Substitution Algebra

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Cross Multiplication



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Inductance in Series

Inductance in Parallel

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Capacitance in Parallel

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A direct proportion is often used in electrical calculations. 

Proportional calculations are useful in electrical circuits. They're easy to set up and easy to solve. They follow this pattern:

If a is to be then c is to d.

Or if a wire is a certain number of ohms for a certain number of feet, then it will be a certain number of ohms for another length.

Example: If 600 feet of a conductor is 60 ohms, what will be the resistance of 800 feet? Kind of easy, eh? You can do this one in your head. 80 ohms. That's because resistance is directly proportional to length.

A formula representation for this concept is A / B = C / D

The calculator technique is referred to as cross multiplication. Depending on which value you need, here are the formulas derived for the unknown:

A = B times (C / D) or A = BC / D
B= A divided by (C / D) or B = AD / C
C= D times (A / D) or C = AD / B
D = BC divided by A or D = BC / A

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