2 Lesson Intro to Devices (Section 2-1) (CLO 2-1) Chapter 2 introduces devices. We draw a distinction between a device something you can buy at Radio Shack, and an element a mathematical model of a device. Devices are inherently nonlinear, but in this course, elements or models are all linear. We also develop two sets of constraints: 1. Element constraints the physical behavior of devices using their linear models, and 2. Connection constraints the behavior of connected devices using Kirchhoffs conservation laws. Both of these constraints must be satisfied in every circuit. This lesson will focus on element constraints the next on connection constraints. Element Constraints Suggest starting with the resistor and explain its i-v characteristics and its model Ohms Law v = I R where R is resistance with units of ohms or i = v G where G is conductance with units of Siemens S. Conductance is a useful tool in discussing duality. This concept is easy for students so do a quick problem and have the students calculate R given i and v, for example. Introduce the open and the short circuits as extreme examples of a resistor and plot their characteristic on an i-v graph. This will be useful later when we set current and voltage sources to zero. Show the relationships for power dissipated in a resistor p = vi=iRi = i2R and p = vv/R = v2/ R. Discuss the power rating of a resistor and have them calculate the maximum current through a 100 , W resistor. Mention that discrete resistors come in specific values. There is a table inside the back cover of their text that what shows those values are depending on the resistors tolerance. Explain that tolerance is the maximum error an actual resistor may vary from their nominal rated value. You might also mention the color code and how to read it. . Now, especially for the CpE folks introduce them to the switch the first two-state device they will learn as an open or short, either/or but never both at the same time. Note that a switch always has p=0 W regardless of what state it is in. Move on to sources. Introduce DC sources and their i-v characteristics and circuit models. Show how the characteristics change if we were to increase or decrease the value of the current/voltage source. Show how their i-v characteristics would look if they were producing zero volts and zero amperes. Tie these characteristics back to an open and a short. Show them how the models would look for ac (time-varying) sources. For now tell them we will concentrate on DC sources until we reach Ch 5. Suggest telling student that there are many, many devices and that we will introduce more and more of them as we progress in the course. It might be useful to introduce the two connection constraints here since the next lesson is crowded. Mention Kirchhoffs laws and their origins. Take time to explain what a node is. It is not simply a solder point. Figure 2-11 serves as a useful example to show the difference between a node and simply the junction of two wires. Show students that they can find a node by drawing a line parallel to the wires and crossing the wires only where the wire connects to each element.