Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Use a meter stick and scissors or wire cutters to measure and cut the wire, respectively.
2. For each lab group, cut one 2-m long 28-gauge (thin) piece, one 2-m long 14-gauge (thick) piece and one 1-m long 28-gauge (thin) piece. Remember, the smaller the gauge number, the thicker the wire.

Safety Precautions

Be cautious of the ends of the wires. Use caution when heating the wire with a Bunsen burner. When heating the wire in the Bunsen burner, hold the wire with pliers. Wear safety glasses when performing this experiment. Please follow all normal laboratory safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The materials should be saved and stored for future labs. The wire may be thrown into the normal laboratory trash according to Flinn Scientific Disposal Method #26a.

Lab Hints

• Enough materials are provided in this kit for 16 students working in pairs or for 8 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period.
• The Background information may be copied for each student before the experiment. Without the Background information, the experiment becomes more discovery-based.
• The unheated wire should be saved and reused. The heated wire may need to be replaced because it will become scorched and corroded with time. A refill supply for the 28-gauge wire is available (Flinn Catalog No. AP7094).
• Needle-nose or “regular” pliers can be used.
• Extra lightbulbs are provided to cover “Murphy’s Law.”
• Make sure students do not cross the wires, hold the wires, or allow them to touch anything metal.
• The 14-gauge (thick) wire is rigid. Bend into a large loop before class so that students will not need to do this during the experiment.
• More advanced classes should use digital multimeters to measure the resistance of each wire and the current flowing through each wire when connected to the battery.
• Students can also experiment with the effect of cooling the wires. Or this experiment can be performed as a demonstration. Cooling the wire in a freezer or with several bags full of ice (the wire should stay dry) unfortunately does not show dramatic results. For best results, use several dry ice blocks or liquid nitrogen and a 2-m long piece of 28-gauge (thin) wire. Use extreme caution when handling the dry ice blocks or liquid nitrogen. Wear insulated gloves and chemical splash goggles. Cooling the 2-m wire with dry ice or liquid nitrogen should decrease the electrical resistance enough so that the lightbulb glows noticeably brighter.

Sample Data

“Simple circuit” lightbulb brightness

The lightbulb glows at a medium intensity. It does not appear to be glowing as brightly as it could. (Note: This experiment uses a 1.5-V battery with a 3.7-V lightbulb so the lightbulb will not burn out, or glow as brightly as it possibly can.)

Long thick wire (2 m) lightbulb brightness

The lightbulb appears to glow about as brightly as it did in the “simple circuit.” It may be slightly dimmer.

Long thin (2 m) wire lightbulb brightness

The lightbulb glows with about half the brightness compared to the “simple circuit” and the 2-m-long thick wire circuit.

Short thin wire (1 m) lightbulb brightness

The lightbulb glows brighter than it did with the 2-m-long thin wire, but not as brightly as it did with the 2-m long thick wire. The brightness is midway between the 2-m-long thick and thin wires.

Heated thin wire (1 m) lightbulb brightness

After the coil was heated for a few seconds in the Bunsen burner flame, the lightbulb went out, indicating no (or very little) current was flowing through the lightbulb.

Cooled (room temperature) thin wire (1 m) lightbulb brightness

After the coil was allowed to cool for a few seconds, the lightbulb illuminated again, starting out dim and eventually becoming as bright as it was before the wire was heated.

Answers to Questions

1. What is the general relationship between the amount of resistance in a circuit and the brightness of the lightbulb?

   A brighter lightbulb indicates less resistance in the circuit. The brightness is proportional to the amount of current flowing through the lightbulb. According to Ohm’s law, when the voltage in the circuit is constant, a higher current (brighter lightbulb) means there is less resistance.

2. How did the length of the thin wire affect the brightness of the lightbulb? What does this mean in terms of the resistance in the circuit?

   The shorter thin wire produced a brighter lightbulb. This means the resistance is lower when the shorter thin wire is used compared to the longer thin wire.

3. How did the thickness of the wire affect the brightness of the lightbulb? What does this mean in terms of the resistance in the circuit?

   The thicker wire produced a brighter lightbulb, indicating more current flowed through the lightbulb. Therefore, a thicker wire will have less resistance compared to a thin wire of the same length (and composition).

4. What are the two physical dimensions that affect the resistance in a wire? How do these physical dimensions affect the resistance in a wire?

   Both the length and the diameter of the wire affect its total resistance. The longer the wire, the more resistance is has (there is more material). However, a wire with a larger diameter will have less resistance compared to a wire of the same composition with a smaller diameter.

5. How did the temperature of the wire affect the electrical resistance of the wire? Explain why the resistance changed.

   The increased temperature of the wire increased the electrical resistance of the wire, thereby preventing most of the current from flowing through the lightbulb. After the wire cooled down, the resistance decreased and current was able to flow through the wire again. This means heating the wire only caused a physical change to the wire and not a chemical change. The electrical resistance increased because the random motion of the electrons increased dramatically when the wire was heated in the flame. The greater electron motion prevented a continuous “linear” motion of electrons along the voltage difference.

Introduction

How do the length and thickness of a wire affect an electrical circuit? What about the temperature of a wire? Learn about some of the properties of the electrical resistance in wires.

Concepts

• Electrical resistance
• Ohm’s law

Background

In a simple circuit, a load is connected between the terminals of a power supply with conductive wires. The materials that compose the load and wires are not perfect conductors and therefore the movement of the electrons between the terminals of the power supply is impeded. This impedance of electron movement is known as electrical resistance. The voltage drop between the terminals of the power supply provides the energy to move the electrons. The movement of the electrons (the current) is hindered by the resistance in the conductive wires and the load. In general, the resistance in an electrical circuit is constant. Therefore, the resistance in an electrical circuit can be written in the form of Equation 1.

ΔV = Potential difference (voltage drop)
I = Current
R = Resistance

Equation 1 can be expressed in its familiar form, known as Ohm’s law:
Electrical resistance uses the SI unit known as the ohm (Ω, the Greek letter omega), which is equal to a volt per ampere. Electrical resistance depends on the type of material, the length and the diameter as well as the temperature of the material. Each type of material (e.g., aluminum, steel, graphite) affects the movement of electrons in its own unique way. This is known as resistivity (ρ). A longer piece of material will have more material for the electrons to travel through, and therefore the electrical resistance will increase. A larger diameter conductive wire will provide more pathways for the electrons to travel, and therefore will result in lower electrical resistance. The resistance of a material can be expressed in the following equation:

ρ = resistivity
L = length
A = cross-sectional area

In general, colder temperatures slow down the movement of the internal electrons of the materials and this helps to reduce the electrical resistance. Higher temperatures cause an increase in the internal vibrations, which results in more electron movement impedance. Semiconductors, however, are an exception to this rule. Many semiconductors are more conductive (have less resistance) at higher temperatures.

Materials

Safety Precautions

Be cautious of the ends of the wires. Use caution when heating the wire with a Bunsen burner. When heating the wire in the Bunsen burner, hold the wire with pliers. Wear safety glasses when performing this experiment. Please follow all normal laboratory safety guidelines.

Procedure

Length Dependence

1. Assemble the lamp receptacle, lightbulb, battery and connector cords as shown in Figure 1.
   {12547_Procedure_Figure_1}
2. Record the brightness of the lightbulb for this “simple circuit” in the Resistance Worksheet. Note: In order to improve the lifetime of the lightbulb, do not connect the circuit for more than 15-second intervals.
3. Connect the 2-m thicker wire between the lightbulb and the battery using a third connector cord, as shown in Figure 2. Note: Make sure the wire does not make contact with itself if the wires cross, do not hold onto the wire, or allow the wire to touch other metal objects. Record the brightness, relative to the “simple circuit” from step 3, in the Resistance Worksheet.
   {12547_Procedure_Figure_2}
4. Disconnect the 2-m thicker wire from the circuit.
5. Connect the 2-m thinner wire between the lightbulb and the battery. Note: Make sure the wire does not make contact with itself if the wires cross, do not hold onto the wire, or allow the wire to touch other metal objects. Record the brightness, relative to the “simple circuit” from step 3, in the Resistance Worksheet.
6. Disconnect the 2-m thinner wire from the circuit.
7. Connect the 1-m thin wire between the lightbulb and the battery. Record the brightness, relative to the “simple circuit” from step 3, in the Resistance Worksheet.

Temperature Dependence

1. Set up a Bunsen burner according to the instructor’s direction.
2. Obtain the 1-m thin wire and a pencil.
3. Carefully wrap the 1-m wire around the pencil, leaving about 10 cm of unwrapped wire at each end (see Figure 3).
   {12547_Procedure_Figure_3}
4. Remove the wire from the pencil and loosen up the coils to make sure the wire loops do not touch each other.
5. Carefully ignite the Bunsen burner and adjust the flame to a doublecone.
6. Connect the coiled wire to the lightbulb and battery as before and observe the glow of the lightbulb.
7. Pick up the wire coil with pliers. Make sure all the wire connections stay intact.
8. Place the coil into the Bunsen burner flame and observe the lightbulb. Note: Do not heat the coil in the Bunsen burner flame for more than 5 seconds (see Figure 4).
   {12547_Procedure_Figure_4}
9. Remove the coil from the flame, turn off the Bunsen burner, and record the observations in the Resistance Worksheet.
10. Allow the coil to cool down for a minute or two and observe the lightbulb. Record the brightness of the lightbulb in the Resistance Worksheet.
11. Consult your instructor for appropriate storage procedures.

Student Worksheet PDF

12547_Student1.pdf