Two multiports are connected by power interactions using Variables
Power variables are classified in a universal scheme and to describe all types of multiports in a common language.
Effort e(t) : voltage, temperature, pressure
Flow f(t) : mass flow, current, entropy flow,
The momentum or impulse p(t), (magnetic flow, integral of pressure, angular momentum, ...
The general displacement q(t), (mass, volume, charge ... )
Energy stored by a spring