Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut and strip the blue PVC-insulated wire so that each group has two separate 1.5-m lengths of wire.
2. Strip about 4 cm of insulation from each end of the 1.5-m lengths of wire. This should result in 16 total lengths of wire, two for each group.

Testing of Apparatus
To test the lightning bells have students bring their setup to a Van de Graaff generator testing station. Connect a metal discharge electrode to the generator terminal.

1. Check that the students have wired their setup correctly.
2. Turn on the Van de Graaff generator.
3. Have students hold the aluminum sphere end of the lightning rod about 6 inches from the Van de Graaff generator and observe the hanging paper clip.
4. Once the test is done, use the discharge electrode to discharge the dome of the Van de Graaff generator and to discharge the metal cylinders.

Safety Precautions

Van de Graaff generators produce a very small current (microamps) and therefore an accidental shock from a Van de Graaff generator may cause pain and be startling, but the shock should not cause serious harm to most individuals, even at a high voltage. When working with a Van de Graaff generator, it is important to have a metal discharge electrode connected to the Van de Graaff generator terminal. This acts as a ground and allows an operator to discharge the generator safely before getting near it. Do not use Van de Graaff generators near flammable gases or vapors. Do not touch a Van de Graaff generator with wet hands or damp clothing. Use a Van de Graaff generator with an ON/OFF switch to prevent accidental shocks when performing experiments.

Lab Hints

• Adjust the distance between the two metal cylinders as needed to ensure the paper clip “rings” the bells continuously.
• Old “tube” TVs may be used as an alternative to charge the lightning bells. Cut out a large sheet of aluminum foil that will cover almost the entire TV screen. Tape the foil to the screen. With wire, connect the aluminum foil to one of the metal tubes. Connect the second metal tube to ground. Turn on the TV. The static electricity that builds up on the screen will be transferred to the aluminum foil and then to the metal tube.
• Static electricity experiments and demonstrations always work best on a dry day. Lower humidity days are better than high humidity days. Air-conditioned air or heated winter air tends to be drier and more conducive to electrostatic demonstrations.

Teacher Tips

• Refer to Flinn Scientific Publication No. 10552, Van de Graaff Generator Safety, at www.flinnsci.com for additional safety information.

Answers to Prelab Questions

1. What does it mean to “ground” an object?

   When an object is grounded, it simply means that it is connected to the Earth or ground in such a way that charge buildup on the object can be neutralized by the relative “infinite” supply of electrons from the Earth.

2. In your own words, define electric charge.

   Electric charge is a fundamental characteristic unit of matter that is used to express the quantity of excess charged particles (electrons or protons) in a system.

3. Read the Safety Precautions. What safety measures should be taken for this activity?

   There should be a metal discharge electrode handy when using a Van de Graaff generator. Great care should be taken to not touch the Van de Graaff generator with wet hands or damp clothing.

Answers to Questions

1. Why is the paper clip initially attracted to the metal cylinder?

   The paper clip is attracted to the metal cylinder because of the large buildup of static charge on the cylinder. The free electric charges on the metal paper clip are attracted to the buildup of static charge.

2. Why does the paperclip immediately repel once it makes contact with the metal cylinder?

   As soon as the paperclip makes contact with the metal cylinder, it acquires the same sign of charge as the cylinder (either positive or negative). This causes the paper clip to repel because like charges repel.

3. What is the law of conservation of charge?

   The law of conservation of charge simply states that electric charge can never be created or destroyed. The total quantity of electric charge in the universe is constant.

4. What impact did Franklin’s electrical discoveries have?

   Franklin’s experiments gave the world a new understanding of electricity. He introduced the concept of conservation of electric charge, gave us the conventional terms of positive and negative charges, and his invention of the lightning rod was used to safely protect many buildings from hazardous storms, something we still do today. Furthermore, his accomplishments boosted his public image in the political sphere of his time and allowed him to secure French aid in the fight against Great Britain.

Introduction

Benjamin Franklin (1705–1790) was a public servant, Founding Father, journalist, philanthropist, scientist and inventor. His scientific and innovative talents led him to discover the basic nature of electricity—that electric charge is conserved. He identified positive and negative electrical charges and through the use of metal bells and his newly invented lightning rod, was able to accurately classify lightning from the clouds as electricity. Reinvent Benjamin Franklin’s lightning bells with this enlightening experiment!

Concepts

• Electricity
• Circuits
• Positive and negative charges

Background

Benjamin Franklin’s global scientific renown had a modest beginning. Franklin’s initial experiments on electricity were done with friends and neighbors. From the years 1747–1750, the results of these experiments were formally described in five letters sent to the Royal Society of London. His work was published by the Society in 1751 in a pamphlet titled Experiments and Observations on Electricity, Made at Philadelphia, in America. In the five letters, Franklin described the effects of sharp points and dull points when used to draw electricity, and introduced the idea that when glass is rubbed with silk it does not create electricity but instead transfers electricity as a consequence of the friction between the two materials. He detailed that whatever amount of electrical “fluid” was added to the glass was equally lost by the silk. This led him to be the first to introduce terms such as plus and minus to describe the electrical states of the glass and silk. Similarly, he began using terms such as charging and discharging when explaining the function of a Leiden jar as well as underlining the importance of grounding when charging or discharging such an object. He assumed that glass was positively charged and the silk negatively charged, conventions we still use today. In fact, had Franklin decided to assign the values in reverse order, we would be referring to electrons as positive charges instead! The concept of electricity being regarded as a single fluid, never created nor destroyed, was profound and is known now as the law of conservation of charge. Electric charge is a fundamental property of matter. The electron has the smallest unit of solitary electric charge and the proton has an equal amount of positive charge. When something is referred to as having a certain amount of “charge” it means that the charged object has either more or less electrons than protons.

In his last two letters, Franklin described his observations on lightning. He postulated that when an electrified cloud passes over a region, it might discharge electricity to high points (e.g., trees, hills, towers). This hypothesis led to the practical advice to not hide underneath trees during thunderstorms and in fact, one is more likely to be safe crouching in an open field! He lastly suggested that tall sharp metallic points might be able to discharge the electricity from clouds and reduce the chances of a structure being damaged by lightning. He proposed the idea for an experiment to be carried out to finally deduce whether clouds are truly electrified and if lightning strikes are the same phenomenon as electrical sparks.

Upon the publication of Experiments and Observations, French naturalist Thomas-Francois Dalibard (1709–1778) carried out many of the Philadelphia experiments. After successfully demonstrating the experiments to King Louis XV, Dalibard was inspired to attempt the experiment proposed by Franklin—to draw electricity from the clouds. In May of 1752, Dalibard successfully drew sparks from a tall iron rod as a thunderstorm passed overhead. This monumental result showed that thunderclouds are electrified and that lightning is indeed an electrical discharge. Once Franklin heard of the success of the experiment, he set out to study the characteristics of lightning electricity by installing a tall insulated rod on the roof of his house. This rod was connected to a metal bell inside his house. Next to this first bell, and separated by a small gap was a second bell that was connected to ground. To connect an object to ground simply means to directly connect the object to the Earth. This allows for the electrically charged object to become neutralized either by adding electrons from the “infinite” supply of the Earth or by transferring electrons to ground (if the object is positively charged). A small metal ball was suspended in between the bells—this ball would then ring the bells whenever an electrical storm passed by. Below is Franklin’s description of the setup.

(The rod was) “fixed to the top of my chimney and extending abut nine feet above it. From the foot of this rod, a wire (the thickness of a goose-quill) came through a covered glass tube in the roof and down through the well of the staircase; the lower end connected with the iron spear of a pump. On the staircase opposite to my chamber door, the wire was divided; the ends separated about six inches, a little bell on each end; and between the bells a little brass ball, suspended by a silk thread, to play between and strike the bells when clouds passed with electricity in them. After having frequently drawn sparks and charged bottles from the bell of the upper wire, I was one night awakened by loud cracks on the staircase. Starting up and opening the door, I perceived that the brass ball, instead of vibrating as usual between the bells was repelled and kept at a distance from both; while the fire passed, sometimes in very large, quick cracks from bell to bell, and sometimes in a continued, dense, white stream, seemingly as large as my finger, whereby the whole staircase was inlightened with sunshine, so that one might see to pick up a pin.”

Franklin’s scientific worldwide fame earlier in his life contributed to his societal influence as a public servant. As a leader of the American Revolution, his standing in the public eye helped him acquire French aid in the war against Great Britain for the American Colonies.

Experiment Overview

The purpose of this laboratory activity is to understand the electrostatic concepts behind a prominent historical scientist’s invention. Benjamin Franklin’s “lightning bells” and famous lightning rod are constructed and tested with a Van de Graaff generator. The transference of electric charge and properties of conductive materials is studied in order to understand what makes the bells ring.

Materials

Prelab Questions

1. What does it mean to “ground” an object?
2. In your own words, define electric charge.
3. Read the Safety Precautions. What safety measures should be taken for this activity?

Safety Precautions

The edges of the metal tubes may be sharp; handle with care. Van de Graaff generators produce a very small current (microamps) and therefore an accidental shock from a Van de Graaff generator may cause pain and be startling, but the shock should not cause serious harm to most individuals, even at a high voltage. When working with a Van de Graaff generator, it is important to have a metal discharge electrode connected to the Van de Graaff generator terminal. This acts as a ground and allows an operator to discharge the generator safely before getting near it. Do not use Van de Graaff generators near flammable gases or vapors. Do not touch a Van de Graaff generator with wet hands or damp clothing. Use a Van de Graaff generator with an ON/OFF switch to prevent accidental shocks when performing experiments.

Procedure

1. Gather two metal tubes, a large paper clip, 30 cm of fishing line, two 1.5-m lengths of insulated wire, red and black connector cords, a roll of tape, iron nail, plastic ruler, a meter stick and a 3-foot long piece of aluminum foil.
2. Tape the iron nail to the meter stick with the pointed end of the nail protruding an inch from the end of the stick. Leave the head of the nail free of tape.
3. Clip a red connector cord to the bottom of the iron nail.
4. Clip one of the ends of one of the 1.5-m long pieces of wire to the other end of the red connector cord.
5. Crunch and roll the aluminum foil into a sphere. It should have a diameter of about 6 cm or larger. Note: Try to make the sphere as round as possible.
6. Poke the end of the nail through the aluminum foil sphere. This apparatus will be used as the lightning rod.
7. Place the metal tubes about 4 cm apart on an even work surface.
8. Cut a 30-cm length of fishing line.
9. Tie one end of the 30-cm length of fishing line to the large paper clip.
10. Tape the other end of the fishing line to the middle of the plastic ruler.
11. Place the ruler flat on top of the metal tubes and adjust how the fishing line is taped so that the bottom of the paper clip is suspended only 2–3 millimeters above the work surface (see Figure 1).
    {14078_Procedure_Figure_1}
12. Clip the black connector cord to the free end of the unused 1.5 m length of wire.
13. Tape the free end of the length of wire to one of the metal tubes.
14. Tape the free end of the wire attached to the assembled “lightning rod” to the other metal tube.
15. Follow teacher instructions for testing of lightning bells. Place the setup next to the Van de Graaff generator.
16. Make sure that the free end of wire attached to the red connector cord (on the lightning rod) is taped securely to a metal tube.
17. Make sure that the free end of wire attached to the black connector cord is taped securely to the second metal tube.
18. Clip the free end of the black connector cord to the ground connection on the base of the Van de Graaff generator.
19. With instructor guidance, turn on the Van de Graaff generator.
20. Hold the aluminum sphere end of the lightning rod about 15 cm from the Van de Graaff generator and observe the hanging paper clip.

Student Worksheet PDF

14078_Student1.pdf