Navigate to: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.
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. 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.
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. 19. Which of the following statements are true statements about electric force? Choose all that apply.
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). 20. Which of the following statements are true statements about electric field? Choose all that apply.
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. 21. Which of the following statements are true statements about lightning rods? Choose all that apply.
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. 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.
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. 23. In diagram IV of Question #22, the pop can acquires the charge that it does because ___.
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.
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. 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.
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. Part B: Diagramming and Analysis28. 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. 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. 30. Construct electric field lines around the following configuration of charges. Include at least six lines per charge. 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). 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.
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. 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. Navigate to:You Might Also Like ...Users of The Review Session are often looking for learning resources that provide them with practice and review opportunities that include built-in feedback and instruction. If that is what you're looking for, then you might also like the following:
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