At some time all of us have done the experiment of "electrifying" our pen by rubbing it on the sleeve of our jersey so that it attracts little pieces of paper. In previous years we have learned that the atoms of a material possess particles with the property we call an electric charge. We know that there are positive charges in the nucleus called protons, and negative charges outside the nucleus called electrons. Could you explain what type of charges have moved when we charge our pen?

The natural unit of electric charge should be the electron. But as it is too small for practical purposes we have adopted another unit called the coulomb, equivalent to the charge of some 6 trillion electrons. We will give the exact definition of this unit in the  topic on electric current, in the section on electrolysis.