Materials Included In Kit

Additional Materials Required

Safety Precautions

Remind students to have caution when handling pins. In the case of a burned resistor, disconnect the circuit and ventilate the room if an unpleasant odor persists. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory and follow all laboratory safety guidelines.

Disposal

All materials may be saved and stored for future use. For resistors that have burned out, wait until they are cool and dispose of in the regular trash.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. All materials are reusable. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• The following table lists the voltages at which each resistor used in this activity will hit its maximum power value.
  {12155_Hints_Table_2}
  A 3 V battery is safe enough for all resistor combinations, however, the 1.8 Ω resistor, denoted with the two gold bands, should not be connected to a battery, except in series with other resistors. Connected in parallel, it will likely burn out.
• Have students memorize the color chart rather than just consulting it, and test them accordingly by not allowing them to consult their charts for some or all of the resistors. One helpful memory trick is to point out that at the values of 2–7, the color code follows the colors of the rainbow.
• If students are having difficulty identifying which end of the resistor has the “first band,” note that all the resistors provided in this lab have a tolerance of ±5%, which is a gold band. This will always be the last band.
• Extra resistors are provided to cover possible accidents and burnouts; the 1.8 Ω resistor in particular comes with 25 as it is the most prone to burn out, as it has the lowest power rating.
• Additional resealable bags are provided; you may wish to keep 15 sets of nine unidentified resistors together for storage and reuse.

Teacher Tips

• This activity serves as an excellent introduction to resistors, and will fit will within any electronics or electricity unit.
• For further study of electronics, consider the Simple Circuits Kit (Flinn Catalog No. AP6302), and the What Is a Capacitor? Kit (Flinn Catalog No. AP7412). An assembled circuit requiring a capacitor can be put together in the Build a Shake Flashlight Kit (Flinn Catalog No. AP7320).
• Extension: If time permits, have students choose any three resistors to connect in parallel and series, but warn them to avoid the resistor with two gold bands. You may wish to have them predict the resistance of their combination before getting started. If the current is below the detection threshold of your multimeters, have students directly measure the resistance instead and calculate the current based on the voltage and resistance.

Answers to Prelab Questions

1. Why is it important to check the expected power across each resistor before connecting it in a circuit?

   Checking the expected power across the resistor will ensure that the power will not exceed the resistor’s power rating. If the power across a resistor exceeds its power rating, it could burn out. Note: Do NOT encourage students to “test” this scenario!

2. A resistor has the following colored bands on it: Brown–Black–Black–Gold. Determine the numerical value of each band. What is the total resistance? The brown band is the first digit, 1. The first black band is the second digit, 0. The second black band is the multiplier, 1. Thus, 10 x 1 = 10 Ω.
3. What resistance range could the resistor from step 2 take?

   The gold band is the tolerance, ±5%, so the resistor could be anywhere from 9.5 Ω to 10.5 Ω.

4. What would be the effective resistance for three of these resistors hooked in series? In parallel?

   Three 10 Ω resistors in series would have an effective resistance of 10 Ω + 10 Ω + 10 Ω = 30 Ω. Three 10 Ω resistors in parallel would have an effective resistance of 1/R_eff = 1/10 Ω + 1/10 Ω + 1/10 Ω = 3/10 Ω. Therefore, R_eff = 10/3 Ω.

Sample Data

Data Table 1. Identifying the Resistors

Data Table 2. Series and Parallel Connections

Answers to Questions

1. Calculate the possible range of each resistor according to the tolerance and fill these values in on the table for Part A.
2. Calculate the percent error for each of the resistor arrangements and fill these values in on the table for Part B. Did the measured resistance fall within the tolerance range for the resistor combination?

   Student answers will vary. All combinations have a percent error lower than their tolerance, except for the second parallel combination.

3. What would be the color code of a 390 kΩ resistor, with 10% tolerance?

   The color code would be Orange-White-Yellow-Silver.

4. What would be the color code of a 6.8 Ω resistor, with 1% tolerance?

   The color code would be Blue-Gray-Gold-Brown.

5. Calculate the approximate current across each resistor in the parallel combination by dividing the voltage by the listed resistance. Compare the listed resistance with the current.

   220-Ω resistor: V/R = I; 2.85 V/220 Ω = 0.013 A
   620-Ω resistor: V/R = I; 2.85 V/620 Ω = 0.0046 A
   1100-Ω resistor: V/R = I; 2.85 V/1100 Ω = 0.0026 A The lower resistor had more current going through it than the higher ones. When a circuit splits as in a parallel connection, more current is able to travel through the paths of least resistance.

6. Compare the effective resistance of both the parallel and series combinations of resistors. Which has a higher resistance?

   The series combination had the higher resistance.

7. Because of its low resistance, the resistor with two gold bands is not recommended for use in a parallel circuit. Why might this be the case? Speculate on how it would fare in a series circuit.

   The low resistance of this resistor would cause more of the current to travel through it than the other resistors. Because these resistors can only dissipate so much power (which is related to current), it is very likely that it would burn out. In a series circuit the current has to travel through all three resistors so the total current in the loop would be limited. As long as the other two resistors have a higher resistance, the low resistance resistor will not have to carry a great deal of current.

8. What would happen if one of the three resistors were removed and replaced with a simple wire in a series circuit? Parallel?

   Replacing a resistor in a series circuit would cause the resistance to drop by the same amount as the resistor. Replacing the resistor in a parallel circuit would be a more dangerous situation because the current must be split between resistors causing most of the current to go through the simple wire with very little resistance. As a result, a short circuit would be created and the wire might burn out, creating a fire or explosion risk.

References

Halliday, D., Resnick, R., & Walker, J, Fundamentals of Physics, 8th ed.; Wiley: Cleveland, OH, 2008.

Introduction

Resistors are common circuit components designed to limit current flow by producing a voltage drop. The higher the resistance, the less current passes through for a given voltage drop. Resistance values are determined by reading the color code on the body of the resistor. Use this code to decipher the value of a number of resistors, and then test your predictions!

Concepts

• Resistance
• Ohm’s law
• Resistor color code
• Series vs. parallel circuits

Background

Resistors are the backbone of any electronic circuit, directing the current in tightly controlled amounts. The purpose of resistors can be compared to water pipes, where the inside diameter of the pipe controls how much water flows through it. Resistors are used to reduce current to delicate circuit components, split voltage to bring power to multiple components, create heat, act as fuses and are used for many other applications. Lightbulbs are forms of resistors and even many heaters use resistors as their heating elements. Resistors come in a wide variety of shapes, sizes, values and types to meet the diverse needs of modern electronics. Electronics as we know it today would not be possible without these devices!

All resistors conduct electricity. The specific resistance value of a resistor determines the amount of current that flows through it, and the combination of resistors in a circuit allows them to control the current flow through the whole circuit. For a given resistor, the applied voltage divided by the resistance gives the current, as shown by Equation 1.

where

V is the voltage drop
I is the current
R is the resistance.

Resistors can be connected in both series and parallel. When resistors are connected in series, the same current goes through each resistor. The effective resistance for resistors in series is simply the sum of the individual resistance values, as shown by Equation 2. where R_eff is the effective or total resistance.

Resistors in parallel will have the same potential difference across each of them. Depending on the resistance of each resistor, the amount of current going through each resistor connected in parallel varies. The total effective resistance is given by Equation 3: Because the voltage across each resistor is the same, the current through each resistor can be easily calculated using Equation 1.

Different types of resistors vary in their application, and each type has its own advantages and disadvantages. The resistors used in this lab are called carbon film resistors. Carbon film resistors are very common, due to their high performance and low cost. They are made with an insulating ceramic rod covered with a conducting carbon film. The carbon is etched away in a spiral shape, allowing the resistance to vary greatly without changing the size of the resistor itself. Two end caps connect to the leads, and the whole body is covered with an insulated coating and painted (see Figure 1). As a voltage is applied across a resistor, some of the energy is dissipated in the form of heat. Electrons are continuously colliding with atoms, forcing them to vibrate and heat the material. Resistors may overheat or even burn if too much current is passed through them. The maximum amount of current they can take is generally given as a power rating. The amount of power a resistor can handle may be found using any form of Equation 4. Carbon film resistors cannot easily dissipate the heat from a high-power circuit. They typically come in power ratings from ⅛ W to 5 W. For power needs greater than 5 W, other types of resistors, such as wire-wound power resistors, must be used. Resistors will heat up before hitting this maximum level, which can decrease the lifetime of a circuit. However, sometimes resistors are used in circuits exactly for the purpose of creating heat—many portable heaters rely on this property to provide warmth.

Because resistors are generally too small for convenient labeling, a color code has been developed using a series of bands for easy identification. Each color is associated with a specific number, and the placement of the band gives information on how that number is used. Table 1 denotes which colors correspond with which numbers. Resistors have no polarity—it does not matter which direction they face in the circuit. For the purposes of identification, however, the order of the bands does matter. One end of a resistor will have the tolerance band, which is usually further removed from the other three and is commonly silver or gold. The tolerance band is considered the “last” band.

The first two bands are the digits. For the sample resistor shown in Figure 2, the first band is violet, which according to Table 1 corresponds to a value of 7. The second band is black, which corresponds to 0. So these simply come together to form “70.” This is then multiplied by the third band—the multiplier. The third band in this example is green, corresponding to a value of 10⁵, or 100,000. Multiplying 70 by 100,000 gives us the resistance value of 7,000,000 Ω or 7 M Ω. Lastly, the tolerance gives the range. The resistance of the resistor may not be exactly the value depicted on the label, but all resistors will have to fall within a certain range of resistance. Since the tolerance band is silver in the sample resistor, the tolerance is ±10%. The value of the resistor then could be anywhere within 10% of 7 MΩ—in this case, the range extends from 6,300,000 Ω (6.3 MΩ) to 7,700,000 Ω (7.7 MΩ). Note that the symbol Ω is used to denote ohms, which is the SI unit for resistance. One ohm has a value of one volt per amp. The symbol MΩ stands for megaohms, or 1 x 10⁶ ohms.

Experiment Overview

In this experiment, nine unidentified resistors will be given. The resistance of each resistor will be decoded. Predictions will be tested by placing the resistors in series and parallel connections, and the theoretical predictions will be compared with the actual values.

Materials

Prelab Questions

1. Why is it important to check the expected power across each resistor before connecting it in a circuit?
2. A resistor has the following colored bands on it: Brown–Black–Black–Gold. Determine the numerical value of each band. What is the total resistance?
3. What resistance range could the resistor from step 2 take?
4. What would be the effective resistance for three of these resistors connected in series? In parallel?

Safety Precautions

If a resistor begins to darken or smoke, immediately disconnect the circuit and do not touch the resistor—it will be hot. Allow it ample time to cool. Handle pins with caution, as they are sharp. Please follow all laboratory safety guidelines.

Procedure

Part A. Identifying the Resistors

1. Obtain one of the unidentified resistors.
2. Identify the tolerance band, which is further removed from the others; it will likely be silver or gold.
3. Starting from the opposite end of the resistor, record the colors of the bands in order in Data Table 1.
4. Using Table 1 from the Background section, fill in the numerical values for each of the bands in the data table.
5. Calculate the resistance of the resistor, and record the result in the table.
6. Repeat steps 1–5 for each “unknown” resistor.
7. Check your answers with your instructor.

Part B. Series and Parallel Connections

8. Obtain the batteries and holder, connector cords, multimeter, and the three resistors with color codes Red–Red–Brown–Gold, Brown–Brown–Red–Gold and Blue–Red–Brown–Gold.
9. Using five connector cords, connect the three resistors, the multimeter and the battery in series (see Figure 3a). Note: A multimeter must be connected in series with a circuit to read current.
   {12155_Procedure_Figure_3a}
10. Record the current indicated by the multimeter on the worksheet.
11. Remove the multimeter from the circuit, and connect the resistors in series directly to the battery. Use the multimeter to measure the voltage across the entire series of resistors. Note: A multimeter must be connected in parallel with a circuit component to read the voltage drop across it.
12. Connect the three resistors in parallel with each other, and in series with the multimeter and battery (see Figure 3b).
    {12155_Procedure_Figure_3b}
    Note: The leads of the resistors may be clipped directly with the alligator clip as in Figure 4a, or using a pin coupler as shown in Figure 4b.
    {12155_Procedure_Figure_4a}
    {12155_Procedure_Figure_4b}
13. Record the current from the multimeter in the worksheet.
14. Remove the multimeter from the circuit and connect the resistors directly to the battery. Use the multimeter to measure the voltage across all three resistors at once. 

Student Worksheet PDF

12155_Student1.pdf