CHARGING AND DISCHARGING IN STATIC ELECTRICITY



As well as lightning flashes, we experience static electricity in a number of ways in everyday life. You may have noticed tiny sparks when taking off clothes made of synthetic fibres. You may have felt a small shock when getting out of a car. An electrostatic charge builds up on the car and then discharge through you when you touch the metal door. You have probably rubbed a ballon on your clothes or hair and seen how it will stick to a wall or ceiling.
        If you rub a plastic ruler with a cloth, both are likely to become electrically charged. You can tell that this is so say holding the ruler and then the cloth close to your hair - they attract the hair. [ If your hair is not attracted, try some tiny scraps of paper instead.] You have observed a static electricity generated by rubbing. You have also observed that a charged object may attract uncharged objects.
   
a.b.
        Now we have to think systematically about how to investigate this phenomena. First, how do two charged objects affect one another? Figure above shows only way of investigating this. A plastic rod is rubbed with a cloth so that become charged. The rod is hung in a cradle so that it is free to move. When the cloth is brought close to it, the rod moves towards the cloth. If a second rod is rubbed at same way and brought close to the first one, the hanging rod moves away. Now we have seen both attraction and repulsion, and suggests that there are two types of static electricity. Both rods have been treated in the same way, so we expect them to have the same type of the electricity. The cloth and the rod must have the different types.
        The two types of static electricity are referred to as positive charge and negative charge. We can explain the experiment shown in the figure by saying that the process of rubbing gives the rod one type of electric charge [say, negative], while the cloth is given the opposite type [say, positive].
The figure shows the two experiment with the charges marked.
        From these experiments, we can also say something about the forces that electric charges exert on each other:

  • like charges repel 
  • unlike charges attract.
['Like charges' means charges that are of same type - both positive, or both negative. 'Unlike charges' means charges of opposite type - one positive and the other negative. People often remember this rules as 'opposite attract'.]

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