Static electricity is the accumulation of electric charges on the surface of a material, usually an insulator or nonconductor of electricity.
The term static means standing still or at rest, and static electricity refers to an electric charge at rest. The result of this buildup of static electricity is that objects may be attracted to each other or may even cause a spark to jump from one to the other.
How is Static Electricity Produced?
One of the simplest ways to produce static electricity is by friction. For example, a static charge can be produced by rubbing a balloon with a piece of wool, as illustrated in Figure 1.
The process causes electrons to be pulled from the wool to the balloon. As a result the balloon ends up with an excess of electrons and a negative charge. At the same time, the wool loses electrons, creating a shortage of electrons and a positive charge. Note that the charged atoms remain on the surface of the material.
Static electricity is different from current electricity that flows through metal wires. Most often the materials involved in static electricity are nonconductors of electricity.
Figure 1 Producing static electricity by friction.
Static electricity is formed when we accumulate extra negatively charged electrons and they are discharged to an object or person. Take, for example, the rubber soles of your shoes and that wool carpet in the living room. When you walk across the carpet, your body builds up a negative charge of extra electrons it can’t get rid of through the insulating soles of your shoes. Then when you reach for the doorknob, you may experience a static electric shock caused by electrons moving from your hand to the metal doorknob (Figure 2).
Static electricity occurs more often during the colder seasons because the air is drier, and it’s easier to build up electrons on the skin’s surface. In warmer weather, the moisture in the air helps electrons move off of you more quickly so the static charge is not as great.
Figure 2 Static electric shock.
The first law of electric charges is illustrated in Figure 3 and states that like charges repel and unlike charges attract. An invisible electrostatic field exists in the space between and around charged balls.
When two like-charged bodies are brought together, their electric fields repel one body from the other. When two unlike-charged bodies are brought together, their electric fields attract one body to the other.
Figure 3 Law of electric charges.
The electrostatic field around two charged objects can be represented graphically by lines referred to as electrostatic lines of force (Figure 4).
Lines are directed away from positively charged objects and toward negatively charged objects. The attractive or repulsive force that is exerted between two charged particles is directly proportional to the strength of their charges and inversely proportional to the square of distance between the two charges. This means that the bigger the charges, the more will be the force; the more distance they are apart, the less the force between them.
Figure 4 Electrostatic field around two charged objects.
If two strongly charged bodies (one positive and one negative) are moved near to each other, before contact is made you actually see the equalization of the charges take place in the form of an arc. Lightning is a perfect example. Cloud-to-ground lightning occurs when the electric charge travels between a negatively charged cloud base and the positively charged ground. A single bolt of lightning delivers about 1 trillion watts of electricity.
Electrostatic discharge (ESD) is the rapid discharge of static electricity from one object to another of a different potential. This is a very rapid event that happens when two objects of different potentials come into direct contact with each other.
Electrostatic discharge is one of the main causes of device failures in the semiconductor industry. Static electricity, so low that you can’t feel it, can wreck havoc with today’s large-scale microelectronic devices.
Methods of protection against ESD include prevention of static charge buildup and safe dissipation of any charge buildup (Figure 5). Packaging materials such as static shielding bags, conductive bags, and electrostatic discharge containers and boxes provide direct protection of devices from electrostatic discharge.
Antistatic footwear and wrist straps when properly worn and grounded keep the human body near ground potential, thus preventing hazardous discharge between bodies and objects.
Figure 5 Methods of protection against ESD.
Charging by Conduction and Induction
The three common ways for a neutral object to become charged are by friction, conduction, or induction. Charging by conduction or contact occurs when a neutral object is placed in contact with an already-charged object.
If the object is negatively charged, electric repulsion will push some of the excess electrons from the charged to the neutral object. If the object is positively charged, electric attraction will pull some electrons from the neutral object to the charged one.
In the example shown in Figure 6, when a rod (that has an excess of electrons) touches a neutral ball, the charge distributes itself over both objects. When they are separated, the ball will now be electrically charged.
Figure 6 Charging by conduction.
Charging by induction involves an already-charged object that is brought close to but does not touch the neutral object.
In the example shown in Figure 7, the negatively charged rod is brought close to an electrically neutral ball. The electrons on the ball are repelled and move to the opposite side of the ball. Touching the negative right side of the ball drains electrons from the ball to ground, giving the ball a net positive charge. The rod is then removed, leaving a positively charged ball.
Figure 7 Charging by induction.
Practical Uses of Static Electricity
There are several useful applications for the forces of attraction between charged particles. For these applications, static electric charges are normally produced by a high-voltage DC (direct current) source.
Electrostatic paint spraying (Figure 8) uses static electricity to attract the paint to the target, reducing paint wastage and improving coverage of the target. The paint and target part are charged with opposite charges so the two attract and the paint sticks to the target. This process produces a uniform cover of paint with excellent adhesion.
Figure 8 Electrostatic paint spraying.
Electrostatic air cleaners or precipitators use positively and negatively charged plates to remove dirt particles from the air. Figure 9 illustrates the operation an electronic air cleaner used in a home-heating system to clean the air as it circulates through the furnace.
- The dirty air passes through a paper filter that removes large dust and dirt particles from the air.
- The air then moves through an electrostatic precipitator consisting of two oppositely charged, high-voltage grids.
- The precipitator works by giving a positive charge to particles in the air and then attracting them with a negatively charged grid.
- Finally, the air passes through a carbon filter, which absorbs odors from the air.
Figure 9 Electrostatic air cleaner.
- Define static electricity.
- Explain why rubbing a balloon with a piece of wool results in a negative charge on the surface of the balloon.
- State the law of electrostatic charges.
- How is the force between two charged particles affected by the strength of their charges and the distance between them?
- Define electrostatic discharge.
- Electrostatic discharge is one of the main causes of device failure in the ________________ industry.
- Name three ways for a neutral object to become charged.
- Charging by ________________ involves an already-charged object that is brought close to but does not touch the neutral object.
- Summarize the operation of an electrostatic paint spraying process.
- Summarize the operation of an electrostatic air cleaner.
- Why does static electricity occur more often during the colder seasons?
Review Questions – Answers
- Static electricity is the accumulation of electrical charges on the surface of a material, usually an insulator.
- Rubbing a balloon with a piece of wool results in a negative charge on the surface of the balloon because electrons are pulled from the wool to the balloon.
- Like charges repel and unlike charges attract.
- The attractive or repulsive force that is exerted between two charged particles is directly proportional to the strength of their charges and inversely proportional to the square of the distance between the two charges.
- Electrostatic discharge is the rapid discharge of static electricity from one object to another of a different potential.
- A neutral object can become charged by friction, conduction, or induction.
- An electrostatic paint spraying process operates by charging the paint and the part to be painted with opposite charges. The paint is attracted to the part. When the paint hits the part, the charge is neutralized and no more paint is applied to that area.
- The electrostatic air cleaner operates by charging the particles in the air with positive charges and then attracting them to a negative grid.
- Because the air is drier, and it’s easier to build up electrons on surfaces.