Related Links

• Science Fair Project Guide

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• How Do Different Materials React to Static Electricity?
• Use a Breadboard to Build and Test a Simple Circuit

Project Summary

Difficulty	6  –  8
Time required	Short (several days)
Prerequisites	You should be familiar with the concepts of atoms, electrons, and voltage. Familiarity with building circuits with a breadboard is helpful, but not required.
Material Availability	All of the materials are available at Radio Shack.
Cost	Low ($20 - $50)
Safety	No issues

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Abstract

Objective

The objective of this science fair project is to build a super-sensitive charge detector and use it to investigate the nature of electric fields created by static electricity.

Introduction

Static electricity is the accumulation of electrical charges on a surface, produced by the contact and separation of dissimilar materials. If you have ever received a shock when touching a doorknob, you have some firsthand experience with static electricity. The sparks created by static electricity can cause real problems, such as when they "fry" an electronic component in a computer.

Objects are usually electrically neutral, meaning they have an equal number of positive charges and negative charges. When two different materials are rubbed against each other, one of the materials often donates electrons to the other one. Electrons are elementary particles that carry a negative charge. If an item gives up electrons to another item, the first item will end up with a net positive charge. On the other hand, the item that now has extra electrons will have a net negative charge.

The materials listed below are ranked in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. If two materials are rubbed together, the one higher on the list will donate electrons and become positively charged.

Triboelectric Series

• Positive (+)
• Human hands
• Glass
• Human hair
• Nylon
• Wool
• Fur
• Lead
• Silk
• Paper
• Cotton
• Steel Neutral (0)
• Wood
• Hard rubber
• Nickel, copper
• Brass, silver
• Gold, platinum
• Polyester
• Plastic wrap
• Polyurethane
• Polyethylene (like Scotch® tape)
• Polypropylene
• Silicon
• Teflon
• Negative (-)

For this science fair project, you will build a simple, but extremely sensitive, charge detector (see Figure 1). When it is assembled, it will be able to sense the changes in the static electricity on your body as you walk over a carpet, when you pet your cat or dog, or when you touch a plastic pen or brush to your hair.

Figure 1. This is a circuit diagram for a solid-state charge detector. It can detect very weak electric fields. The circuit has three components: a 9-volt battery, a light-emitting diode (LED), and a field-effect transistor (FET), labeled MPF-102 in the diagram. The field-effect transistor has three leads: a source (S), a gate (G), and a drain (D).

As shown in Figure 1, the circuit has three components: a 9-volt battery, a light-emitting diode, and a field-effect transistor (FET). The field-effect transistor has three leads: a source (S), a gate (G), and a drain (D). A thorough description of how the field-effect transistor works would require delving into advanced electrical engineering, but the essential features can be seen in Figure 2. The field-effect transistor has a channel of N-type semiconducting material that allows electrons to carry a current. When the battery is connected to the transistor, a voltage is applied across this N-type semiconducting material. The material is call "N-type" because negative charges—electrons—are the charge carriers.

In semiconductors, electrons and holes act as charge carriers. The more-abundant charge carriers are called majority carriers. In N-type semiconductors they are electrons, while in P-type semiconductors they are holes.

Field-effect transistors are normally on devices, meaning that with no negative electric field, they allow maximum current to flow. In the middle of the N-channel is a region of P-type semiconductor. Around the P-type material, there is a depletion zone. There are fewer electrons in the depletion zone, so the bigger the depletion zone is, the higher the resistance. In the presence of a negative electric field, the depletion zone gets bigger, the current is decreased, and so the LED on the circuit is turned off.

The field-effect transistor circuit can be compared to a water faucet. In this analogy, the voltage is like the water pressure, and the field-effect transistor is like a faucet. When you open a faucet, water flows because of water pressure. The water will keep flowing until the faucet is closed. In the circuit, electrons flow through the FET and LED. The electrons flow because of the voltage supplied by the battery. Because electrons are flowing through the LED, it glows red. When the FET is "closed"—by bringing an object with a negative charge near the FET—electrons cannot flow through the circuit, and the LED light dims.

Figure 2. Shown is a schematic of an N-channel junction field-effect transistor. It is made from a single piece of N-type semiconductor constricted in the middle by P-type material forming the gate. Varying the gate voltage modulates the current flow through the device. When the gate voltage is made more negative, it constricts the current path in the region of the gate, increasing its resistance and reducing the current flow.

Terms, Concepts and Questions to Start Background Research

Before you begin this science fair project, you should be familiar with these concepts:

• Static electricity
• Charge
• Electron
• Triboelectric series
• Electric field
• Transistor
• N-type semiconductor
• P-type semiconductor
• Depletion zone
• Solid state

Questions

• Which holes in the breadboard are connected to each other?
• How does distance affect the strength of the electric field?
• In your own words, explain how the field-effect transistor compares to a water faucet.

Bibliography

This website is the source for the circuit used in this project. The website also has some entertaining variations of the basic project.

• Beaty, W.J. (2008, May). Ridiculously sensitive charge detector. Retrieved May 29, 2008 from http://www.eskimo.com/~billb/emotor/chargdet.html

Check out these websites for additional information.

• Zavisa, J. (2000, April 1). How Van de Graaff Generators Work. Retrieved June 2, 2008 from http://science.howstuffworks.com/vdg.htm
• Nave, R. (2008, June). Junction Field Effect Transistor. Retrieved June 2, 2008 from http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/fet.html

Materials and Equipment

You can find most of the parts below at Radio Shack: http://www.radioshack.com

• Breadboard, about 3"x2", Radio Shack part # 276-003
• 9-volt battery
• N-channel field-effect transistor, MPF-102, Radio Shack part # 276-2062
• Red light-emitting diode (LED), Radio Shack part # 276-041
• Battery connector, Radio Shack part # 270-325
• Clear tape
• Plastic pen
• Several objects from the triboelectric series list above
• Lab notebook

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies receives no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is any Amazon.com or Barnes&Noble.com link.) If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

Experimental Procedure

Notes Before You Begin: To start this science fair project, you should be familiar with the concepts of electrons, charges, and electric fields. You will be using a field-effect transistor, so some knowledge of semiconductors would be helpful, but is not essential. You can easily build the circuit on a solderless breadboard. See the Science Buddies webpage Use a Breadboard to Build and Test a Simple Circuit. Avoid touching the gate wire, which is the wire used as the antenna, since this could send a damaging charge into the transistor. Do not connect the battery directly to the LED, as this will burn it out. In the circuit, the FET acts as a resistor to limit current through the LED.

1. Push the battery connector wires into the breadboard. In Figure 3, below, the red wire is inserted at E2. (All of the holes in row 2, from A to E, are now connected to the positive lead from the battery). Insert the black wire into the breadboard (hole E4). All row 4 holes from A to E are now connected to the negative battery lead.
2. Don't connect the battery to the battery holder yet. Connect the battery on the last step to avoid burning out either the LED or the FET.
3. Connect the middle transistor lead (the source) to the red battery lead. This is in hole A2 in the example below.
4. Insert the drain lead from the FET (left-most wire when viewing with the flat end in front) into the breadboard (A3 in the diagram).
5. The gate wire serves as the antenna, so it is not connected to the breadboard. Bend it so that it points away from the other leads.
6. Note that the LED has a flat region on one side. Connect the LED lead closest to the flat region to the black battery wire (negative). This is hole C4 in the example.
7. Connect the other LED lead to the drain lead of the FET. This is hole C3 in the example.
8. Double-check the connections, then connect the battery. The LED should glow red.
9. Use clear tape to hold down the battery, and also to secure the two leads from the battery.
10. You are now ready to experiment with electric fields!
11. Choose two objects from the triboelectric series and predict which objects will have a negative and which will have a positive charge after they are rubbed together. Write your predictions down in your lab notebook.
12. Now rub the two objects together. For example, run a plastic pen through your hair and hold the pen near the antenna (the unattached gate wire). Record your observations in your lab notebook.
    Notes:
    1. The charge on an object will dissipate if you handle it, especially if your hands are slightly damp. By handling the object, the charged particles are transferred from the object to your skin. You can keep the charge from being lost by isolating the charged object; for example, by suspending the object with string made out of nylon or some other insulating material.
    2. Remember, objects with a negative charge will make the LED get dimmer as they are brought near the antenna. Objects with a positive charge will make the LED brighter. It is harder to see the LED getting brighter since is already pretty bright, but it is possible if you are careful.
    3. The LED might turn off after the circuit is exposed to a strong positive field. It can be turned on again by waving a negatively charged object (such as a plastic pen that has been run through your hair) near the antenna.
13. Try each pair of items at least three times. Are your results consistent? Record your observations in your lab notebook.
14. Try walking across a carpeted floor then bringing your hand near the circuit. Try this wearing tennis shoes, just socks, or your bare feet. What happens? Why?
15. Try rubbing one end of a plastic ruler with a piece of cloth. How far does the negative charge extend over the ruler? (You can "neutralize" the ruler between trials by touching it all over with your hands. This works best if you have slightly damp socks on.)

Figure 3. Picture of completed charge detector circuit.

Variations

• Attach an insulated wire to the antenna to increase the sensitivity of the charge detector (for example, a 24-inch insulated test jumper lead with alligator clip, Radio Shack part # 278-1157). Investigate how the length of the wire affects the sensitivity.
• Keep track of the humidity and record how it affects your static electricity experiments.
• Build the circuit using solder to make the connections. One option is to strip the plastic top off of the battery connector and solder the components onto the exposed plus and minus terminals. See a picture here.
• Electrify an object, for example, by running a pen through your hair. Now support the pen with different materials and place it near the antenna to see which is best at keeping the object charged.
• Can you detect electric fields that are created when you pet your cat or dog?
• Electrify an object, hold it near the antenna to dim the LED light, and then pass objects between the object and the antenna. What happens?

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

David Whyte, PhD, Science Buddies

• Scotch® is a registered trademark of 3M.

Last edit date: 2009-03-13 14:16:00

Career Focus

If you like this project, you might enjoy exploring careers in Electricity & Electronics.

	Electrician Electricians are the people who bring electricity to our homes, schools, businesses, public spaces, and streets—lighting up our world, keeping the indoor temperature comfortable, and powering TVs, computers, and all sorts of machines that make life better. Electricians install and maintain the wiring and equipment that carries electricity, and they also fix electrical machines.			Electrical and Electronics Engineer Just as a potter forms clay, or a steel worker molds molten steel, electrical and electronics engineers gather and shape electricity and use it to make products that transmit power or transmit information. Electrical and electronics engineers may specialize in one of the millions of products that make or use electricity, like cell phones, electric motors, microwaves, medical instruments, airline navigation system, or handheld games.
	Electrical Engineering Technician Electrical engineering technicians help design, test, and manufacture electrical and electronic equipment. These people are part of the team of engineers and research scientists that keep our high-tech world going and moving forward.			Semiconductor Processor What do traffic lights, lasers, and microchips have in common? They are made from special materials called semiconductors. Semiconductors have helped revolutionize technology. If you enjoy hands-on work and are interested in participating in cutting-edge semiconductor technology, then a career as a semiconductor processor maybe of interest to you!

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