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#181853 11/03/08 10:23 PM
Joined: Jul 2008
Posts: 5
E
Erick Offline 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?

Erick #181854 11/03/08 11:17 PM
Joined: Jan 2005
Posts: 5,370
Likes: 1
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.

Erick #181855 11/03/08 11:22 PM
Joined: Jul 2006
Posts: 27
C
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; 11/03/08 11:26 PM.
canuck #181856 11/04/08 12:32 AM
Joined: Jul 2008
Posts: 5
E
Erick Offline OP
New Member
Thanks very much you guy's are great! Erick

Erick #181858 11/04/08 01:13 AM
Joined: Jul 2004
Posts: 625
S
Member
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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