Problem
State whether the following statements are true or false, justifying your answer: (a) A neutral body contains the same number of protons and electrons. (b) Rubbing a body with a cloth creates new charges out of nothing. (c) In a conductor, the conduction electrons are free to move throughout the volume of the material. (d) Two charges of the same sign attract each other if they are close enough.
Solution
(a) True. A body is neutral when its total charge is zero: the number of protons (charge ) equals the number of electrons (charge ), so the charges cancel exactly.
(b) False. Electric charge is conserved: rubbing does not create charges out of nothing, it transfers electrons from one body to the other. One body becomes positively charged because it loses electrons, the other negatively charged because it gains them; the total charge of the system remains unchanged.
(c) True. This is precisely the definition of a conductor: the conduction (valence) electrons are not bound to individual atoms and can move freely throughout the volume of the material, making the flow of current possible.
(d) False. The Coulomb force between charges of the same sign is always repulsive, at any distance. Only charges of opposite sign attract each other.
Links
Topics: Elettrostatica Concepts: Carica elettrica · Conduttori in equilibrio elettrostatico