AN EXPERT SYSTEM FOR UNDERSTANDING EXPRESSIONS FROM ELECTRIC CIRCUIT ANALYSIS ABSLRACT

important that students master the skills it requires. Most of cuaiit analysis is concerned with writing expressions that embody transtoamations, conservation laws and the electrical behaviors of various devices This paper describes work in progress or. an expert system that can examine the expressions that students write, diagnose the errors in them and suggest corrections. The expert system is implemented in OPS5 and uses the strategies of divide and conquer, forward chaining and hypoihesize and-test The system can now recognise most of the Common errors that students make in dealing with linear resistive circuits and operates at about the level of a competent human grader. 1 CIRCUIT ANALYSIS The general form of a circuit analysis problem is: • given the configuration of a circuit and two subsets of its variables (one designated as known, the other as unknown); • solve for the unknown variables in terms of the known variables. By "configuration " we mean P specification of the type and location of each element in the uicuit. By "variable " we mean the value of a circuit element or a voltage or a current. An example of a circuit problem is shown in Figure 1. The first stage in solving a circuit analysis problem is to assemble a set of equations relating the known and unknown variables. The second stage is to solve for the unknowns. Programs embodying numerical and symbolic procedures for the second stage are widely available. Therefore, in this paper we will address only the first stage. Equations relating the known and unknown variables are generated by invoking Kirchhoff's conservation laws (the net current entering each and every node is zero; the net voltage around each and every loop is zero) and by using branch relations (that describe the electrical behavior of the elements in the circuit- for instance, voltage is equal to resistance multiplied by current in the case of a resistor). Many mor