1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Watts, amps, and voltage

  1. Jul 16, 2014 #1
    Hi, I am a bit confused about what exactly is being produced when someone uses a hand generator. I know P=R(I^2). Does this mean that if I have a hand crank that can produce 1 amp I can use a 200 ohm resistor and produce 200 watts of power. If this is the case it seems as though producing any amperage can generate an arbitrarily large amount of power.
  2. jcsd
  3. Jul 16, 2014 #2


    User Avatar
    Gold Member


    Yeah, if you were strong enough and the generator could handle in, you could produce thousands of horsepower just by turning the crank. Do you reckon you're strong enough? Good luck with that.
  4. Jul 16, 2014 #3
    Sounds about right. But of course to produce that arbitrarily high power will require an even higher amount of power from your hand doing the cranking. You will always be limited by the power of your physique. As the resistance is increased the difficulty in turning the crank and maintaining 1 amp will get harder and harder.
  5. Jul 16, 2014 #4
    You might be able to produce 200 Watts of power - by supplying at least 200 Watts or work to the crank.
    Of course, you going to need a good size resistor to dissipate 200 Watts.

    Then you figure that with a 1MΩ resistor you'll still be able to force 1 amp through.

    If you do, the voltage will be V = RI = 1 million volts. Your hand crank generator probably can't handle that voltage. It will just arc over and start to damage itself.

    But you can get one that will hold up. So now we'll choose the resistor.
    A common resistor size is 1/4 Watts. For this experiment, that resistor would simply vaporize.
    The wattage you are looking to generate is P = R(I^2) = 1 Tera watt.
    So you will need to do one of two things, either find a 1 TW resistor or only run the experiment for a few picoseconds.

    Now we need to talk about your arm and the mechanical limits of your generator. Are you up to turning that generator with 1TW of power (assuming no Kryptonite).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook