1 How does the mass of an object change when it acquires a positive charge? 1. Increases 2. More information is needed. 3. Decreases 4. Doesn t change 2 Why do clothes often cling together after tumbling in a clothes dryer? 1. gravitational force 2. air pressure 3. electrical force 4. The dryer heat caused some of the fabrics to melt together. 5. The clothes are electrically neutral. 3 The five thousand billion freely moving electrons in a penny repel one an Why don t they fly off the penny? 1. They cause a jam when they try to fly away. 2. The electrons attract each 3. They are attracted to the five thousand billion positively charged protons in the atomic nuclei of atoms in the penny. 4. The shell of the penny prevents the electrons from flying. 5. They don t have enough speed. 4 Object A and object B are initially uncharged and separated by a distance of 2 meters. Suppose 10,000 electrons are removed from object A and placed on object B, creating an electric force between A and B. The electric force is 1. attractive. 2. zero. 3. repulsive. 5 An additional 10,000 electrons are removed from A and placed on B. By what factor does the electric force change? Quest Chapter 32 What happens to the object when it acquires a positive charge? How does that affect the mass of the object? Hmmm Clothes rubbing together under dry conditions?...wait, the spark of knowledge will come Think Is this any different than the electrons flying around a nucleus? What happens to the balanced charges on A and B when this change occurs? What will be the result? forces. Before #4, the charges are balanced. After #4, A is +10k charges and B is -10k charges. Now, the numbers are 10k more. What happens
6 When combing your hair, you scuff electrons from your hair onto the comb. Is your hair then positively or negatively charged? What about the comb? 1. Neither is charged. 2. positively charged; negatively charged 3. negatively charged; positively charged 4. Both are positively charged. 5. Both are negatively charged. 7 Three small spheres carry equal amounts of electric charge. They are equally spaced and lie along the same line. + + What is the direction of the net electric force on each charge due to the other charges? From a balanced (uncharged) condition, electrons are moved. The object losing e - s will have what type of charge and the object gaining e - s will have what type of charge? Draw arrows representing the direction in which the charges would want to move based on their sign. 8 Two charged particles of equal magnitude ( Q and +Q) are fixed at opposite corners of a square that lies in a plane. A test charge +q is placed at a third corner. What is the direction of the force on the test charge due to the two other charges? 9 If you double the charge on one of two charged objects, how does the force between them change? 1. Double 2. Quadruple 3. Halve 4. Does not change 5. Triple 10 How does the magnitude of the electrical force between a pair of charged objects change when the objects are moved twice as far apart? 1. Reduces to one half of original value 2. quadruples 3. Doesn t change 4. Reduces to one quarter of original value 5. doubles Draw arrows representing the direction in which the charges would want to move based on their sign. forces. What would happen to F e if one charge was doubled? forces. What would happen to F e if the radius was doubled?
11 How does the magnitude of the electrical force between change a pair of charged particles when they are brought to half their original distance of separation? 1. Reduces to one quarter of original value 2. Doesn t change 3. quadruples 4. Reduces to one half of original value 5. doubles 12 Two equal charges exert equal forces on each What if one charge has twice the magnitude of the other? 1. The bigger charge will exert a force four times as strong. 2. The bigger charge will exert a force twice as strong. 3. The forces will be equal. 4. The smaller charge will exert a force twice as strong. 5. The smaller charge will exert a force four times as strong. 13 How is Coulomb s law similar to Newton s law of gravitation? How is it different? 1. Both forces proportional to the same constant; electrical forces may be either attractive or repulsive, whereas gravitational forcesare only attractive. 2. Both forces vary inversely as the squareof the separation distance between the two objects; electrical forces are only present onearth, whereas gravitational forces can exist everywhere. 3. Both forces are proportional to the product of the mass of the two objects; electrical forces may be either attractive or repulsive, whereas gravitational forces are only attractive. 4. Both forces vary inversely as the square of the separation distance between the two objects; electrical forces may be either attractive or repulsive, whereas gravitational forces are only attractive. 5. Both forces are proportional to the product of the masses of the two objects; electrical forces are only present on earth, whereas gravitational forces exist everywhere. 6. Both forces are proportional to the same constant; electrical forces are only present on earth, whereas gravitational forces exist everywhere. Same hint as #10 only halved. forces. Does the equation care which charge is bigger? You have to compare the two equations.
14 An electroscope is a simple device consisting of a metal ball that is attached by a conductor to two thin leaves of metal foil protected from air disturbance in a jar. When the ball is touched by a charged body, the leaves that normally hang straight down, spread apart. Why? 1. The charge transfers to the leaves through the glass. Since the leaves have different charges, they are pushed away from each 2. None of these 3. The charge transfers to the two leaves through the glass. Since the leaves have identical charges, they are pushed away from each 4. The charge transfers to the leaves through the metal ball. Since the leaves have different charges, they are pushed away from each 5. The charge transfers to the leaves through the metal ball. Since the leaves have identical charges, they are pushed away from each 15 When one material is rubbed against another, electrons jump readily from one to the Why don t protons do that? 1. Electrons can attract each other while protons repel each 2. Electrons are much heavier than protons. 3. Electrons travel at the speed of light while protons move very slowly. 4. Electrons aremuch lighter than protons. 5. Electrons are easily dislodged from the outer regions of atoms, but protons are held tightly within the nucleus. 16 When a positively charged conductor touches a neutral conductor, the neutral conductor will 1. lose protons. 2. gain electrons. 3. gain protons. 4. stay neutral. 5. lose electrons. Why would the leaves separate? Is there a force acting on them? What is the nature of the force? Remember your atomic structure. Crawford s Couch Potato Theory Things seek the lowest energy level (or concentration) possible.
17 There is an old saying that the lightning never strikes the same place twice. Is this true? 1. Yes 2. No 18 If you are caught outdoors in a thunderstorm, why should you not stand under a tree? 1. The tree has more resistance than the air and thus is likely to be hit by lightning. 2. A tree attracts electrically polarized air molecules that can have a harmful effect on your body. 3. A tree is likely to be hit by lightning because you and the tree now form a polarized system. 4. A tree is likely to be hit by lightning because it provides a path of less resistance between the cloud overhead and the ground. 5. A tree is likely to accumulate an electric charge that can kill you if you touch the tree. 19 Is it necessary for a charged body to actually touch the ball of the electroscope for the leaves to diverge? 1. None of these 2. No; the charged particles will attract or push electrons out of the ball. 3. No; the charged particles will move through the air. 4. Yes; charged particles transfer to the ball only with contact. 5. Yes; particles can t move through the air. 20 Can an object be charged negatively with the help of a positively charged object? 1. Yes, by letting the two objects touch each other 2. Yes, by rubbing the two objects together 3. No; negative charges can only be obtained with other negatively charged objects. 4. Yes, by bringing the positively-charged object near the object to be charged, then discharging the far side 5. Yes, by bringing the positively-charged object near the object to be charged, then discharging the near side Lightning rods Mountain tops Hmmm Why should you not stand next to dynamite when its fuse is lit? What does a tree present to the charged clouds in the sky? Do charges have to ever touch to have electrical forces act on them? What does a positive charge do to negative charges? Do they have to touch? How would you get rid of any positive charge?
21 1) Two uncharged metal balls, X and Y, stand on glass rods and are touching. X Y 2) A third ball, carrying a positive charge, is brought near the first two. + X Y 3) Then the first two balls are separated from each other, + X Y 4) and the third ball is finally removed. X Y When this is all four steps are done, it is found out that 1. Balls Y and X are both positive. 2. Balls Y and X are still uncharged. 3. Balls Y and X are both negative. 4. BallY is positive and ball X is negative. 5. BallY is negative and ball X is positive. 22 1) Two uncharged metal balls, X and Y, each stand on a glass rod and are touching. Y X 2) A third ball carrying a positive charge, is brought near the first two. + Y X 3) While the positions of these balls are fixed, ball X is connected to ground. + Y X 4) Then the ground wire is disconnected. + Y X 5) While X and Y remain in touch, the ball carring the positive charge is removed. Y X 6) Then ball X and Y are separated. Y X After these procedures, the signs of the charge qx on X and qy on Y are 1. qx is positive and qy is negative. 2. qx is negative and qy is negative. 3. qx is negative and qy is positive. 4. qx is neutral and qy is neutral. 5. qx is neutral and qy is negative. 6. qx is neutral and qy is positive. 7. qx is positive and qy is neutral. 8. qx is positive and qy is positive. 9. qx is negative and qy is neutral. What is the electron distribution at first? What happens to the electrons on X and Y when the charged object is brought near? What happens when X and Y are separated? What is the charge distribution on X and Y now? Is this similar to #21? What is different? What does the ground wire do when the charged object is brought near? So, what is positive? What is negative? What happens when the wire is disconnected? So 2, what are the charge signs on X and Y?
23 A neutral ball is suspended by a string. A positively charged insulating rod is placed near the ball, which is observed to be attracted to the rod. This is because 1. there is a rearrangement of the electrons in the ball. 2. the string is not a perfect conductor. 3. the ball becomes negatively charged by induction. 4. the ball becomes positively charged by induction. 5. the number of electrons in the ball is greater than in the rod. 24 If a negatively charged rod is held near an uncharged metal ball, the metal ball 1. becomes negatively charged. 2. becomes polar. 3. is unaffected. 4. Effect cannot be determined. 5. becomes positively charged. 25 What keeps an inflated balloon from falling down if you rub it against your hair and place it against a wall? 1. When you rub the balloon against your hair, it will remove some mass from the balloon and make it lighter. 2. When you rub the balloon against your hair, the balloon may have some oil attached to it, which can be sticky. 3. Rubbing leaves a balloon electrically charged; the charged balloon polarizes the wall. 4. Rubbing distorts the atoms inside the ballon and polarizes it. 5. Rubbing polarizes the air inside of the balloon. Will the electrons be attracted or repelled by the rod? If so, is this a permanent change? In other words, does the ball gain or lose charge? Same hint as #23. What does rubbing do to the balloon? What does placing the balloon against the wall do to the wall?