Charging by conduction
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It should always be remembered that whenever you have to charge a body, it must be insulated from the earth. It means that the body should be supported on a stand which is made of an insulating material which does not allow charges to pass into the earth.
Fix the metal sphere 'Y' to be charged on an insulating stand. Bring the charged rod 'X' with a positive charge in contact with 'Y'. From 'Y' some of the free electrons flow towards the rod 'X' to neutralize the positive charge on it. As a result, there will be a deficiency of electrons on the sphere 'Y' and surplus of positive charges. Thus 'Y' gets positively charged. As charging by conduction depends upon flow of free electrons, this method cannot be used to change an insulator where the electrons are firmly bound.
The sphere could be charged negatively in a similar fashion by bringing a negatively charged rod in contact with it. Charging by conduction produces similar charge.
Charging by Induction
In this method, electric charges are induced without the two bodies coming in contact with each other.
Two metal cans A and B are placed on insulating tiles in such a way that the surfaces of the two touch.
A positively charged body R is now brought near B. Keeping R in position the can A is separated from the can B by moving the tile below A. While moving the can A, care should be taken that it is not touched. Then R is removed completely.
An Alternative method of charging body by induction is the earthing process.
- A positively charged acetate strip is brought near a neutral, insulated conductor. The negative charge is attracted to the positively charged body at the end 'A' and the positive charge is repelled and collects at the end 'B'.
The conductor is then earthed by touching the end 'B' with a finger.
This closed path allows electrons to flow from the earth to neutralize the charge at 'B'. The acetate strip is then removed after the finger has been taken from the conductor.
Now the conductor is left with a negative charge and it will get distributed on the conductor. You will notice that the kind of charge induced on the conductor is opposite to that on the charged strip.
Similarly the figure below shows the various states of charging a metallic sphere positively.
