Resistors and Multimeters
I. Purpose
The purpose of this exercise is to teach you about resistors, electrical resistance, multimeters, and wiring up electrical circuits. First, you will build some resistors of different shapes and sizes and measure their resistance using a multimeter. Then you will find out what happens when you connect together resistances in series and see how the total resistance adds. In the last part, you will examine what happens when you connect resistors in parallel.
II. References
Read this write-up before coming to the lab. If you have had no prior experience with electrical circuits, you should do some additional reading about current, voltage, resistance and Ohms law. Most introductory physics texts have at least one chapter on such things; see for example Chapter 22 from "Physics for Scientists and Engineers-Volume 2" by Tipler. There are also many books written for people who want to build circuits and want a more practical-minded discussion, see for example: "Getting Started in Electronics" or "Engineers Mini-Notebook: Formulas, Tables and Basic Circuits" both by Forest M. Mims (Radio Shack);
III. Equipment
#2 pencil resistor board multimeter
II. Building your own resistors
(1) Resistors can be made from many different materials and come in a wide variety of shapes and sizes. The most basic requirement is that the material from which a resistor is built has to let electrical current flow through it, i.e. it is an electrical conductor. In practice, most resistors are made from metal wire, thin-films of metal, or amorphous carbon which is a non-crystalline semiconductor. Very roughly speaking, a semiconductor is a relatively poor conductor of electricity.
(2) In this lab, you will start by constructing your own resistors using a pencil and a piece of paper. The pencil "lead" is actually made of graphite, which is a form of carbon. Graphite is a semiconductor and so can be used to construct resistors. Begin by using your pencil to fill in the two rectangular regions below on this paper. To get a good consistent resistor, you have to put down a fairly uniform coating of graphite - shade in the region darkly and you shouldn't have a problem. However, don't spend more than a couple of minutes total on this.
II. Measuring Resistance
(1) Set up a small table on your spreadsheet with columns for resistance’s R1, R2, R3 and R4.
(2) Use your Ohmeter to measure resistor R1. Switch to the W scale and place a lead at each end of the first resistor. Let things settle down for a few seconds. You should find a value between 100k W and 5k W. If the meter shows a much larger resistance or indicates that the resistance is off scale, then go back and shade in the resistors more with your pencil.
(3) Measure the first resistor R1 three times, each time picking up the leads and putting them down again near the end of the resistor. This should only take you about a minute to do. Record your values in your spreadsheet.
(4) You should find a noticeable spread in the data. What's causing this variation? Briefly explain.
(5) Repeat your measurements for resistor R2. If R2 looks unusually large, just shade it in some more, but don't worry about it if its not too different.
