Int. J. Elect. Enging. Educ., Vol. 33, pp. 353-372. Manchester D.P.• 1996. Printed in Great Britain ANALYSIS OF SOME PHYSICAL PHENOMENA AND PROCESSES BY EQUIVALENT ELECTRICAL CIRCUITS BRANIMIR RELJIN*, SLAVICA RISTICt and MILESA SRECKOVIC* * Faculty of Electrical Engineering, University of Belgrade t VTI, Belgrade 1 INTRODUCTION Circuit theory is the fundamental engineering discipline that covers all electrical engineering. Using electrical models of physical devices (for example, the physi- cal resistor and capacitor, the transistor, the motor, etc.) and then forming and solving the basic circuit equations (the relations corresponding to Kirchhoff's laws and those between currents and voltages describing circuit elements), the circuit theory permits us to predict the electrical behaviour of physical circuits. Advanced methods in circuit theory, such as node analysis and modified node analysis, are very useful for computer-aided implementations. Started in the mid-1960s with the computer program ECAP, a number of routines for circuit analysis are now available: such as TINA, MCAP, SPICE, etc. The MicroSim Corporation offers the PSpice program for IBM-type personal computers - the version of this is a computer package named DESIGN CENTER 1 operating with schematic entry. At the Faculty of Electrical Engineering in Belgrade the program named SALEC 2 • 3 (Symbolic Analysis of Linear Electric Circuits) was developed, which has almost the same syntax as PSpice but the output results are symbolic variables, not numerical ones. All of these programs save time and money in the process of circuit design and analysisv". Once the equations describing the circuit are written by an engineer, the solving of them is made by computer and it is not necessary to know (or to understand) how the program works. (It is similar to driving a car: we can drive but we need not know how the engine works.) After this brief introduction to circuit analysis, it is clear that many non- electrical problems in physics (and not only in physics) can be efficiently resolved using appropriate electrical models. Some characteristicexamples will be considered in this paper. 2 ELECTRICAL MODELS OF SOME NON-ELECTRICAL PHYSICAL PHENOMENA Case 1 Simple mechanical-to electrical analogs Consider the mechanical system as in Fig. l(a) in which the block with mass m is tied to the wall by a spring and is driven by a horizontal force 1.( t). Let 353