# Circuit fundamentals: Passive elements – resistance

Resistance is an ideal electric circuit component, which is characterised by energy losses in a circuit as heat, some mechanical work, or electromagnetic radiation.

Resistance is marked with that letter r, and it’s measurement unit is Ohm. It is an ideal electric circuit component, where the irreversible process of electromagnetic energy absorption goes. Mathematically explained:

$r=\frac{u}{i}$

The eqation above is called Ohm’s Law. R always has a positive value. Another characteristic

$g=\frac{1}{r}$

is called conductance, and is measured with Simens (sim).

Instantaneous power, on the resistor, is the following:

${P}_{r}=u*i=r*{i}^{2}=g*{u}^{2}$

Here you see how instantaneous power is related to voltage and conductivity.

Generally speaking voltage on the resistor is related to current which goes through the resistor.

$U=I*R$

This relation is known as volt-ampere characteristic. I general, it is not linear.