ECN Forum Forums General Topics of the Trade General Discussion Area need some help

Guy's I am a crazy with embarrassment but here goes: An electric heater is rated for 2400 watts at 240 volts. What power is consumed when the heater is operated at 120 volts? I=P/E, R=E/I, I=E/R, P=EI, I know the steps but with unequal voltages?? man I am so stumped, I know the answer it's (600) but don't know the formula Ohms law out the window to a point what am I missing?

Watts = Amps times Volts
Volts = Amps times Ohms
therefore .. Watts = Amps times amps times Ohms
so ... amps times amps is a 'squared' relationship
that is, reducing the volts by half, and you cut the amps by four (2x2). Thus, 1/4th the heat.

First, use this formula to calculate the resistance @ 240 Volts

p = e^2.......p = watts e = volts r = ohms(resistance)
.....---
.....R


2400 = (240)^2
.........------
...........R

R = ((240) x (240))
.....-------------
.....2400

R = 24 ohms

Then use the same formula to calculate the power at 120 volts:

p = (120)^2
.....------
.....24

p = 600 watts

Here are some more formulas that are helpful:
http://www.the12volt.com/ohm/ohmslaw.asp#pie

Thanks very much you guy's are great! Erick

However, remember that the resistance of the heating element is likely to change with temperature, so 600W is only an approximation.