Ohm's Law

• There is a direct relationship between the amount of electromotive force applied to a circuit and the amount of current in the circuit. In other words, if the EMF in a circuit increases by 10%, the current in the circuit will also increase by 10%.
• There is also a direct inverse relationship between the amount of resistance in a circuit and the current in a circuit. In other words, if the resistance in a circuit increases by 10%, the current in the circuit will decrease by 10%.

He formulated a mathematical equation to represent these relationships. The equation is known as Ohm's law.

• E = I x R.

This equation can be manipulated with algebra to produce two other formulas:

• I = E/R
• R = E/I

If you will recall from Chapter One, "E" represents Electro-motive force (EMF), "I" represents Current, and "R" represents resistance. In typical automotive diagnostics, most of Ohm's law calculations are not very useful; however, the resistance calculation is extremely useful because a live circuit resistance determination can be made.

The Ohm's Law solving circle below will help you to remember Ohm's Law and use it to solve circuit calculation problems. To use the solving circle, cover the letter you want to solve for. For example if you wanted to solve for E, cover the E. I and R are left on the bottom, multiply the two together to find the answer for E.

IMPORTANT: All ohm's law calculations must be performed with the same units of measurement. To prevent errors in your calculations (see chapter 7), convert all numbers to base units first.