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

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?



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


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]

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]

New Member

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]


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




