4.2 Linearity Property (1)
It is the property of an element describing a linear relationship
between cause and effect.
A linear circuit is one whose output is linearly related (or
directly proportional) to its input.
Homogeneity (scaling) property
v = i R       k v = k i R
Additive property
v1 = i1 R and v2 = i2 R
  v = (i1 + i2) R = v1 + v2