Chapter 9: STATIC ELECTRICITY

When a positively charged strip is brought near the electroscope, it induces a separation of charge (Figure 2). The ball on the top of the electroscope becomes negatively charged and the two leaves become positively charged

Since the two leaves have the same charge, they repel each other and spread out. When the charged object is taken away, there is no longer an induced separation of charge and the leaves return to their original position.

To ensure that the electroscope is neutral...

it needs to have all the excess charges removed, or discharged. The simplest way to discharge any object, is to ground the object. An object is grounded if it is connected to Earth by a conductor, such as a copper wire. Earth is so immense that it removes excess static charge from an object. When you touch the ball of the electroscope briefly, you ground the electroscope by removing any excess charge from it

What are some effects of electric charge?

lightning,

What charge has no overall/net charge? (equal amounts of +/-; cancels out)

neutral charge

Charging by induction happens when...

objects are charged without touching or making any direct contact. -An example of this process is the buildup of dust on the screen of a television or computer monitor. When a screen is turned on, it begins to build up a charge. When a neutral dust particle comes near the screen, the screen induces an opposite charge on the near side of the dust particle and a similar charge on the far side. The dust is attracted to the screen. -(a) A neutral metal sphere (b) When a negatively charged bar comes near the sphere, the charge on the bar causes, or induces, the electrons on the sphere to change their position.

Charging by conduction occurs when...

objects touch and an electric charge is transferred from one object to the other. -For example, if you walk across a carpet and get a spark by touching a metal doorknob, you are transferring some of your charge to the doorknob by conduction. -(a) A neutral metal sphere(b) When a negatively charged bar contacts the sphere, some of the extra electrons move to the sphere, giving it a negative charge.

The law of electric charges states...

opposites attract, likes repel

An insulator is a...

substance in which the electrons are so tightly bound to the atoms making up the material that they are not free to move to a neighbouring atom. e.g. oil fur wool rubber glass plastic pure water

As the charge builds up on the dome...

the electric force between the dome and the discharge ball increases. Therefore, the electric force increases as the charge increases. This is called a direct relationship because as one value increases, the other value also increases.

The Electrostatic shows...

the order at which some materials are more likely than others to gain or lose electrons

Electrostatics

the study of static electricity charge

Some materials, such as carbon, silicon, and germanium, are semiconductors because

they allow electrons to move although there is some resistance e.g. carbon germanium silicon selenium polyacetylene silicon carbide

A conductor...

allows electrons to flow freely from one atom to another e.g. silver copper gold aluminum tungsten nickel

Static electricity charges can be...

built up in objects

Electrostatic Precipitators British Columbia generates most of its electrical energy using hydroelectricity and natural gas thermal power plants. However, almost half of the world's electrical energy is generated by burning coal. Burning coal produces pollutants and ash particles that contribute to global warming, acid rain, and pollution. Electrostatic precipitators are used to remove the ash particles from the smoke. Electrostatic precipitators can also be used to remove wood dust in industrial plants and pulp mills. Figure 3 shows how an electrostatic precipitator works. The exhaust from burning coal passes through negatively charged plates, where the ash particles become negatively charged. The negatively charged ash particles then pass between positively charged plates, which attract the ash. The ash is then shaken or scraped off the plates and removed.

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Fabric Softener Sheets and Static Cling tumbling of clothes in a dryer presents the perfect conditions for charging by friction. The clothes are made of different insulating materials, which rub against each other (friction) in a very dry air (non-conducting) environment as they tumble around. Wool socks might end up negatively charged, while a cotton shirt might become positively charged. The socks will then be attracted to the shirt. Fabric sheets contain a chemical that is attracted to negatively charged clothes. The chemical is slightly oily and coats the surface of the clothes, leaving them feeling soft. Thus, the fabric sheet not only neutralizes the negative charges, but also reduces friction between clothes by making the surfaces slightly oily

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Static electricity is at the heart of the laser printing process (Figure 2). When you send a document from a computer to be printed, the computer translates the material into a language that the laser printer understands. The printer then sends the information to a laser device, which emits light that is reflected onto a drum. To begin, the entire drum is given a positive charge. The light from the laser causes negative electrostatic charges on the drum in the shapes of the letters and images. In this way, the laser "writes" the images on the drum. The drum is rolled onto a reservoir of toner, which is a fine powdery How Far Away Is the Lightning? You can determine the distance to lightning knowing that the speed of light is 300 000 km/s and the speed of sound is about 330 m/s. If you count the seconds between seeing the flash of lightning and hearing the sound of the thunder, and then divide the number by three, you can determine the approximate distance to the lightning in kilometres. For example, if you see a flash and then count 12 s before you hear thunder, you know that the lightning is about 4 km away. KNOW? Did You Figure 1 Scientists do not completely understand how thunderclouds become charged. To learn more about how lightning is formed, go to www.science.nelson.com GO GO 09-SciProbe9-Chap09 2/8/07 10:46 AM Page 288NEL 9.5 Applications of Static Electricity 289 mixture of black pigment and plastic. The toner is given a positive charge so that it sticks to the negative areas of the drum and is repelled from the positive areas. This is similar to putting glue on some areas of a soda can and rolling the can over flour. The flour only sticks to the glue-coated areas of the can. While the toner is stuck on the drum, the paper, which has been given a strong negative charge, rolls under the rotating drum. Since the paper is more negative than the drum, the force of attraction between the toner and the paper is greater, and the paper picks up the black toner from the drum. The negative charge is removed from the paper to stop it from sticking to the drum, and then the paper passes through a fuser. The fuser heats the toner and melts it onto the page. The completed page is sent to the output tray, and your print job is finished. The remaining toner is removed from the drum, and the printer is ready for another page.

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What is a static charge?

An electric charge at rest; the charges stay until the object discharge them

Static charges tend to last longer indoors on winter days when the heated air is very dry. Why?

Dry air is an insulator, and moist air is a poorer insulator than dry air.

What is a Faraday cage? A Faraday cage is named after British scientist Michael Faraday, who built the first one in 1836 and explained how it operated.

Faraday showed that when discharges from an electrostatic generator struck the outside of a room that he had coated with metal foil, an electroscope inside the room did not react. This proved that there was no excess charge on the inside of the room. Automobiles and airplanes behave like a Faraday cage; if lightning hits a car, it does not affect the people sitting in the car. Cell phones and radios do not work in elevators and buildings made of metal because the building acts like a Faraday cage. If a conductor has a sharp point, that area receives a greater concentration of charge. This is why a lightning rod that is placed on the top of a building has a pointed end. The lightning rod gets a concentrated positive charge if a negatively charged cloud is above the building. A conductor goes from the rod into the ground. The lightning passes through the rod and down the conductor into the ground below the building instead of going through the building.

The metal of an electroscope is a conducting material.

However, it can hold a static charge because it is not in direct contact with its surroundings. This is known as being isolated. When an isolated conductor, such as a metal ball, is given a charge, the charge spreads out evenly over the surface of the ball because of the repulsion of like charges. In fact, if a person were to get inside a metal ball, the person would be perfectly safe from the static charge on the surface of the ball

How can charges be created? (3 ways)

Induction, Conduction & Friction

In 1795, Charles Coulomb, a French physicist, was the first person to recognize the relationships between electric force, size of charge, and distance. These relationships form the basis for Coulomb's law and may be simplified as follows:

The strength of the electric force increases with increasing electric charges and decreases with increasing distance. The SI unit for size of the electric charge is called the coulomb (C) in his honour.

as the distance between the dome and the discharge ball decreases, the electric force increases.

This is called an inverse relationship because as one value decreases, the other value increases.

Lightning is a form of static electricity that can be harmful...

during the water cycle, moisture evaporates and condenses to form clouds in the atmosphere. Clouds contain millions of water droplets that collide with other droplets as they rise and fall. During these collisions, electrons are transferred from the rising water droplets to the falling water droplets. The negatively charged water droplets collect in the lower part of the cloud, giving it a negative charge. The positively charged water droplets continue to rise, carrying the positive charge to the top of the cloud. This causes the cloud to acquire a separation of charge. As the collisions between droplets continue to occur, and the charges at the top and bottom of the cloud increase, the separation of charge in the cloud becomes more intense. Eventually, the electrons at the surface of Earth are repelled by the strong negative charge at the lower part of the cloud. This causes Earth's surface to acquire a strong positive charge. In a complicated procedure, the air around the cloud forms a pathway from the cloud to the surface of Earth. This pathway is about 5 cm wide and usually lasts less than a second. Once the pathway is complete, moving electrons flow between Earth and the cloud in an attempt to neutralize the charge separation. The moving electrons excite the surrounding air, causing it to emit light—a lightning flash—and to heat to a temperature of about 3000 °C. The air rapidly expands, sending out a shock wave that we hear as the crack or rumble of thunder. The total energy produced in a large thunderstorm is equivalent to an atomic bomb being detonated!

What is the only charge that can move?

electrons; protons remain in the nucleus of an atom.

Van de Graaff generator? developed in 1930 by Robert J. Vande Graaff at Princeton University

is a device that separates large quantities of electric charge. It is used to research static electricity and to make static charges available for experimentation. An electric motor turns a rubber belt on a roller past a negatively charged comb. As the belt goes past the charged comb, it picks up the negative charges and carries them up to the collecting comb. The negative charges leave the belt at the collecting comb. They are conducted to the large hollow metal sphere where they spread across the outside surface of the sphere. is able to produce very large electric charges, which can be used to investigate electric forces. A force is a push or pull acting on an object and is usually measured in newtons (N). If the force on an object is unbalanced, it can cause a change in the motion, or acceleration, of the object. The electric force, which is also called the electrostatic force, is the force that exists between static charges. The electric force can either pull the static charges together (force of attraction) or push the static charges apart (force of repulsion).

Charging by friction occurs when...

two objects are rubbed together. -For example, large amounts of electric charge build up on clothes as they tumble against each other in a dryer. Friction causes electric charges to be transferred from one object to another. -For example, the hair and the comb have equal numbers of protons and electrons, and are both neutral. When they are rubbed together, the polyethylene atoms that make up the comb have a stronger attraction for electrons than the atoms that make up the hair. The rubbing brings more of the hair in contact with the comb, allowing the transfer of significantly more electrons. The excess negative charge builds up on the comb, while the hair becomes positively charged because it loses some electrons. -(a) The comb and the hair are both neutral (b) After being rubbed together, the comb is negatively charged and the hair is positively charged. Only the electrons move during the transfer of electric charge.

induced charge separation is...

when a positively charged object is brought toward a neutral object, the nearest end of the neutral object moved toward the positively charged object. This is because the positively charged object causes the electrons in the neutral object to shift slightly toward the positive object.

how can electrons can be transferred from one object to another object?

when two objects rub against each other, one object loses electrons and the other object gains electrons. Both objects become charged.