15.2 Conductors and insulators  (Page 3/3)

Some materials are made up of molecules which are already polarised. These are molecules which havea more positive and a more negative side but are still neutral overall. Just as a polarised polystyrene ball can be attracted to a charged rod, these materialsare also affected if brought close to a charged object.

Water is an example of a substance which is made of polarised molecules. If a positively charged rod is brought close to a stream of water, the molecules can rotateso that the negative sides all line up towards the rod. The stream of water will then be attracted to the rod since opposite charges attract.

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.