# How do excess charges move in an insulator?

I am currently studying intro into electrostatics and reading my notes from teacher that stated, "an insulator holds on tightly to its outer electrons and does not permit the flow of electric charges through them". Now i understand this well, however i saw an example in my physics textbook which showed a negatively charged insulator (glass rod) which had been put into contact with a copper wire which connected the rod to a metal ball (essentially the metal conducting ball and rod were connected by the copper wire), the excess electrons on the rod would then transfer down the copper wire and to the metal ball.

Now given the definition of a insulator not permitting the movement of charge through it, how can charges move from the rod and down the copper wire?. This seems to me to contradict the definition of an insulator since the insulator would be holding tightly on to the excess charges.

• The electrons are not moving through the glass rod when they transfer to the copper wire. They exist on the outer surface. – Mitchell Jul 15 '17 at 5:32
• Oh i see, so if you then had electrons that exist on the surface of the rod that is not in contact with the copper wire , those electrons would remain fixed on the rod and only those electrons on the surface who come into physical contact with the wire are transferred. Is that correct? – MathLearner Jul 15 '17 at 5:52
• Insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule. If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging. The particles of the insulator do not permit the free flow of electrons; subsequently charge is seldom distributed evenly across the surface of an insulator. It should be understood that insulators can be charged. For more info : physicsclassroom.com/class/estatics/Lesson-1/… , I strongly suggest you to read this. – Mitchell Jul 15 '17 at 5:59
• yes i finished reading that physicsclassroom link, however i am just concerned with charged insulators transferring charge to a conductor. As an example say the glass rod had electrons placed at the top of the rod, the middle of the rod and at the botttom of the rod (i.e at the surface of those locations), now if you wrapped a copper wire around the middle of the rod what would happen to the charges at the top surface of the rod and at the bottom surface of the rod? my guess would be they would remain fixed in place but im unsure. – MathLearner Jul 15 '17 at 6:42
• They will stay there. The website I mentioned should clear all your doubts. Read it all, It is worth the time. – Mitchell Jul 15 '17 at 6:46