EECE 210 Lecture Circuit Variables, Spring 2012

Chapter 1 Circuit Variables Introduction Electrical systems pervade our lives; they are found in homes, schools, workplaces, and transportation vehicles everywhere. Circuit Theory Three basic assumptions permit us to use circuit theory to study a physical system represented by an electric circuit. These assumptions are as follows: Electrical effects happen instantaneously throughout a system  We can make this assumption because we know that electric signals travel at or near the speed of light.  Thus, if the system is physically small, electric signals move through it so quickly that we can consider them to affect every point in the system simultaneously.  A system that is small enough so that we can make this assumption is called a lumped-parameter system. The net charge on every component in the system is always zero  Thus no component can collect a net excess of charge, although some components, can hold equal but opposite separated charges. There is no magnetic coupling between the components in a system Circuit Analysis: An Overview  A commonly used mathematical model for electrical systems is a circuit model.  The elements that comprise the circuit model are called ideal circuit components.  An ideal circuit component is a mathematical model of an actual electrical component, like a battery or a light bulb.  It is important for the ideal circuit component used in a circuit model to represent the behavior of the actual electrical component to an acceptable degree of accuracy. Circuit Analysis: An Overview  Circuit analysis is based on the variables of voltage and current Voltage and Current  The concept of electric charge is the basis for describing all electrical phenomena.  The charge is bipolar, meaning that electrical effects are described in terms of positive and negative charges.  In circuit theory, the separation of charge creates an electric force (voltage), and the motion of charge creates an electric fluid (current).  Whenever positive and negative charges are separated, energy is consumed. Voltage created by the seperation The voltage between two ends of a path is the total energy required to move a unit electric charge along that path. Current Electric Power vi dt dq dq dw   ) )( ( The assignments of the reference polarity for the voltage and the reference direction for the current is arbitrary. As a simplifying convention choose the assigned polarity of the voltage across a Basic circuit element in such a way that the current passing through the element is in the direction of voltage drop. + - Passive Sign Convention  Whenever the reference direction for the current in an element is in the direction of the reference voltage drop across the element, use the positive sign in any expression that relates the voltage to the current (for ex: V = IR, P = IV).  Otherwise, use a negative sign. (V = -IR, P = -IV).