<< Click to Display Table of Contents >> Navigation:  Part 2: Electrostatic Transducers I > 2.4 Intensive and extensive variables

So far, we have seen that the energy stored in an electrostatic transducer is defined by 4 variables: two variables in the electrical domain, u and q, and two variables in the mechanical domain, Fext and x. We should note that the four variables are of two dissimilar types which are called intensive and extensive variables.

Extensive variables represent a quantity which is typical for the corresponding domain. In the case of electrostatic transducers, the extensive variables are the amount of charge q on the capacitor plates and the distance x over which the moveable plate has moved. The increase in energy stored in a transducer as a result of work done at the ports depends on the change in the extensive variables (dq and dx) multiplied by the corresponding intensive variable (u and Fext). In a sense the intensive variables are a measure for how difficult it is to change the extensive variables. E.g. the work needed to add a certain charge dq to a capacitor depends on the voltage u between the plates, i.e. on the amount of charge that is already present on the capacitor.

The time derivatives of the extensive variables are called flow. The intensive variables are often called effort.