Chapter 20Chapter 20

Up to and including ElectroscopesUp to and including Electroscopes

SummarySummaryLauren LarsonLauren Larson

Definitions and Key WordsDefinitions and Key Words

Charge - A property of elementary particles that determines the Charge - A property of elementary particles that determines the strength of its electric force with other particles possessing strength of its electric force with other particles possessing charge. Measured in coulombs, or in multiple of the charge on charge. Measured in coulombs, or in multiple of the charge on the proton.the proton.

Charged - Possessing a net negative or positive charge. Charged - Possessing a net negative or positive charge. Insulator - A material that does not allow the passage of electric Insulator - A material that does not allow the passage of electric

charge. Ceramics are good insulators.charge. Ceramics are good insulators. Conductor - A material that allows the passage of electric Conductor - A material that allows the passage of electric

charge. Metals are good conductors. charge. Metals are good conductors. Grounding - Establishing an electrical connection to the earth in Grounding - Establishing an electrical connection to the earth in

order to neutralize an object.order to neutralize an object. Conservation of Charge - In an isolated system, the total charge Conservation of Charge - In an isolated system, the total charge

is conserved.is conserved.

Electrical Properties Electrical Properties (p417)(p417)

This section goes through materials tat can This section goes through materials tat can transfer charge: conductors like metal, bodies, transfer charge: conductors like metal, bodies, and moisture. Also materials that cannot and moisture. Also materials that cannot transfer charge: insulators like silk. To lose the transfer charge: insulators like silk. To lose the charge on an object, you ground it by either charge on an object, you ground it by either touching the ground with the object or holding touching the ground with the object or holding the object, where it is charged, while touching the object, where it is charged, while touching the ground. All conductors can be used to gain the ground. All conductors can be used to gain contact with the ground in order to lose charge. contact with the ground in order to lose charge. Ex. Metal wires on fuel trucks.Ex. Metal wires on fuel trucks.

Two Kinds of Charge Two Kinds of Charge (p418-419)(p418-419)

Like charges repel and opposite charges Like charges repel and opposite charges attract.attract.

Ex: Balloon and Wool = When the balloon Ex: Balloon and Wool = When the balloon is charged by rubbing the balloon on it, the is charged by rubbing the balloon on it, the balloon will attract to the wool or other balloon will attract to the wool or other oppositely charged objects. However, if a oppositely charged objects. However, if a different balloon is also rubbed by the wool different balloon is also rubbed by the wool the two balloons will repel as they are like the two balloons will repel as they are like charges.charges.

Conservation of Charge Conservation of Charge (p 419-420)(p 419-420)

In an isolated system the total charge is conserved. In an isolated system the total charge is conserved. When you rub a piece of wool on a balloon no charge is When you rub a piece of wool on a balloon no charge is lost. The negative charge is now on the wool and the lost. The negative charge is now on the wool and the positive charge on the balloon. When you add up the positive charge on the balloon. When you add up the charge in the system it is the same as it was before the charge in the system it is the same as it was before the wool touched the balloon. With the modern view of wool touched the balloon. With the modern view of physics this makes a lot of sense. Objects have –physics this makes a lot of sense. Objects have –electrons, +protons, and neutral neutrons. Electrons and electrons, +protons, and neutral neutrons. Electrons and protons have the same amount of charge, (+/-) 1.6 X protons have the same amount of charge, (+/-) 1.6 X 10^-19.SO when you rub the wool and the balloon 10^-19.SO when you rub the wool and the balloon together, the wool gets some electrons form the balloon, together, the wool gets some electrons form the balloon, but the total charge is still the same.but the total charge is still the same.

Induced Attractions Induced Attractions (p420-422)(p420-422)

Not only do opposite charges attract, but Not only do opposite charges attract, but objects with no charge are equally objects with no charge are equally attracted to items with either a + or – attracted to items with either a + or – charge.charge.

Ex: The balloon, after charged by the Ex: The balloon, after charged by the wool, will attract to objects with no charge.wool, will attract to objects with no charge.

The Electroscope The Electroscope (p422-424)(p422-424) Electroscopes can be used to Electroscopes can be used to

discover if an object is charged discover if an object is charged or not. If an object is charged or not. If an object is charged when it is placed on the ball when it is placed on the ball the pieces of foil will also be the pieces of foil will also be charged and repel each other. charged and repel each other. To discover if a charge is To discover if a charge is negative or positive hold an negative or positive hold an object where the charge is object where the charge is known to the ball. If the foils known to the ball. If the foils move further apart they are the move further apart they are the same charge as the object same charge as the object used, if they move closer used, if they move closer together they are the opposite together they are the opposite charge. charge.

Subtle PointsSubtle PointsLeah StarrLeah Starr

Moisture is a Moisture is a conductor; this is why conductor; this is why electrical effects vary electrical effects vary from day to day. from day to day. (p417)(p417)

Ex: A person is far Ex: A person is far more likely to shock more likely to shock or be shocked in or be shocked in winter than in summer winter than in summer because the air is because the air is dryer.dryer.

We can ground a We can ground a conducting object by merely conducting object by merely touching it, however we touching it, however we must rub an insulated object must rub an insulated object in order to ground it. (p417)in order to ground it. (p417)

Like charges are repelled by Like charges are repelled by one another, however an one another, however an uncharged object and a uncharged object and a charged object, as well as charged object, as well as oppositely charged objects will oppositely charged objects will attract one another.attract one another. (p418)(p418)

An object will be uncharged if An object will be uncharged if it has equal amounts of it has equal amounts of positive and negative charges. positive and negative charges. Not because it has no charge. Not because it has no charge. An object is considered to An object is considered to have a positive charge when it have a positive charge when it has an excess of protons, or has an excess of protons, or deficiency of electrons.deficiency of electrons. (p420)(p420)

Charges are not lost Charges are not lost through rubbing, they are through rubbing, they are transferred.transferred. (p420)(p420)

An uncharged object still An uncharged object still attracts a charged object. This attracts a charged object. This occurs even within an occurs even within an insulated object, the insulated object, the movements of the charges are movements of the charges are not as extreme. not as extreme.

An electroscope will not tell us An electroscope will not tell us what kind (+ what kind (+ oror -) of a charge is -) of a charge is present, only that there is one.present, only that there is one. (p422)(p422)

An electroscope can be An electroscope can be charged through charged through inductioninduction. In . In this case then a like charge this case then a like charge remains within the remains within the electroscope and the pieces of electroscope and the pieces of foil continue to repel one foil continue to repel one another.another. (p423)(p423)

Conceptual QuestionsConceptual QuestionsLouise KleszykLouise Kleszyk

Question #20.6Question #20.6

Why is it easier to Why is it easier to demonstrate electrostatic demonstrate electrostatic phenomena in Fairbanks, phenomena in Fairbanks, Alaska, than in Honolulu, Alaska, than in Honolulu, Hawaii?Hawaii?

What are the differences between What are the differences between Alaska and Hawaii?Alaska and Hawaii?

Hawaii is:Hawaii is:

Close to the equator.Close to the equator.

Warm and Humid.Warm and Humid.

An enjoyable Tourist An enjoyable Tourist DestinationDestination

Alaska is:Alaska is:

Close to the North Close to the North Pole. Pole.

Cold and Dry.Cold and Dry.

Also an enjoyable Also an enjoyable tourist destination.tourist destination.

Which factor matters?Which factor matters?

HUMIDITY!HUMIDITY!

Water conducts electricity.Water conducts electricity.

Humid air allows objects to lose charge.Humid air allows objects to lose charge.

What does this all mean?

Although a great place to catch a tan, Hawaii is not an ideal place to study

and observe electrostatic phenomena.

Question #20.20Question #20.20

When a charged When a charged comb is brought near comb is brought near bits of paper, the bits bits of paper, the bits are first attracted to are first attracted to the comb and then the comb and then repelled. Describe repelled. Describe how the charge on how the charge on the comb and the bits the comb and the bits of paper changes of paper changes during the process.during the process.

What do we know about charges?What do we know about charges?

LIKE LIKE charges repel one another.charges repel one another.

OPPOSITEOPPOSITE charges attract. charges attract.

Here’s the deal:Here’s the deal:

The comb has a charge. (+ or -).The comb has a charge. (+ or -).The paper has no charge.The paper has no charge.Because they are opposite they attract.Because they are opposite they attract.And then some of the charge transfers to And then some of the charge transfers to

the paper.the paper.Because after this they are similarly Because after this they are similarly

charged they begin to repel one another.charged they begin to repel one another.

And because there are no And because there are no exercises for this section:exercises for this section:

More Conceptual Questions!More Conceptual Questions!Alicia WellerAlicia Weller

Question #20.1Question #20.1

A handheld glass rod can be charged by A handheld glass rod can be charged by rubbing it with silk or a plastic bag while rubbing it with silk or a plastic bag while holding it in your hands. Would you holding it in your hands. Would you conclude from this that glass is a conclude from this that glass is a conductor or an insulator? Why?conductor or an insulator? Why?

Your body is a conductor.Your body is a conductor.

If glass was a conductor the charge would If glass was a conductor the charge would run through your body and be grounded.run through your body and be grounded.

So, the glass rod must be an insulator!So, the glass rod must be an insulator!

Question #20.21Question #20.21

Why are neutral objects attracted to both Why are neutral objects attracted to both negatively and positively charged objects?negatively and positively charged objects?

The induced charges on the near side of the The induced charges on the near side of the neutral object are always opposite from the neutral object are always opposite from the charged object near it. charged object near it.

This produces attraction no matter what the This produces attraction no matter what the charge.charge.

This is like Sonali’s example of the people in the This is like Sonali’s example of the people in the restaurant when men in uniform walk by. The restaurant when men in uniform walk by. The girls migrate to the window and the boys migrate girls migrate to the window and the boys migrate away.away.

The End!The End!