## Static Electricity Review

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17. Which of the following charging methods work without ever touching the object to be charged to the object used to charge it? Choose all that apply.

 a. charging by friction b. charging by contact c. charging by induction

 Answer: C In charging by induction, a charged object is brought near an object without touching it. The presence of the charge object induces electron movement and a polarization of the object. Then conducting pathway to ground is established and electron movement occurs between the object and the ground. During the process, the charged object is never touched to the object being charged.

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18. Which of the following charging methods result in charging an object opposite to the charge on the object used to charge it? Choose all that apply.

 a. charging by friction b. charging by contact c. charging by induction

 Answer: AC In charging by friction, the two objects being rubbed acquire opposite types of charge as electrons are transferred between objects. In charging by contact, both objects acquire the same type of charge. In charging by induction (as described in the answer to Question #17), the objects acquire the opposite type of charge.

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19. Which of the following statements are true statements about electric force? Choose all that apply.

1. Electric force is a contact force.
2. Electric forces can only act between charged objects - either like-charged or oppositely-charged.
3. Electric forces between two charged objects increases with increasing separation distance.
4. Electric forces between two charged objects increases with increasing quantity of charge on the objects.
5. If object A attracts object B with an electric force, then the attractive force must be mutual - i.e., object B also attracts object A with the same force.
6. A doubling of the quantity of charge on one of the objects results in a doubling of the electric force.
7. A tripling of the quantity of charge on both of the objects results in an increase in the electric force by a factor of 6.
8. A doubling of the separation distance between two point charges results in a quadrupling of the electric force.
9. A tripling of the separation distance between two point charges results in an electric force which is one-sixth of the original value.

 Answer: DEF a. Electric force is a non-contact force (or field force); it can act over separation distances even when the objects do not touch. b. An electrical attraction can even occur between a charged object and a neutral object. The neutral object is first polarized and then the attraction can occur. c. As separation distance increases, the electric force decreases; there is an inverse relationship. d. As the charge on any of the two objects increases, the electric force increases; there is a direct relationship. e. Forces - even electric forces - come in pairs. For every action, there is an equal and opposite reaction force. Indeed, to say that "object A attracts object B" is to automatically infer that object B also attracts object A. f. Whatever alteration is in made of the charge, the same alteration is made of the electric force. Doubling the charge will double the force. g. Whatever alteration is in made of the charge, the same alteration is made of the electric force. Tripling the charge will triple the force; tripling the other charge will triple the force again. Tripling the force (x3) two times will have the net effect of increasing the force by a factor of 9 (not 6). h. Force is inversely proportional to the square of the distance. Whatever alteration is made of the distance, the inverse square alteration is made of the force. So increasing distance by a factor of 2 (doubling it) will decrease force by a factor of 4. i. Force is inversely proportional to the square of the distance. Whatever alteration is made of the distance, the inverse square alteration is made of the force. So increasing distance by a factor of 2 (doubling it) will decrease force by a factor of 9 (3^2).

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20. Which of the following statements are true statements about electric field? Choose all that apply.

1. The electric field strength created by object A is dependent upon the separation distance from object A.
2. The electric field strength created by object A is dependent upon the charge on object A.
3. The electric field strength created by object A is dependent upon the charge of the test object used to measure the strength of the field.
4. The electric field strength about charged Object A is the force per charge experienced by a test charge placed at some location about Object A.
5. As the distance from Object A is doubled, the electric field strength created by object A increases by a factor of 4.
6. As the charge of Object A is doubled, the electric field strength created by object A increases by a factor of 2.
7. As the charge of Object A is doubled and the distance from object A is doubled, the electric field strength created by object A decreases by a factor of 2.
8. Object B is used to test the electric field strength about Object A; as the charge of Object B is doubled, the force which it experiences is doubled but the electric field strength remains the same.
9. Object B is used to test the electric field strength about Object A; as the separation distance between Object A and Object B is doubled, the force which it experiences decreases by a factor of 4 but the electric field strength remains the same.
10. The electric field strength inside of a closed conducting object (e.g., inside the sphere of the Van de Graaff generator) is zero.
11. For irregularly shaped objects, the electric field strength is greatest around points of highest curvature.
12. The electric field is a vector which points in the direction which a positive test charge would be accelerated.
13. Electric fields are directed inwards in regions around negatively-charged objects and outwards in regions around positively-charged objects.
14. Units on electric field are Newtons/Coulomb (N/C).

 Answer: ABDFGHJKLMN a. TRUE The E vector depends upon two factors - the charge of the object creating the field and the separation distance from that charge. b. TRUE The E vector depends upon two factors - the charge of the object creating the field and the separation distance from that charge. c. FALSE An object can be used to measure the strength of an electric field; such an object is called a test charge. The test charge encounters an electric force. The electric field is the ratio of the force divided by the charge on the test charge. If the quantity of charge on the test charge is increased, the force is increased proportionately but the ratio remains the same. d. TRUE An object can be used to measure the strength of an electric field; such an object is called a test charge. The test charge encounters an electric force. The electric field is the ratio of the force divided by the charge on the test charge. e. FALSE The electric field strength is inversely proportional to the square of the distance of separation from the charge. If the distance is increased by a factor of 2, then the E is decreased by a factor of 4. f. TRUE The electric field strength is directly proportional to the quantity of charge on the object which creates the field. Doubling the charge (Q) will double the electric field strength. g. TRUE The electric field strength is directly proportional to the quantity of charge on the object which creates the field and inversely proportional to the square of the distance of separation from that charge. Doubling the charge will have the effect of increasing the E by a factor of 2; doubling the distance will have the effect of decreasing the E by a factor of 4. The combined effect of these two factors will result in decreasing the E by a factor of 2. h. TRUE The electric field strength (E) created by object A does not depend upon the charge on the object used to test it. The electric field force (F) would be doubled by the doubling of the charge. But the electric field strength (E) is the force per charge - twice the force on twice the charge yields the same electric field strength. i. FALSE The electric field strength created by object A does depend on how far from object A that the test charge is. Twice the separation distance means one-fourth the electric force (the inverse square law) and therefore one-fourth of the electric field. j. TRUE This is a unique property of the electric field inside a closed conductor. k. TRUE This is another unique property of the electric field. This property explains how lightning rods work. Being pointed, the electric field strength is large around the points (parts of highest curvature). l. TRUE This is the customary convention used to define the direction of the electric field around any charged object. m. TRUE Because electric field direction is defined as the direction which a positively-charged object would accelerate, the direction would be toward negatively charged objects (+ test charges are attracted) and away from positively-charged objects (+ test charges are repelled). n. TRUE Since electric field is the force per charge exerted upon a test charge, the units would be force units per charge units - Newtons/Coulomb.

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21. Which of the following statements are true statements about lightning rods? Choose all that apply.

1. Lightning rods are placed on homes to reduce the risk of lightning damage to a home.
2. Any metallic object which is placed on the roof of a home and grounded by an appropriate conducting pathway can serve as a lightning rod.
3. Most lightning rods are pointed as a decorative feature.
4. To be totally effective, a lightning rod must stretch high into the sky and draw charge from the lowest clouds by the method of contact.
5. Lightning rods are capable of reducing the excess charge buildup in clouds, a characteristic of dangerous thunderstorms.
6. Charge is incapable of passing from clouds to a lightning rods since the air between the clouds and the lightning rod has an insulating effect.
7. The electric field strength about the points of lightning rods are very high.

 Answer: AG a. TRUE The role of lightning rods is to divert charge around your home to the ground in the event of a possible lightning strike. b. FALSE If the lightning rod does not have the characteristic protrusion up into the air above the building it protects, then it might fail to do what it does best - divert charge around the building and preventing it from being conducted through the home. c. FALSE Lightning rods are pointed because the electric field around a sharply curved object is high and serves to ionize air around it; this provides a conducting pathway from the cloud to the ground. The pathway allows charge to slowly flow between cloud and ground, thus preventing a sudden lightning discharge. In more recent years, scientists have found that a blunt tip on the lightning round provides and equal if not greater measure of protection. d. FALSE Lightning rods do not need to contact the clouds to discharge them. The strong electric fields around the points serves to turn air from an insulator to a conductor (by ionizing particles in the air). This provides a conducting pathway between the cloud and the lightning rod. e. FALSE This statement is a good description of the classical dissipation theory of lightning rods.  It was once believed (beginning with Ben Frnaklin) that lightning rods served to dissipate or remove electrostatic charge buildup in clouds by removing the charge gradually over time.  Scientists generally agree that that dissipation theory is not an accurate view of the role of lightning rods.  It is believed that lightning rods serve to divert electrostatic charge around a home during a lightning strike. f. FALSE If this were true, then there wouldn't be any lightning strikes.  The role of the lightning rod is to actually make the pathway between the cloud and the ground more conductive by ionizing the air around the rod and providing a more conductive path from cloud to ground through the lightning rod. g. TRUE This explains why lightning rods are often pointed. Though as discussed in more detail in c. above, blunt-tipped lighning rods have been found to provide equal or even greater protection as pointed-tip lightning rods.

 Useful Web Links Electric Fields and Conductors || Lightning [ #17 | #18 | #19 | #20 | #21 | #22 | #23 | #24 | #25 | #26 | #27 | #28 | #29 | #30 | #31 | #32]

22. Charged balloons are used to induce a charge upon neutral metal pop cans. Identify the type of excess charge which would be present on cans G, H, I, J, K, L, M, N and P as shown in the diagrams below.

 a. positive b. negative c. neutral
 Diagram Type of Charge on... I. G: positive (A) H: negative (B) II. I: negative (B) J: positive (A) II. K: positive (A) L: negative (B) M: positive (A) IV. N: negative (B) V. P: neutral (C)

 Answer: See table above. I. The presence of the negatively-charged balloon repels electrons which are present in can G. Many of these electrons move into can H. This leaves can G charged positively (it lost electrons) and can H charged negatively (it gained electrons). II. The presence of the negatively-charged balloon next to can J repels electrons which are present in can J. Many of these electrons move into can I. Similarly, the presence of the positively-charged balloon next to can I will attract electrons present in both cans. This combined effect leaves can J to become charged positively (it lost electrons) and can I to become charged negatively (it gained electrons). III. The presence of negatively-charged balloons on each end of the collection of cans causes electrons to be repelled from the balloons. Many electrons leave the two cans on the ends and enter the can in the middle in order to distant themselves from the balloons. This leaves Can L charged negatively (it gained electrons) and Cans K and M charged positively (they lost electrons). IV. The presence of the positively-charged balloon will attract electrons towards itself. There is a migration of electrons from the ground (hand) towards the balloon. This leaves can N charged negatively. V. Electrons in can P will be attracted to the positively-charged balloon. This will polarize the can. But unless there is a source of electrons connected to can P, there is no way for the can to develop an overall charge. The can remains neutral.

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23. In diagram IV of Question #22, the pop can acquires the charge that it does because ___.

1. electrons move from the balloon to the pop can
2. electrons move from the pop can to the balloon
3. protons move from the balloon to the pop can
4. protons move from the pop can to the balloon
5. electrons move from the pop can to the hand (ground)
6. electrons move from the hand (ground) to the pop can
7. protons move from the pop can to the hand (ground)
8. protons move from the hand (ground) to the pop can

 Answer: F Any explanation which involves the movement of protons can be quickly ruled out since protons are bound in the nucleus and incapable of moving about during ordinary electrostatic experiments. Electrons cannot move from the balloon to the pop can (nor vice versa) since there is no connecting path between these two objects. Air is a relatively good insulator and prevents the movement of charge between these two objects. What happens is that electrons in the hand sense the electric pull of the balloon and are drawn towards it, entering the pop can and imparting a negative charge to it.

 Useful Web Links Charging by Induction [ #17 | #18 | #19 | #20 | #21 | #22 | #23 | #24 | #25 | #26 | #27 | #28 | #29 | #30 | #31 | #32]

24. A negatively charged balloon will be attracted to a neutral wooden cabinet due to polarization. Which one of the following diagrams best depict why this occurs?

 Answer: D The molecules of the wooden cabinet are neutral (which rules out diagrams A, B and E) yet polarized. The molecules polarize in such a way so as to make its forward side most appealing to the balloon. This means that the positive poles of the wood molecules are nearest the negative charge of the balloon. Only diagram D shows this orientation.

 Useful Web Links Polarization [ #17 | #18 | #19 | #20 | #21 | #22 | #23 | #24 | #25 | #26 | #27 | #28 | #29 | #30 | #31 | #32]

For Questions #25 - #27, identify the type of charge on objects A-D based on the electric field lines shown for each configuration of charges.

 a. positive b. negative c. neutral
 25. A is positive 26. B is negative 27. C is positive and D is negative.

 Answer: See diagrams above. The direction of an electric field is the direction which a positively charged test charge would be accelerated. The electric field lines point in this same direction. As such, electric field lines are always directed away from + charges and towards - charges (or infinity). From this principle, it can be reasoned that A is positive, B is negative, C is positive and D is negative.

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### Part B: Diagramming and Analysis

28. A neutral conducting sphere is charged by induction using a positively-charged balloon. What will be the charge of the following sphere in step e if steps a - d are followed? _________

Explain how the object acquires this charge. Show the type and location of excess charge on the conducting sphere in each step of the diagram (where appropriate).

 Answer: See diagram above. In b., the presence of the positively-charged balloon serves to polarize the sphere. Electrons in the sphere are drawn towards the positively-charged balloon. This leaves a separation of charge in the sphere, with the excess negative charge on the left and the excess positive charge on the right. In c., the sphere is touched to ground and electrons from the ground are drawn upwards and into the right side of the sphere. It is the presence of the + charge on the right side of the sphere which attracts these electrons to itself. At this point the sphere is charged negatively. In d., this excess negative charge is shown distributed on the left side of the sphere, still drawn towards the positively-charged balloon. In e,. the balloon is pulled away and there is a movement of electrons about the sphere so that the excess negative charge is uniformly distributed.

 Useful Web Links Charging by Induction [ #17 | #18 | #19 | #20 | #21 | #22 | #23 | #24 | #25 | #26 | #27 | #28 | #29 | #30 | #31 | #32]

29. A negatively-charged balloon is held above (without touching) a neutral electroscope. The presence of the nearby balloon causes the needle of the electroscope to deflect.

a. Draw the location and type of excess charges on the "polarized" neutral electroscope.

b. Explain how the balloon has induced the temporary charge upon the electroscope (i.e., describe the direction of electron movement).

 Answer: See diagram at right. The negative balloon repels electrons from the plate to the needle and base. Since like charges are repelled, electrons in the top of the electroscope (the plate) are repelled and forced downwards to the bottom of the electroscope (the needle and base). This causes the temporary separation of charge or polarization of the electroscope.

 Useful Web Links Polarization || Charging by Induction

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30. Construct electric field lines around the following configuration of charges. Include at least six lines per charge.

 a. b. c. d.

 Answer: See diagrams above Electric field lines begin at either infinity or a + charge and are drawn either to infinity or to - charges. Avoid crossing lines. At the surface of charges, the lines should be directed radially inwards (for - charges) or radially outwards (for + charges).

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31. Use an unbroken arrow to show the direction of electron movement in the following situation. The arrow should extend from the source of the electrons to the final destination of the electrons.

 a. A positively charged balloon is touched to a neutral conducting sphere. Electrons move from the sphere to the + balloon. b. A glass rod is rubbed with a piece of wool. The wool has the greater electron affinity. Electrons move from the glass rod to the wool. c. A positively-charged balloon is held near a neutral conducting sphere. The sphere is then touched on the opposite side. Electrons move from the hand to the sphere. d. A negatively-charged rod is held near a neutral conducting sphere. The sphere is touched on the opposite side. Electrons move from the sphere to the hand.

 Answer: See diagrams above. In a, c, and d, the movement of electrons is governed by the principles that opposites attract and like-charged objects repel. Electrons (being negative) are attracted to positively charged objects and repelled by negatively charged objects. In b, the more electron-loving substance (highest electron affinity) gains the electrons from the object with which it is rubbed.

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32. A negatively-charged object creates an electric field which can be measured at various locations in the region about it. An electric field vector is drawn for point X. Use your understanding of the electric field - distance relationship to draw E vectors for points A - E. (The length of the arrow should be indicative of the relative strength of the E vector.)

 Answer: See diagram above. The electric field vector points in the direction which a positive test charge would be accelerated. This would result in a vector directed towards a negatively charged object. Thus all E vectors point towards the negatively-charged object. The E vector is inversely dependent upon square of the distance. Thus, the points which are further away have the smallest E vectors.

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