Ohm's Law is the mathematical relationship among electric current, resistance, and voltage. The principle is named after the German scientist Georg Simon Ohm.

In direct-current (DC) circuits, Ohm's Law is simple and linear. Suppose a resistance having a value of *R* ohms carries a current of *I* amperes. Then the voltage across the resistor is equal to the product *IR*. There are two corollaries. If a DC power source providing *E* volts is placed across a resistance of *R* ohms, then the current through the resistance is equal to *E/R* amperes. Also, in a DC circuit, if *E* volts appear across a component that carries *I* amperes, then the resistance of that component is equal to *E/I* ohms.

Mathematically, Ohm's Law for DC circuits can be stated as three equations:

*E = IR*

*I = E/R*

*R = E/I*

When making calculations, compatible units must be used. If the units are other than ohms (for resistance), amperes (for current), and volts for voltage), then unit conversions should be made before calculations are done. For example, kilohms should be converted to ohms, and microamperes should be converted to amperes.

*This was last updated in*September 2005

*Posted by:*Margaret Rouse

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