Ohm's Law

E = IR

E is electromotive force, also called voltage, and measured in volts.

I is intensity, also called current, and measure in amperes, shortened to amps.

R is resistance and measured in ohms. The greek letter omega (Ω) is the symbol for ohms.

Ohm's Law is a fundamental concept for electronics that you will constantly refer to, either explicitly through calculation, or intuitively as you scan a schematic or take measurements of a circuit.

Typically you will know at least two of the variables and you will want to solve for the other.

The formula can be rearranged just like an algebraic equation to solve for the missing variable.

I = E/R

If you have a 12V battery and a 6Ω resistor, hooking them together will produce a current of 2A.

Voltage and resistance are easy to measure than current, so this arrangement of the formula gets used frequently.

R = E/I

If you have a 12V power supply and an unknown resistor, you can measure the current to derive the resistance. If current measures 1A, you must be using a 12Ω resistor. Of course you can also just measure a single resistor with an ohmmeter, so this formula usually gets used when solving for resistances that aren't plain resistors. For instance, a transistor or vacuum tube can behave like resistors while in-circuit. This resistance isn't something we can measure out-of-circuit, so we have to rely upon Ohm's Law to derive it by measuring the current and voltage.

E = IR

Back to where we started. If you are observing a 2A current in a circuit made of a 10Ω resistor and a power supply, then the power supply must be set to 20V.