Up to and including Electroscopes Chapter 20 Up to and including Electroscopes

Summary Lauren Larson

Definitions and Key Words Charge - A property of elementary particles that determines the strength of its electric force with other particles possessing charge. Measured in coulombs, or in multiple of the charge on the proton. Charged - Possessing a net negative or positive charge. Insulator - A material that does not allow the passage of electric charge. Ceramics are good insulators. Conductor - A material that allows the passage of electric charge. Metals are good conductors. Grounding - Establishing an electrical connection to the earth in order to neutralize an object. Conservation of Charge - In an isolated system, the total charge is conserved.

Electrical Properties (p417) This section goes through materials tat can transfer charge: conductors like metal, bodies, and moisture. Also materials that cannot transfer charge: insulators like silk. To lose the charge on an object, you ground it by either touching the ground with the object or holding the object, where it is charged, while touching the ground. All conductors can be used to gain contact with the ground in order to lose charge. Ex. Metal wires on fuel trucks.

Two Kinds of Charge (p418-419) Like charges repel and opposite charges attract. Ex: Balloon and Wool = When the balloon is charged by rubbing the balloon on it, the balloon will attract to the wool or other oppositely charged objects. However, if a different balloon is also rubbed by the wool the two balloons will repel as they are like charges.

Conservation of Charge (p 419-420) In an isolated system the total charge is conserved. When you rub a piece of wool on a balloon no charge is lost. The negative charge is now on the wool and the positive charge on the balloon. When you add up the charge in the system it is the same as it was before the wool touched the balloon. With the modern view of physics this makes a lot of sense. Objects have –electrons, +protons, and neutral neutrons. Electrons and protons have the same amount of charge, (+/-) 1.6 X 10^-19.SO when you rub the wool and the balloon together, the wool gets some electrons form the balloon, but the total charge is still the same.

Induced Attractions (p420-422) Not only do opposite charges attract, but objects with no charge are equally attracted to items with either a + or – charge. Ex: The balloon, after charged by the wool, will attract to objects with no charge.

The Electroscope (p422-424) Electroscopes can be used to discover if an object is charged or not. If an object is charged when it is placed on the ball the pieces of foil will also be charged and repel each other. To discover if a charge is negative or positive hold an object where the charge is known to the ball. If the foils move further apart they are the same charge as the object used, if they move closer together they are the opposite charge.

Subtle Points Leah Starr

Moisture is a conductor; this is why electrical effects vary from day to day. (p417) Ex: A person is far more likely to shock or be shocked in winter than in summer because the air is dryer.

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

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

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

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

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

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

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

Conceptual Questions Louise Kleszyk

Question #20.6 Why is it easier to demonstrate electrostatic phenomena in Fairbanks, Alaska, than in Honolulu, Hawaii?

What are the differences between Alaska and Hawaii? Hawaii is: Close to the equator. Warm and Humid. An enjoyable Tourist Destination Alaska is: Close to the North Pole. Cold and Dry. Also an enjoyable tourist destination.

Which factor matters? HUMIDITY! Water conducts electricity. 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 #20.20 When a charged comb is brought near bits of paper, the bits are first attracted to the comb and then repelled. Describe how the charge on the comb and the bits of paper changes during the process.

What do we know about charges? LIKE charges repel one another. OPPOSITE charges attract.

Here’s the deal: The comb has a charge. (+ or -). The paper has no charge. Because they are opposite they attract. And then some of the charge transfers to the paper. Because after this they are similarly charged they begin to repel one another.

And because there are no exercises for this section: More Conceptual Questions! Alicia Weller

Question #20.1 A handheld glass rod can be charged by rubbing it with silk or a plastic bag while holding it in your hands. Would you conclude from this that glass is a conductor or an insulator? Why?

Your body is a conductor. If glass was a conductor the charge would run through your body and be grounded. So, the glass rod must be an insulator!

Question #20.21 Why are neutral objects attracted to both negatively and positively charged objects?

The induced charges on the near side of the neutral object are always opposite from the charged object near it. This produces attraction no matter what the charge. This is like Sonali’s example of the people in the restaurant when men in uniform walk by. The girls migrate to the window and the boys migrate away.

The End!