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12.1 Conductors and insulators

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Two identical, metal spheres have different charges. Sphere 1 has a charge of - 9,6 × 10 - 18 C . Sphere 2 has 30 excess electrons. If the two spheres are brought into contact and then separated, what charge will each have? How many electrons does this correspond to?


  1. We need to determine what will happen to the charge when the spheres touch. They are metal spheres so we know they will be conductors. This means that the charge is able to move so when they touch it is possible for the charge on each sphere to change. We know that charge will redistribute evenly across the two spheres because of the forces between the charges. We need to know the charge on each sphere, we have been given one.

  2. This problem is similar to the earlier worked example. This time we have to determine the total charge given a certain number of electrons. We know that charge is quantized and that electrons carry the base unit of charge which is - 1,6 × 10 - 19 C . The total charge will therefore be:
    Q 2 = 30 × − 1,6 × 10 − 19 C x = 4,8 × 10 − 18 C

  3. As the spheres are identical in material, size and shape the charge will redistribute across the two spheres so that it is shared evenly. Each sphere will have half of the total charge:
    Q = Q 1 + Q 2 2 x = − 9.6 × 10 − 18 + ( − 4,8 × 10 − 18 ) 2 x = 7,2 × 10 − 18 C
    So each sphere now has:
    7,2 × 10 − 18 C
    of charge.

  4. We know that charge is quantized and that electrons carry the base unit of charge which is - 1,6 × 10 - 19 C .

  5. As each electron carries the same charge the total charge must be made up of a certain number of electrons. To determine how many electrons we divide the total charge by the charge on a single electron:
    N = − 7,2 × 10 − 18 − 1,6 × 10 − 19 x = 45 electrons

Summary

  1. Objects can be positively charged, negatively charged or neutral .
  2. Objects that are neutral have equal numbers of positive and negative charge.
  3. Unlike charges are attracted to each other and like charges are repelled from each other.
  4. Charge is neither created nor destroyed, it can only be transferred.
  5. Charge is measured in coulombs (C).
  6. Conductors allow charge to move through them easily.
  7. Insulators do not allow charge to move through them easily.

The following presentation is a summary of the work covered in this chapter. Note that the last two slides are not needed for exam purposes, but are included for general interest.

End of chapter exercise

  1. What are the two types of charge called?
  2. Provide evidence for the existence of two types of charge.
  3. Fill in the blanks: The electrostatic force between like charges is while the electrostatic force between opposite charges is .
  4. I have two positively charged metal balls placed 2 m apart.
    1. Is the electrostatic force between the balls attractive or repulsive?
    2. If I now move the balls so that they are 1 m apart, what happens to the strength of the electrostatic force between them?

  5. I have 2 charged spheres each hanging from string as shown in the picture below.
    Choose the correct answer from the options below: The spheres will
    1. swing towards each other due to the attractive electrostatic force between them.
    2. swing away from each other due to the attractive electrostatic force between them.
    3. swing towards each other due to the repulsive electrostatic force between them.
    4. swing away from each other due to the repulsive electrostatic force between them.

  6. Describe how objects (insulators) can be charged by contact or rubbing.
  7. You are given a perspex ruler and a piece of cloth.
    1. How would you charge the perspex ruler?
    2. Explain how the ruler becomes charged in terms of charge.
    3. How does the charged ruler attract small pieces of paper?

  8. [IEB 2005/11 HG] An uncharged hollow metal sphere is placed on an insulating stand. A positively charged rod is brought up to touch the hollow metal sphere at P as shown in the diagram below. It is then moved away from the sphere.
    Where is the excess charge distributed on the sphere after the rod has been removed?
    1. It is still located at point P where the rod touched the sphere.
    2. It is evenly distributed over the outer surface of the hollow sphere.
    3. It is evenly distributed over the outer and inner surfaces of the hollow sphere.
    4. No charge remains on the hollow sphere.

  9. What is the process called where molecules in an uncharged object are caused to align in a particular direction due to an external charge?
  10. Explain how an uncharged object can be attracted to a charged object. You should use diagrams to illustrate your answer.
  11. Explain how a stream of water can be attracted to a charged rod.
  12. An object has an excess charge of − 8,6 × 10 − 18 C . How many excess electrons does it have?
  13. An object has an excess of 235 electrons. What is the charge on the object?
  14. An object has an excess of 235 protons. What is the charge on the object?
  15. Two identical, metal spheres have different charges. Sphere 1 has a charge of − 4,8 × 10 − 18 C . Sphere 2 has 60 excess electrons. If the two spheres are brought into contact and then separated, what charge will each have? How many electrons does this correspond to?
  16. Two identical, insulated spheres have different charges. Sphere 1 has a charge of − 96 × 10 − 18 C . Sphere 2 has 60 excess electrons. If the two spheres are brought into contact and then separated, what charge will each have?
  17. Two identical, metal spheres have different charges. Sphere 1 has a charge of − 4,8 × 10 − 18 C . Sphere 2 has 30 excess protons. If the two spheres are brought into contact and then separated, what charge will each have? How many electrons or protons does this correspond to?
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Source:  OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7
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