Electrical engineers communicate with symbols. Circuit diagrams are abstract representations of real circuits and are composed of symbols for the various elements in the circuit. Here is the circuit symbol for a resistor.
The symbol for a resistor should include definitions for the voltage across the resistor and the current through the resistor. The definitions include:
It's important to remember that the voltage across the resistor, and the current through the resistor are related by Ohm's Law: Vr = RIr when the polarities are as shown below.
We need to be very precise when we consider Ohm's law because the polarities are very important.
Q11. Which is the correct expression for Ohm's law when polarities are defined as shown below?
Q12. Which is the correct expression for Ohm's law when polarities are defined as shown below?
Q13. Which is the correct expression for Ohm's law when polarities are defined as shown below?
While it is true that a piece of metal, like a wire for example, is a resistor of sorts, you need to know that today resistors are made with specific values, and they often take a common form. Typically they look like the one shown at the right - only they' be pretty small, maybe a half to three quarters of an inch long for the body.
A resistor is typically formed from some sort of resistive material and put into a cylindrical form. Usually the resistor will have a brownish body with several stripes painted on the resistor body. Those stripes are in a code that will tell you the value of the resistors (in ohms). Here's what's important.
The resistor shown above is 1000 ohms = 10x102
Notice how cleverly we put certain parts of the result in bold text and colored it. The bolded text corresponds to the stripes, and the colors are shown on the bold text. Here is the color code.
Given a resistor, to calculate the value of the resistance you use the three stripes. (If there are four stripes, just use the first three. The last stripe tells you how accurate the resistance value is.) Here is the algorithm.
In this section you'll learn a little about how to measure resistances. You'll need to have an ohmmeter, a digital multimeter or a data acquisition unit. When you use any of those instruments to measure a resistance, the same thing happens. It's just that a digital multimeter can make voltage and current measurements, while a data acquisition unit can measure frequency and temperatures.
We'll also assume that you're in a lab running these lessons and that you have a lab notebook that you are using. (You should always have a lab notebook for lab work!)
An ohmmeter measures resistance, and gives you a value of the measured resistance in ohms, kilohms or megohms. Many ohmmeters look like the following diagram.
There's an internal source that provides a voltage. That source may be a battery or a small power supply. The source drives a voltage divider - two resistors in series. One of those resistors is internal to the meter, and the other resistor is the resistor being measured. An internal meter measures the voltage across the resistance being measured and converts that voltage into a resistance reading. The resistance being measured is connected to the ohmmeter terminals, and the terminals are often colored black and red.
All you have to do to measure a resistance is to connect your resistor to the ohmmeter as shown at the right, and be sure that the ohmmeter (or DVM or DAU) is set to measure resistance. Don't get uptight about which lead goes on which end of the resistor. It doesn't matter. (The resistor is a "bilateral" element and should be the same either way!)
Here's the way you connect the ohmmeter (or digital voltmeter or data acquisition unit) to the resistor. Here we're using the same resistor as was used in the questions above. The ohmmeter shown here includes all of the circuitry shown above including a power supply or battery and an internal resistance. To measure the resistance it applies a small voltage across the resistance.
At this point, you are ready to start the first laboratory exercise on resistors.
The resistance of a resistor is determined by several physical properties of the resistor. We're going to limit ourselves to resistors that have a constant cross section - like a wire. Here are the properties.
R = rL/A
You may be tempted to conclude that there are no serious "What If?" questions for resistors. Actually, there are many questions about these devices. Note the following characteristics of resistors.
