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#181853 - 11/03/08 11:23 PM need some help  
Erick  Offline
New Member
Joined: Jul 2008
Posts: 5
Ut
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?


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#181854 - 11/04/08 12:17 AM Re: need some help [Re: Erick]  
renosteinke  Offline
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Joined: Jan 2005
Posts: 5,316
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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.


#181855 - 11/04/08 12:22 AM Re: need some help [Re: Erick]  
canuck  Offline
Member
Joined: Jul 2006
Posts: 27
Canada
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



Last edited by canuck; 11/04/08 12:26 AM.

#181856 - 11/04/08 01:32 AM Re: need some help [Re: canuck]  
Erick  Offline
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Joined: Jul 2008
Posts: 5
Ut
Thanks very much you guy's are great! Erick


#181858 - 11/04/08 02:13 AM Re: need some help [Re: Erick]  
SolarPowered  Offline
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Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
However, remember that the resistance of the heating element is likely to change with temperature, so 600W is only an approximation.



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