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To model a physical system, individual components are identified and their constitutive equations are determined. In general, these constitutive relationships are obtained from experimental measurements of the component's "through" and "across" variables; through variables (r) are measured by an instrument in series with the component, while across variables (a) are obtained from an instrument in parallel. For electrical systems, the through and across variables are current and voltage, respectively. For mechanical systems, force and displacement (or its derivatives) play the role of through and across variables, respectively. Note that through and across variables may be tensors of any order, including scalars and vectors.
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