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by gfretwell  10/19/20 03:43 PM






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need some help
#181853
11/03/08 11:23 PM

Joined: Jul 2008
Posts: 5
OP
New Member

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?



Re: need some help
[Re: Erick]
#181854
11/04/08 12:17 AM

Joined: Jan 2005
Posts: 5,316
Cat Servant Member

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.



Re: need some help
[Re: Erick]
#181855
11/04/08 12:22 AM

Joined: Jul 2006
Posts: 27
Member

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.



Re: need some help
[Re: canuck]
#181856
11/04/08 01:32 AM

Joined: Jul 2008
Posts: 5
OP
New Member

Thanks very much you guy's are great! Erick



Re: need some help
[Re: Erick]
#181858
11/04/08 02:13 AM

Joined: Jul 2004
Posts: 625
Member

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




