When applying a voltage U, in any conductor an electric current of intensity** i**. For most conductors these two quantities are directly proportional, ie as one increases so does the other.

Thus:

This constant is called **resistance** **electric ** conductor (R), which depends on factors such as the nature of the material. When this proportionality is maintained linearly, we call the conductor **ohmic**having its value given by:

Being R constant, as stated in Ohm's 1st Law: *For ohmic conductors the intensity of the electric current is directly proportional to the voltage (ddp) applied at its terminals.*

Electrical resistance can also be characterized as the "difficulty" encountered in passing electric current through a conductor under a certain voltage. In SI the unit adopted for this magnitude is the **ohm (Ω)**, named after German physicist Georg Simon Ohm.

You can also define a quantity called Electrical Conductance. **(G)**, as the ease that a current has to pass through a conductor under the given voltage, that is, it is equal to the inverse of the resistance:

And its unit, adopted by SI is siemens (S), where: