Ohm's Law


Equations
(Eq1)    
v = iR
Ohm's law
(Eq2)    
v = −iR
Ohm's law


Nomenclature
vvoltage in volts
icurrent in amperes
Rresistance


Details

For purposes of circuit analysis, current in the resistor must be referenced to the terminal voltage. This can be done in two ways: either in the direction of the voltage drop across the resistor or in the direction of the voltage rise across the resistor, as shown:

v = iRv = iR

Therefore, the two possible equations for voltage are:

(Eq1)    
v = iR

and:

(Eq2)    
v = −iR

The algebraic signs used in Eq1 and Eq2 are a direct consequence of the
passive sign convention. Eq1 and Eq2 are known as Ohm's law. Ohm's law is the algebraic relationship between voltage and current for a resistor. In SI units, resistance is measured in ohms. The Greek letter omega (Ω) is the standard symbol for an ohm.

This is the most basic equation in electricity, and it should be known well. In this formula, resistance is the opposition to current flow. Note that a larger resistance results in smaller current for any given voltage. Some texts use a large V or E for voltage and a large I for current.