Static electric charges occur often in everyday life. Almost everyone has received a shock after walking across a carpet and then touching a metal object or after sliding across a car seat and touching the door handle. Almost everyone has combed their hair with a hard rubber or plastic comb and then used the comb to attract small pieces of paper or other lightweight objects. Static electric charges cause clothes to stick together when they are taken out of a clothes dryer. Lightning is, without doubt, the greatest display of a static electric discharge.
Static electricity can harm sensitive electronic components. Components subject to damage by static electricity are often packages in special wrapping. Care should be taken when opening the packaging. It is generally a good idea to touch a grounded object to make certain that your body is discharged before handling these components.
Although static charges can be a nuisance, or even dangerous, they can also be beneficial.
Static electricity examples
Some examples of static electricity are:
Running a comb through your hair transfers electrons to the comb, giving it a negative charge. When the comb is brought close to bits of paper, charge separation is induced on the paper bits. The attractive electrical force accelerates the paper bits upward against the force of gravity.
Copy machines operate on the principle of static electricity. The copy machine uses an aluminum drum coated with selenium. Selenium is a semiconductor material that changes its conductivity with a change in light intensity. When selenium is in the presence of light, it has a very high conductivity. When it is in darkness, it has very low conductivity.
A high-voltage wire located near the drum causes the selenium to have a positive charge as it rotates. The drum is in darkness when it is charged. An image of the material to be copied is reflected on the drum by a system of lenses and mirrors. The light portions of the paper reflect more light than the dark portions. When the reflected light strikes the drum, the conductivity of the selenium increases greatly, and negative electrons from the aluminum drum neutralize the selenium charge at that point. The dark area of the paper causes the drum to retain a positive charge.
A dark powder that has a negative charge is applied to the drum. The powder is attracted to the positively charged areas on the drum. The powder on the neutral areas of the drum falls away. A piece of positively charged paper passes under the drum and attracts the powder from the drum. The paper then passes under a heating element, which melts the powder into the paper and causes the paper to become a permanent copy of the original.
The manufacture of sandpaper also relies on the application of static electricity. Grains of sand receive a static charge to make them stand apart and expose a sharper edge.
Electronic air filters
Electronic air filters—precipitators—use static charges to attract small particles of smoke, dust, and pollen. The precipitator uses a high-voltage DC power supply to provide a set of wires with a positive charge and a set of plates with a negative charge. As a blower circulates air through the unit, small particles receive a positive charge as they move across the charged wires. The charged particles are then attracted to the negative plates. The negative plates hold the particles until the unit is turned off and the plates are cleaned.
Static electricity is often used in spray painting. A high-voltage grid is placed in front of the spray gun. This grid has a positive charge. The object to be painted has a negative charge. As the droplets of paint pass through the grid, the positive charge causes electrons to be removed from the paint droplets. The positively charged droplets are attracted to the negatively charged object. This static charge helps to prevent waste of the paint and at the same time produces a uniform finish.
When static electricity occurs in nature, it can be harmful. The best example of natural static electricity is lightning. A static charge builds up in clouds that contain a large amount of moisture as they move through the air. It is theorized that the movement causes a static charge to build upon the surface of drops of water. Large drops become positively charged, and small drops become negatively charged. Notice that both positive and negative charges can be contained in the same cloud. Most lightning discharges, or lightning bolts, occur within the cloud. Lightning discharges can also take place between different clouds, between a cloud and the ground, and between the ground and the cloud.
Whether a lightning bolt travels from the cloud to the ground or from the ground to the cloud is determined by which contains the negative and which the positive charge. Current always flows from negative to positive. If a cloud is negative and an object on the ground is positive, the lightning discharge travels from the cloud to the ground. If the cloud has a positive charge and the object on the ground has a negative charge, the discharge travels from the ground to the cloud. A lightning bolt has an average voltage of about 15,000,000 volts.
Automobiles and trucks
The static charge that accumulates on automobiles as they move through dry air can cause dangerous conditions under certain circumstances. For that reason, trucks carrying flammable materials such as gasoline or propane use a drag chain. One end of the drag chain is attached to the frame of the vehicle, and the other end drags the ground. The chain is used to provide a path to the ground while the vehicle is moving and to prevent a static charge from accumulating on the body of the vehicle.
The static charge that accumulates on a person’s body as he or she walks across a carpet can cause a painful shock when a metal object is touched and discharges in the form of an electric spark. Most carpets are made from man-made materials that are excellent insulators such as nylon.
When the clothes made up of nylon are rubbed against some other fabric or against the wearer’s skin, static electricity is formed. This electrostatic force developed between the skin and the clothing particles is accountable for the chattering sound caused while removing such clothes.
In the winter, the heating systems of most dwellings remove moisture from the air and cause the air to have low humidity. The dry air combined with an insulating material provides an excellent setting for the accumulation of a static charge. This condition can generally be eliminated by the installation of a humidifier. A simple way to prevent the painful shock of a static discharge is to hold a metal object, such as a key or coin, in one hand. Touch the metal object to a grounded surface, and the static charge will arc from the metal object to the ground instead of from your finger to the ground.
The static charge that accumulates on clothes in a dryer is caused by the clothes moving through the dry air. The greatest static charges generally are built upon man-made fabrics because they are the best insulators and retain electrons more readily than natural fabrics such as cotton or wool.
The dust particles floating freely in the air get polarized by the television screen. Due to this, the charged dust particles stick to the television screen. This is the reason why a layer of dust gets deposited on the screen minutes after cleaning the previous layer. This interaction between the dust particles and the screen is known as the electrostatic interaction. If a person moves his/her hand even a few centimeters above the screen, he/she can easily feel the electrostatic force in action.