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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