When to use the Formula Wheel formulas

[xstcleod]

On page 6, 17, and 18 in Mike's NEC Exam Prep guide the formula for Power is P=I(squared)xR, which is the formula on the 12 formula "Formula Wheel (pg.15).

However, on pages 14 and 15 (and others) the power formula is P=IxE.

What determines which formula to use? The questions are all asking for the same thing : power loss, or consumption and/or cost.

Also, on the Formula wheel, in the Ohms Law circle (I=ExR) formulas there some formulas (#'s 1,3,6,9) which have one of the symbols which are "squared".

Can anyone explain to me the reasoning for this and/or when to use each, respective, formula?

Thanks!

 

[bob]

On page 6, 17, and 18 in Mike's NEC Exam Prep guide the formula for Power is P=I(squared)xR, which is the formula on the 12 formula "Formula Wheel (pg.15).
However, on pages 14 and 15 (and others) the power formula is P=IxE.
What determines which formula to use? The questions are all asking for the same thing : power loss, or consumption and/or cost.
Also, on the Formula wheel, in the Ohms Law circle (I=ExR) formulas there some formulas (#'s 1,3,6,9) which have one of the symbols which are "squared".
Thanks!

The formulas are different forms of the same equation. Ohm's law can be written in different forms.
1. P=I(squared) x R is used for calculating conductor losses
2. P=I x E calculates power consumed.

Basic Ohm's law is 1. I = V/R 2. V = I x R 3. Watts = I x V.

Take Eq 2 and substitute for V in Eq 3 then Watts = I x I x R or I(squared)xR
These equations can be manipulated to do what you want.

 

[cadpoint]

You have a known quantity the inner circle, the four letters. Keep in mind this is a summary of other products. This one section is now isolated, by just using the letter one usually stays in the quarter.

Then your usually given another item of quantity that is part of an equation inside this quarter. Solve the equation.
Line up your equation A=B/C type equation and solve it. (for example)

Some time your given only the inner circle, sometime one inner and one outer item, sometimes just two outer circle items.

Now you can also jump around the wheel and use the upper right outer circle to use another part of a quarter to work to proof toward your desired inner circle proof(for example). Sometimes your given several unrelated products or numbers that will not create a desired answer of the inner circle, in this case you might have to use additional quaters of the math calculations to use other quaters formula’s.

Just remember what your proofing /solving for. Always answer the question based on all parts that will be useable and relevant from the question and use all the items accordingly.

Sorry I can’t talk about the book side of your thread but I hope that this helps just the same.

 

[480sparky]

Choose the one of the four in the center that you need to calculate. Now choose the three formulas along the outside, using the two variables you have, to calculate what you need.

 

[LarryFine]

The wheel is actually a combination of Ohm's law and "Watt's Law." The formulae with squares or square roots are actually two steps combined.

The formula you should use is whichever includes the known quatities on one side of the equation, and the unknown quantity on the other side.