Answers to Electrostatics Review questions (these may not be identical to your packet questions)

1.          When two dissimilar objects are rubbed together, one object loses electrons and the other object gains electrons, so one object gets a net positive charge and the other gets a net negative charge.

2.          Charging by conduction involves contact with the source of the charge.  Charging by induction is charging indirectly.  The source and the object never touch when charging by induction.

3.          The field lines are drawn from positive sources and toward negative sources, and show the direction a positive test charge would move if placed in the electric field.

4.          Electric potential is the work required to move a test charge from one point to another.  It is the potential energy per charge.

5.          The potential is high.

6.          Positive

7.          Positive

8.          The force is dependent on the charge of the two objects and the square of the distance between them.

9.          The electrostatic force is stronger.  The gravitational force dominates because most objects are electrically neutral and since the earth’s mass is so large, the gravitational effects become significant.

10.      Friction charging does not involve grounding.  Charging by conduction does not involve grounding.  Charging by induction does not need grounding, but grounding can be used.

11.      The force is quadrupled.

12.      Volts (or Joules / Coulomb).  N/C.  Coulomb.  Joules.

13.      A neutral object can never be repelled by an object.  It can be attracted to a charged object.

14.      Electric field lines never cross because they show the direction of the net force, and there cannot be more than one net force at a single location.

15.      Like charges in the metal leaves cause them to repel each other.

16.      The electric field is zero inside a conductor.  All the charges are on the surface of the conductor.

17.      Halved.

18.      There is friction charging between the rubber belt and a scraper which deposits its charges onto the large globe.  The hair (or bunny fur) is connected to the VanDeGraaf and thus gains extra electrons by conduction.   The  negatively charged hairs repel each other and are light enough so that the electrostatic forces overcome the gravitational forces.  The hair lifts up.

19.      Draw a diagram of the problem.  Like charges repel and unlike charges attract.

20.      Reverse the direction of the electric field lines.

21.      A conductor is something that freely gives up or accepts electrons (charge is redistributed in a conductor).  An insulator is a material that doesn’t allow electrons to move easily.

22.      Grounding is neutralizing an object by sharing its charge with the earth.

23.      They have negative energy.

24.      No.  The hanging object could be charged or neutral.

25.      Electrons are less massive than protons (2000 times smaller).

26.      When the charges are next to each other the potential energy is zero.  After they are moved, the potential difference is positive.

27.      A charged object (source) is brought near a neutral object and the neutral object becomes polarized.  Then, when the source is brought in contact with the object, electrons are transferred.  When the source is taken away from the object the object has a net charge that is the same as the source.

28.      A charged object (source) is brought near a neutral object and the neutral object becomes polarized.  Another neutral object (or ground) is brought in contact with the polarized object and the charges are transferred to the neutral object (or ground).  The neutral object (or ground) is taken away from the other object first.  Then the source is taken away from the object the object has a net charge that is the opposite of the source.

29.   Electric potential energy (PE) is the total amount of energy, in Joules, needed to put the test charge at its location.  The electric potential (V) is the energy per charge, in volts or joules per coulomb; for that you do not need to know how much the test charge is.  

30. Neither potential (V) nor potential energy (PE) are vectors.  Just add them with regular math.

31. Doubles.  Quadruples.

32. Trees with thin bark, like ash and beech, get their outer bark wet through in the rain; if struck by lightning, the lightning will go down the wet bark to the ground.  The water will vaporize from the heat of the electricity, but it does not affect the tree within.  Trees with thick bark, such as oak and hickory, do not get their outer bark wet through.  Thus, if they are struck by lightning, there isn't the easy outer conduit to the ground.  Instead, the electricity enters the tree and takes the next best path--the sap, which heats up, vaporizes, and expands, pushing aside the bark and outer layers of the tree.  This makes those trees explode.

33. Gas trucks used to drag chains to ground the charges that build from friction with the road, but it was determined that the chains created more friction charging than they grounded.  Now gas trucks tend to have pointed sides to dissipate the charges into the air like lightning rods on houses.

34. A bell is struck by lightning and becomes charged.  A nearby neutral ball is polarized and attracted to the charged bell.  It strikes the bell and is charged negatively by conduction.  Then the bell and ball repel one another, pushing the smaller ball away and making it strike a second, grounded bell.  Now neutral, the ball returns to its original spot and will be polarized again and attracted again so long as there is still sufficient charge remaining in the lightning-struck bell.