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15.2 Conductors and insulators

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Conductors and insulators

All atoms are electrically neutral i.e. they have the same amounts of negative and positive charge inside them. By convention, the electrons carry negative charge and the protons carrypositive charge. The basic unit of charge, called the elementary charge, e , is the amount of charge carried by one electron.

All the matter and materials on earth are made up of atoms. Some materials allow electrons to move relatively freelythrough them (e.g. most metals, the human body). These materials are called conductors .

Other materials do not allow the charge carriers, the electrons, to move through them (e.g. plastic, glass).The electrons are bound to the atoms in the material. These materials are called non-conductors or insulators .

If an excess of charge is placed on an insulator, it will stay where it is put and there will be a concentration of charge inthat area of the object. However, if an excess of charge is placed on a conductor, the like charges will repel each otherand spread out over the outside surface of the object. When two conductors are made to touch, the total charge on them is shared between thetwo. If the two conductors are identical, then each conductor will be left with half of the total charge.

Charge and electrons

The basic unit of charge, namely the elementary charge is carried by theelectron (equal to 1.602 × 10 - 19 C!). In a conducting material (e.g. copper), when the atoms bond to form the material, some of the outermost, loosely boundelectrons become detached from the individual atoms and so become free to move around. The charge carried by these electrons canmove around in the material. In insulators, there are very few, if any, free electrons and so the charge cannot move around in thematerial.

Interesting fact

In 1909 Robert Millikan and Harvey Fletcher measured the charge on an electron. This experiment is now known as Millikan's oil drop experiment. Millikan and Fletcher sprayed oil droplets into the space between two charged plates and used what they knew about forces and in particular the electric force to determine the charge on an electron.

I have 2 charged metal conducting spheres which are identical except for having different charge. Sphere A has a charge of -5 nC and sphere B has a charge of -3 nC. I then bring the spheres together so that they touch each other. Afterwards I move the two spheres apart so that they are no longer touching.

  1. What happens to the charge on the two spheres?
  2. What is the final charge on each sphere?

  1. We have two identical negatively charged conducting spheres which are brought together to touch each other and then taken apart again. We need to explain what happens to the charge on each sphere and what the final charge on each sphere is after they are moved apart.

  2. We know that the charge carriers in conductors are free to move around and that charge on a conductor spreads itself out on the surface of the conductor.

    1. When the two conducting spheres are brought together to touch, it is as though they become one single big conductor and the total charge of the two spheres spreads out across the whole surface of the touching spheres. When the spheres are moved apart again, each one is left with half of the total original charge.
    2. Before the spheres touch, the total charge is: -5 nC + (-3) nC = -8 nC. When they touch they share out the -8 nC across their whole surface. When they are removed from each other, each is left with half of the original charge:
      - 8 nC / 2 = - 4 nC
      on each sphere.
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Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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