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Posted by Sherman in Idaho [72.47.9.37] on Thursday, December 06, 2012 at 15:20:14 :
In Reply to: Amps x volts = watts = heat = work.... posted by Chris Case [75.36.46.237] on Wednesday, December 05, 2012 at 16:00:47 :
I you were designing a horn from scratch, that's how it would be. For a given amount of watts (loudness), if you had twice the volts to work with, you'd set the resistance of the coil such that you'd have half the amps.
But when you're using a 6 volt component (starter motor, blower motor, or horn) on 12 volts, the device doesn't somehow automatically rewind itself with twice as much wire of half the size in order to adjust to the higher voltage. Instead, the equation that matters is I equals V divided by R. R is the resistance of the wire. The resistance didn't change, so when you double the V (voltage) you double the I (current). Since watts equals amps times volts, you now have double the amps as well as double the volts, so you have four times the power and four times the heat. If you try it with a light bulb, when you end up with is a flash bulb. With a motor or a horn, there's some compensation via a thing called "back EMF" which applies to moving electromagnetic things and keeps the current from increasing quite as much as the simple equation would lead you to believe, and some salvation in the fact that a starter motor or a horn are only used briefly and have plenty of time to cool off between uses.
But in general, doubling the voltage means 4 times the heat, unless you were to totally rewind the thing for the higher voltage.
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