is about 1000 watts, so you would see 250 watts.

    How long would that take? difficult to say. At least 4 times 1 minute, but longer as there are thermal losses that are fixed. Probably, at a guess, more like 10 minutes or longer. 

    at 12 volts? that would be 1/20th the voltage and 1/20² or 1/400 of the power. It would never reach a boil, would barely get warm. 1000 watts/400 = 2.5 watts, tiny amount of heat, less than that of a lit match.

    • Bill1 week agoReport

      Sorry Bill, Richard's answer seemed easier to understand. Wish I could award two best answers.

  • 3 weeks ago

    If you want to boil the kettle you have to melt it first.

    • Richard
      Lv 7
      3 weeks agoReport

      When someone says the kettle has boiled, don't they mean water in the kettle?

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  • 3 weeks ago

    It depends on what type of oven it is, gas, electric, or induction.

    • Bill1 week agoReport

      billrussell42 - you assume that people read the question and understand what they have read.

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