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TOPIC 13: Charges

Tip 1:

Some students have this WRONG idea: They think that a negatively charged conductor is a conductor that simply contains some electrons; whereas a positively charged conductor is a conductor that contains some protons, or positive charges, or whatever.

Hence they are surprised when they discovered that a positively charged conductor also has electrons in them; and a negatively charged conductor also has protons in them.

What happens is that a conductor (before it is charged), contains billions of protons and electrons. However, the number of protons is equal to the number of electrons, so the positive and negative charges balance out and the conductor is not charged, eg. a metal sphere may contain 1 billion protons and 1 billion electrons, and remain uncharged.

When we add extra electrons to the sphere (eg. we add 3 electrons), it will now contain 1 billion protons and (1 billion & 3) electrons. The 1 billion protons' positive charges will cancel out the 1 billion electrons' negative charges. But the extra 3 electrons' negative charges have no one to cancel. Hence the metal sphere has a net negative charge of 3 electrons, and is considered to be a negatively charged sphere.

But that doesn't mean this negative metal sphere contains no protons (it still has 1 billion protons).

Similarly, if we remove 3 electrons from the sphere, it will now contain 1 billion protons but only 999,999,997 electrons. The 999,999,997 electrons' negative charges will cancel out the positive charges of 999,999,997 protons. But there is an excess of 3 protons whose positive charges are not cancelled out. Hence this sphere now has 3 excess positive charges, and is considered to be a positively charged sphere.

But that doesn't mean this positive metal sphere contains no electrons (it still has 999,999,997 electrons).

Note: Only electrons can move in and out of a conductor. Protons are fixed in their positions and cannot move.