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.
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 ohmichaving 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: