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.
- Positive (+)
- Human hands
- Human hair
- Neutral (0)
- Hard rubber
- Nickel, copper
- Brass, silver
- Gold, platinum
- Plastic wrap
- Polyethylene (like clear tape)
- 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.
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
- 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.
- 9 V battery
- 9 V battery snap connector
- 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).
- Lab notebook