Static electricity

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 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. Items that have a net negative or positive charge are surrounded by an invisible electric field, which attracts opposite charges and repels similar charges.

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 clear tape)
Polypropylene
Silicon
Teflon
Negative (-)

For this project, you will build a simple, but extremely sensitive, charge detector. When it is assembled, it will be able to sense the changes in the static electricity on your body as you walk over carpet, when you pet your cat or dog, or when you touch a plastic pen or brush to your hair. Figure 1 shows a circuit diagram for the charge detector. Circuit diagrams are schematics that electrical engineers use to represent circuits. Each symbol represents a different electronic component. However, do not worry if you do not understand the circuit diagram. The procedure of this project will provide step-by-step instructions for how to build the circuit on a breadboard.

Figure 1. This is a circuit diagram for a solid-state charge detector. It can detect very weak electric fields.

As shown in Figure 1, the circuit has three components: a 9 V battery, a light-emitting diode (LED), 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. 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, whereas in P-type semiconductors they are holes. 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.

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 gate pin—electrons cannot flow through the circuit, and the LED light dims.

Simplified diagram of an outside magnetic field closing the inner gate of a junction field-effect transistor

Figure 2. Schematic of an N-channel junction field-effect transistor (JFET, a sub-type of FET). 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 and Concepts

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

Static electricity
Charge
Electron
Electric field
Triboelectric series
Circuit diagram
Light-emitting diode (LED)
Field-effect transistor (FET)
Majority carrier
N-type semiconductor
P-type semiconductor
Depletion zone

Materials and Equipment

Electronic Sensors Kit (1).
You will need the following items from the kit.
- Breadboard
- 9 V battery
- 9 V battery snap connector
- Transistor
- 100 kΩ resistor
- Red LED

You will also need the following items (not included in the kit):

Several objects from the triboelectric series listed in the Background. This project works best with objects on opposite ends of the series (for example, human hair and a plastic item). It will be difficult to generate detectable charge with materials that are very close to each other in the series (for example, wood and cotton).
Ruler
Stopwatch
Tape
Lab notebook

Assembling Your Charge Detector Circuit

Important: your Sensor Kit contains two parts that look very similar: a transistor and a Hall effect sensor. They are both small black plastic parts with three metal legs. This project requires the transistor. When viewed from the top, it is bigger than the Hall effect sensor and rounded on one side, as shown in Figure 3. Make sure you use the transistor, or your circuit will not work!

Figure 3. Hall effect sensor (left) and transistor (right) viewed from the top.

Assemble your charge detector circuit on a breadboard, as shown in the slideshow and described in Table 1.

Click through the slideshow for step-by-step instructions.

Part	Picture	Breadboard Symbol	Location
Transistor			A1, A2, A3. Writing must face to the right.
LED			Long lead in B3 Short lead in B4
100 kΩ resistor			One lead in C1
9 V battery and snap connector			Red lead to E2 Black lead to E4

Table 1. List of circuit components and their locations. Source material for breadboard symbol images credit Fritzing.org.

Measuring Electric Fields

Learn how to use your charge detector. Once you have finished assembling it, the LED should be on (see Figure 4). Try rubbing different objects from the triboelectric series against each other and bringing them near the “antenna” (the free lead of the 100 kΩ resistor). What happens? Here are some tips for using the circuit:
1. When you rub two objects together, the one lower on the triboelectric series (meaning it has a negative charge) should cause the LED to go out when you bring it near the circuit. The object higher on the series (with a positive charge) may cause the LED to get slightly brighter, but this can be difficult to see since the LED is already on. This project works best if you bring negatively charged objects near the circuit.
2. 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 quickly placing it on an insulating surface (like a wooden tabletop) next to the circuit, or by suspending the object with string made out of nylon, or some other insulating material.
3. The LED might turn off completely after the circuit is exposed to a strong positive field. You can “reset” the circuit by tapping the resistor leads with your finger, or by waving a negatively charged object (such as a plastic pen that has been run through your hair) near the antenna.
4. See the FAQ if you have trouble with your circuit.

Figure 4. Top: the LED is on when no objects are near the antenna. Bottom: the LED turns off when a negatively-charged object (in this case, a plastic cup that was just rubbed against human hair) is brought near the antenna.

Select one material from the positive end of the triboelectric series (human hair works well) and an assortment of materials in the neutral and negative parts of the series. You will rub all the other materials against the first one.
If possible, prepare all your materials in one place, on a single work surface, so you can do the experiment without walking around. Moving around (especially on carpet) can cause static electricity to build up on your body, and this can affect your results.
Rub two materials together and immediately place the negatively charged one directly next to the antenna. Use a stopwatch to time how long it takes the LED to come back on. Repeat this for each of your materials (make sure you rub each material the same number of times), and do at least three trials for each material.
Set up a ruler to measure the distance from the antenna. Rub two materials together and then slowly move the negatively charged one toward the antenna until the LED goes out. Record the distance at which the LED goes out. Again, repeat this for each of your other materials and do at least three trials. Remember that the objects might lose charge as you hold them, so you might need to devise a technique to move the objects closer to the antenna without touching them. For example, you could slide them on a wooden block or suspend them from a string.
Analyze your data. Which material holds its charge the longest? Which one generates the strongest electric field? If you rank the materials for each category, is the order the same? Are your results consistent with what you would expect based on the triboelectric series?

Static electricity

Triboelectric Series

Terms and Concepts

Materials and Equipment

Sample Course Demo