# Ohm's LAW

In addition to defining terms, we need to understand some basic principles that are applied in testing electrical devices. One formula that all computer professionals should know is Ohm's Law. From this formula, or a derivation of this formula, all basic power calculations can be performed.

Ohm's Law states that the current (electrons) flowing through a conductor, or resistance, is linearly proportional to the applied potential difference (volts). A conductor is any medium, usually metal, that allows the flow of electrical current. Resistance is any device or media that resists the flow of electrons. In mathematical terms, this means:

Resistance: R=V/I

Current: I=V/R

Volts: V=IR

In these formulas, R = Resistance in Ohms (W), V = Voltage, and I = Current in Amperes.

NOTE
In some cases you may find E used instead of V in formulas expressing Ohm's Law. The E stands for "electromotive force," and is often used by engineers as a more precise technical term for their measurements.

By memorizing any one of these formulas, the other two can be easily derived using simple algebra. For example, the voltage (potential energy of the circuit) is equal to the amperage (the current or flow of electricity) multiplied by any resistance to that flow of electricity. The more resistance there is in a circuit, the lower the current flow for a given voltage.

## PCs and Electrical Power

That PCs use electrical power to operate is no surprise, even to the casual user. The technician must understand the different types of electrical energy and how they work inside the PC. A PC's electrical power can come from a wall outlet, in the form of alternating current, or from a battery in the form of direct current.