Definition: The word static means at rest. Electricity can be at rest. The generation of static electricity can be demonstrated in many ways.
When stroking the fur of a cat, you will notice that its fur is attracted to your hand as you bring your hand back over the cat. You will also hear a crackling sound. If this is done at night, you may see tiny sparks. The sound is caused by the discharge of static electricity.
When we stroke the fur with our hand, the friction between the cat’s fur and our hand excites the atoms. Some atoms lose electrons while others gain electrons. The sparks are created as the atoms attempt to neutralize themselves by gaining back the lost electrons.
You can generate a static charge of electricity by walking across a wool or nylon rug with plastic-soled shoes.
After walking across such a rug, you receive the surprising experience of discharging several thousand volts of static electricity to a metallic object such as a door handle. This condition is especially present on cold winter days when the humidity is quite low.
You can also experience a similar discharge when sliding across the seat of a car covered with certain types of upholstery. The friction of your clothing against the seat leads to a discharge when you touch the earth and the metallic frame of the car at the same time.
The force of attraction and repulsion of charged particles was studied by the French scientist, Charles A. Coulomb.
Because an atom, and electrons, in particular, are so very small, a charge of just a few electrons, say a dozen, is almost impossible to measure. Consequently, Coulomb developed a practical unit for measurement of an amount of electricity. It is known as the coulomb.
One coulomb represents approximately 6.24 × 1018 electrons (6,240,000,000,000,000,000).
While the coulomb is used to describe the flow of electricity, it is not used to describe static charges. It is impractical to describe the very small difference in charges between two bodies using values so large.
The field of force surrounding a charged body is called the electrostatic field or dielectric field.
The field can exhibit a positive or negative charge depending on a gain or loss of electrons.
Two charged masses are shown in Figure 1. Lines represent the electrostatic fields of opposite polarity and the attractive force existing between the masses. In Figure 2, two charged masses are shown with like polarities.
Figure 1. The electrostatic fields of unlike charged bodies show attractive forces.
Figure 2. The electrostatic fields of like-charged bodies repel each other.
A repulsive force exists between the charged masses due to the electrostatic fields. The field is strongest very close to the charged body.
The field strength diminishes at a distance inversely proportional to the square of the distance. Figure 3 illustrates the concept of strength is inversely proportional to the square of the distance.
Figure 3. The strength of the force between two charged bodies is inversely proportional to the square of the distance.
When two electrostatic fields are joined together, the electrons flow from the mass with an excess of electrons to the mass that has a need of electrons. Figure 4 illustrates this principle.
The excess electrons flow from the body that is negatively charged to the positively charged body that has the electron deficiency. This transfer of electrons can be accomplished by touching the two bodies together or by connecting them with a material that supports the flow of electrons between the two bodies. This connecting material is known as a conductor because it “conducts” electricity.
Figure 4. When two charged bodies are connected with a conductor, excess electrons will flow through the conductor from the mass having a surplus of electrons to the mass having a deficit of electrons.
Charges can be transferred in two ways:
One way is by direct contact. When a charged body such as a glass rod touches another body such as the top of an electroscope, the electroscope takes on part of the charge of the rod.
Another way of transferring a charge is induction. A charge is induced by bringing a charged object near another object. The glass rod needs only be brought near the top of the electroscope to charge it.
When an object is charged by induction, the object takes on the opposite charge as the rod. When the rod touches the object, the object takes the same charge as the rod. Refer back to Experiment 1-2.
Static Electricity Applications
The principles of static electricity are used in industry to reduce air pollution. One piece of equipment used in reducing pollution is called an electrostatic precipitator. Most precipitators are divided into two parts, a charging section, and a collecting section.
The charging section can be designed in many different ways. It can be an assembly of parallel rods or wires, a screen pattern, or a bank of piping. Regardless of the physical design, the electrical principle is the same in all cases.
Figure 5 illustrates the operation of an electrostatic precipitator. A grid of electrodes is installed into the stream of pollutants. The electrodes are charged between 45,000 and 75,000 volts forming an ionized field or a corona around each electrode.
A corona is an area of ion or static charge surrounding a high voltage conductor.
The pollution particles become negatively charged as they pass through the corona. The negatively charged particles are then attracted to the positively charged collection plates.
After the collection plates become laden with pollution particles, they must be cleaned. This principle of removing particles (pollutants) from the air is used in industries such as concrete, paper, chemical, and coal-fired power generation.
Figure 5. An electrostatic precipitator places an electrostatic charge on particles and then collects the particles on an oppositely charged plate
The application of electrostatics is also gaining popularity in residential use. Electrostatic filters are a part of the latest air-conditioning systems.
The principle of attraction and repulsion is also applied in the painting industry and in the manufacture of sandpaper.
The painting industry sprays a positively charged mist of paint onto a negatively charged surface such as the panels of an automobile. This procedure reduces the amount of overspray and saves paint.
The sand-paper industry charges the backing paper with a positive static charge and the silica crystals (sand or some other abrasive) with a negative charge. The result is an even spread of granules over the entire surface of the paper.