Ohm's Law and the Basis of Resistor Technology

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Every electrical engineer has seen the Ohm’s Law triangle, but what does it really mean? And why is it integral to resistor technology?

The History of Ohm's Law


In 1827 German physicist George Ohm explained his theory of electricity, outlining the relationship between voltage, current and his constant of proportionality, resistance. He determined that the current through a conductor between two points is directly proportional to the voltage across those two points. The subsequent modern Ohm’s Law defines the voltage required for any given circuit as the product of the current and the resistance of that circuit, or Voltage = Current * Resistance.


V = I * R

The Ohm’s Law Triangle

Every electrical engineer has seen the Ohm’s Law triangle at some point in their education. It’s the “go-to” for teachers when trying to help visually explain the formula. As a quick refresher, the triangle is a visual representation of the mathematical relationship between voltage (V, sometimes represented as U or E), resistance (R), and current (I) in a circuit. This triangle is an easy tool for new engineers to remember the three main aspects of electricity.

0916 ohms law fig 1

Ohm’s law triangle includes three sections: The top half must always be voltage. The bottom half is then split into two smaller halves for current and resistance – current is usually on the left with resistance on the right, but the order doesn’t really matter. It just seems that because most people remember the formula as V = I*R, they write it in the triangle as such.

To solve for one of the variables, cover the letter being solved for and use the remaining line separation to give you the mathematical expression. For example, when solving for resistance (R), cover the R and all that is left is V and I. Then use the line that separates those two variables as an indication to use division. The same is true when solving for current (I). The only tricky part is when solving for voltage (V); the line separating I and R then represents multiplication because the items are next to each other.

The three resulting variations of Ohm’s Law are:


V = I * R

        I = V / R

          R = V / I

This leads to easy manipulation of a circuit. For example, if resistance were to decrease in a circuit with the voltage held constant, the current increases.

In the end, Ohm’s Law is not very complicated, but it is essential for circuit design. If two out of the three values are known, the missing value can be easily calculated. The inner workings of every circuit, no matter how simple or complicated, rest on this cornerstone of electrical engineering.

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