Basic theory/formula's

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Okay, I remember the E, I, and R formula according to Ohm's law, but can some one remind me of the Watt formula? I know E x I = W, but how do I figure resistance from 100 watts and 120 volts?
 

charlie

Senior Member
Location
Indianapolis
Re: Basic theory/formula's

Don't forget that you can substitute from E=IR to P=IE and vice-versa. An example would be to substitute for E, P=I(IR) becomes P=I^2R.
 

bennie

Esteemed Member
Re: Basic theory/formula's

The logical approach by only using one formula is...120 times 120 = 14400 divided by 100 = 144 ohms.
 

physis

Senior Member
Re: Basic theory/formula's

I find that an Ohm's law equation can always be solved by remembering one rule:

I x R = E can be changed to

E / R = I or

E / I = R

It might be clearer to use numbers:

2 X 3 = 6 can be changed to

6 / 3 = 2 or

6 / 2 = 3

Given any two variables the third can always be deduced. I also think of this as being able to "unmultiply" something by dividing it in the opposite direction. :cool:

100w / 120v = .833333A and you need resistance
that's E / I = R or 120v / .833333A = 144ohms

[ May 09, 2003, 03:54 PM: Message edited by: physis ]
 

jmc

Member
Location
Massachusetts
Re: Basic theory/formula's

Watts = volts x amperes, P = E x I. however, this is only where you have a purely resistive load or unity power factor, no inductive loads. Where inductive loads are supplied, watts = volts x amperes x power factor, P = E x I x PF.
 

jro

Senior Member
Re: Basic theory/formula's

Get yourself an Ugly's book, for under $20, it is a handy reference book full of electrical formulas, and other useful info.
 
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