Transitional Science 8/9

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Transitional Science 8/9

9.2 pg. 281 CYU Suggested Answers

1. When two materials are rubbed together, electrons are transferred from one material to the other (diagram should show one object losing electrons and the other object gaining electrons). The electrostatic series can be used to predict which material will become negative and which will become positive.

2. Charging by conduction is where a charged object is brought into contact with a neutral object and the charge is transferred to the neutral object.

3. Charging by induction is where a charged material is brought near another material and causes the electrons in the material to move or shift slightly.

4. Charging by contact always produces the same charge since the charge is shared.

5. A negatively charged object has an excess of electrons. When it touches the pith ball, some of the excess electrons are transferred by conduction to the pith ball. Since both the object and the pith ball are now negatively charged, they repel each other.

6. Charging by conduction involves direct contact and produces the same charge on the object charged. An example of charging by conduction is touching a metal sphere with a charged vinyl strip. Charging by induction does not involve direct contact and produces an opposite charge on the object. An example of charging by induction is bringing a charged vinyl strip close to a metal sphere.

7. No, a positively charged object cannot negatively charge another object by conduction since the charge is shared. Induction can produce a temporary separation of charge in an object. If the object is then separated into two parts, then the charges can be made permanent.

8. Friction between your feet and the sheets produces equal, but opposite, charges. Skin and the material of the sheets must be apart from each other in the electrostatic series. As the charge builds up, there is enough charge to produce a discharge flash between your feet and the sheets. Pulling tape from a roll acts similarly to friction and causes a buildup of static charge. When the charge is large enough, there is a spark.

9. To determine the static charge on a sock, take an object with a known charge on it (such as a negatively charged pith ball), and see if it attracts or repels the sock. Like charges repel and opposite charges attract. A negative pith ball on a thread would attract a positively charged sock and repel a negatively charged sock.

10.

9.3 pg. 284 CYU Suggested Answers:

1. A conductor is a material that electricity will flow through. A conductor has free electrons inside it that are able to move. Some conductors are copper, mercury, or any other metal. An insulator is a material that will not allow electricity to flow through it. The electrons in an insulator are not able to move. Examples of insulators are glass, rubber, and plastics.

2. Copper, aluminum, gold, and sodium are conductors.

3. Grounding a charged object provides a path for the excess charge to Earth. If the object is charged negatively, then the excess electrons are passed to Earth. If the object is charged positively, then Earth makes up the deficiency of electrons. Essentially, the excess charge is shared between the two objects. However, Earth is so vast that the smaller object is completely neutralized.

4. The sphere is left with a negative charge.

5. An effective lightning rod must be made of a conducting material; it must be higher than the object it is protecting (for example, a metallic rod on top of the spire of a church); and it must have good contact with Earth.

6. When the student touches the electroscope with a negative vinyl strip, the electroscope is given a negative charge. This causes the leaves to spread apart. When the student brings a positively charged acetate strip near the top of the electroscope, the electrons are attracted from the leaves up to the top of the electroscope. Since there is less charge on the leaves, the amount of repulsion is less and the leaves will be less spread out.

7. (a) The effect is more noticeable on cold winter days because heated air is usually less humid than warm summer air. The dry air is a better insulator and allows a larger charge to build up on your body.

(b) The wooden part of the door is an insulator and does not conduct the excess charge away from you.

8. Rub the acetate strip with the paper towel. The acetate strip now has a positive charge. The first method is to touch the positive acetate strip to the top of the electroscope. The electroscope has been given a positive charge by conduction. The second method is to bring the positive acetate strip near, without touching, the top of the electroscope. This will induce a negative charge in the ball on the top of the electroscope. Now momentarily ground the ball of the electroscope with your finger. Electrons from your finger will move to the ball. When the acetate strip is removed, there will be a negative charge left on the electroscope by induction.

9. Since object X loses electrons, it will have a positive charge. Object Y has gained electrons, so it has a negative charge. If object X is brought near an electroscope, the positive charge will attract electrons from the leaves up into the ball at the top of the electroscope. Since that will leave the leaves with a positive charge, the leaves will spread apart.

10. Using the electrostatic series, sulfur will gain a positive charge since sulfur is above wool. The positive sulfur will attract electrons from the leaves toward the top of the electroscope. Since the leaves have lost electrons, the leaves will have a positive charge.