Technical Article

Friction as a Source of Electrical Energy

April 21, 2021 by Alex Roderick

This series provides a look at different phenomena that can produce electrical energy. In this article, the fourth in our series, we’ll discuss friction. Specifically, we’ll discuss lightning and static electricity.

When we move a charged particle against an electric field, the energy we consume is stored in the particle as electrical potential energy.  There are six different phenomena acting as sources of this energy: friction, pressure, light, heat, chemical reaction, and magnetism. See Figure 1. Magnetism is the most common usable source of electrical energy. Chemical action is the next most common usable source.

 

Figure 1. The six sources of electrical energy are friction, pressure, light, heat, chemical reaction, and magnetism.
Figure 1. The six sources of electrical energy are friction, pressure, light, heat, chemical reaction, and magnetism.

 

This series provides a look at each of these electrical sources of energy. In this article, the first of our series, we’ll discuss friction.

 

What is Friction?

The triboelectric effect is the generation of a static charge by friction. Friction is the resistance to motion that occurs when two surfaces move against each other. A static charge is an electrical charge at rest. 

Friction causes electrons from one material to flow to another material to create negative and positive static charges. A negative static charge is the accumulation of excessive electrons on a body. A positive static charge is a deficiency of electrons on a body. 

For example, a glass rod, when rubbed with a silk cloth, gives up some of its electrons to the silk. In this process, the silk becomes negatively charged, and the glass rod becomes positively charged. Friction is seldom used for producing electrical energy. 

When a person with a negative static charge contacts an object with a positive static charge, all of the excess electrons flow (jump) to the object. All methods of electrification require electrons to move from one body to another. Electrostatic discharge (ESD) is the movement of electrons from a source to an object. 

Electrostatic discharges as small as 100 V have been known to damage semiconductors. People normally do not feel an electrostatic discharge until the discharge reaches 3000 V. Precautions must be taken to discharge static charges before handling semiconductor devices. A wrist strap attached to a static line that is grounded will discharge static electricity from the body. See Figure 2.

 

Figure 2. A wrist strap connected to the ground is used to prevent electrostatic discharges from damaging electronic components.
Figure 2. A wrist strap connected to the ground is used to prevent electrostatic discharges from damaging electronic components.
 

Small electrostatic discharges (the static shock) are unpleasant and may be costly but are not life-threatening. 

On the other hand, an aircraft in flight accumulates a static charge due to the friction between its surface and the passing air. These static charges may cause interference with radio communications and, under certain circumstances, can cause physical damage. When refueling aircraft, bonding and grounding techniques are employed to discharge any static charge buildup before beginning the fueling process. This eliminates the possibility of an explosion due to an electrostatic discharge. 

 

Lightning 

Lightning is a transient, high-current electrostatic discharge that occurs in the atmosphere. Lightning produces a very high flow of electrical current. Lightning normally has a duration of less than one second and is composed of many distinct strokes that occur in such rapid succession that they appear as a single flash of light.

Before lightning can occur, an electrical charge that can break down the insulation quality of the air must accumulate. Most experts believe that this electrical charge forms as different kinds of ice interact in a cloud. At the same time, updrafts of warm air and downdrafts of cold air flowing through ice crystals or droplets of water in the cloud cause a negative charge to form at the bottom of the cloud and a positive charge to form at the top. 

Thunderstorm clouds are generally negatively charged at the base and positively charged in higher regions. See Figure 3. Under certain conditions, this charge may be reversed or distributed differently in the cloud.

 

Figure 3. Power return strikes from Earth to cloud.
Figure 3. Power return strikes from Earth to cloud.

 

Although lightning may occur between clouds, between clouds and the ground, or between clouds and the clear air above them, over half of all lightning flashes occur within a single cloud.

 

Lightning Protection

During his experiments with lightning, Benjamin Franklin observed that lightning strikes normally occur at points of high elevation. Based on his observations, Franklin concluded that lightning was a form of electricity. He invented the first lightning rod system in 1752. 

The lightning rod system does not prevent lightning strokes. Rather, it provides a means of controlling the path of the electrostatic discharge with conductors. These conductors provide a lower resistance path for the electron flow than that provided by the structure under protection.

 

Figure 4. High-voltage surge arresters are used to protect electrical power distribution equipment from power surges caused by lightning strikes.
Figure 4. High-voltage surge arresters are used to protect electrical power distribution equipment from power surges caused by lightning strikes.
 

To control lightning strikes, air terminals (copper rods) are fastened vertically at the apex of buildings to intercept electrostatic discharges. Electrostatic discharges are directed from the air terminals through heavy copper conductors to the ground (earth). The copper rods must be of sufficient height for the lightning to strike them before it strikes the building. 

The area of protection of a lightning rod is a cone-shaped space that has a base radius equal to the height of the rod. It is possible, however, for lightning to strike within the protected area. See Figure 5.

 

Figure 5. A lightning rod system protects a cone-shaped area that has a base radius equal to the height of the rod.
Figure 5. A lightning rod system protects a cone-shaped area that has a base radius equal to the height of the rod.

 

Lightning rods are not designed to take a direct hit from a lightning stroke. The current from a lightning stroke is so great that it can destroy the system. Instead, a lightning rod system is designed to slowly neutralize the charge by providing a conductive path for electrons. These currents are normally within the design capabilities of the system. This prevents the buildup of a large enough potential needed to break down the air and start the lightning stroke. 

Power lines are often shielded by an overhead conductor that is grounded at fixed intervals. Grounding masts are often used with this conductor. The system works in the same manner as a lightning rod. 

In addition, electric power and communication lines are normally protected with surge and lightning arresters. These devices may be air gaps that are not affected by the normal potential of the conductors under protection. However, during a voltage surge or lightning strike, the air gap breaks down and acts as a conductor to the ground. This keeps the potential of the protected conductors at a safe level.

Once the high potential has been neutralized, the air gap returns to its high resistance state. The distribution of power and communications are instantly restored. 

 

Static Electricity Applications 

Electrostatic discharge is not used in most applications of static electricity. Instead, the ability of charges to repel or attract each other is of greater use. This ability is used in electrostatic precipitators, cathode ray tubes, and manufacturing processes.

An electrostatic precipitator is a device that uses electricity to remove particles from flue gases. The operating principle of an electrostatic precipitator is that unlike charges attract each other. In an electrostatic precipitator, contaminated flue gas containing particles is passed through a grid containing positively charged electrodes and negatively-charged plates. 

The particles become positively charged as they pass by the electrodes. Voltage potentials as high as 120 kV are often necessary to charge the particles. On each side of the electrodes are negatively charged collector plates. The particles are repelled by the electrodes because their charges are the same, and they are attracted to the collector plates because their charges are different. 

The collector plates are periodically vibrated to shake off the particles. The clean flue gas passes out of the precipitator into the smokestack or clean air return plenum with more than 99% of the contaminants removed. See Figure 6.

 

Figure 6. An electrostatic precipitator uses static electricity to remove contaminants from flue gases before releasing the gases into the atmosphere.
Figure 6. An electrostatic precipitator uses static electricity to remove contaminants from flue gases before releasing the gases into the atmosphere.

 

Electrostatic Spray Painting 

Static electricity is also used in various manufacturing processes. Electrostatic spray painting is one application that has been particularly successful. 

In the electrostatic spray painting process, paint is charged as it leaves the spray gun with a charge opposite in polarity to the object being painted. The opposite charges of the paint and object attract each other. 

This process results in a uniform coat of paint on the object. It is particularly useful for painting irregular surfaces. In addition, little paint is lost to the area surrounding the charged object.

 

Sandpaper

The manufacturing of sandpaper also uses electrostatic principles. With sandpaper, an adhesive is applied to the underside of the paper. Negatively charged abrasive particles are attracted to the underside due to the positively charged plates located on the top side of the paper. See Figure 7. 

 

Figure 7. In the manufacturing of sandpaper, negatively charged abrasive particles are attracted to the underside of the paper due to the positively charged plates located on the top side of the paper
Figure 7. In the manufacturing of sandpaper, negatively charged abrasive particles are attracted to the underside of the paper due to the positively charged plates located on the top side of the paper

 

The sandpaper is dried by a heater and cut to size. The deposit on the paper is uniform, with very little waste material.