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Section 32.1 Conductors and Insulators

You already know that metals are good conductors and plastic is a bad conductor. That is how you are protected from current in wires. Any material that can carry electricity readily is a conductor. For instance, ions in an ionic solution carry electricity between two electrodes, which is how electricity flows in a battery. That makes ionic solution a good conductor.

Different conductors carry electricity in a different way. For instance, metals carry electricity through moving electrons but ionic solutions and plasma carry electricity via ions and other charged particles.

One way for distinguishing a conductor from an insulator is to notice what happens when you try to create a potential difference between two points on the object. When you do that to a conductor, charges flow and create a current. Ordinarily, this does not happen in an insulator.

However, if you create enormous potential difference, electrons can flow even in an insulator.

For instance, even though air is an insulator, electrons flow from ground to the positively charged clouds. Enormous potential difference between the ground and clouds creates very high electric field, which is sufficient to knock electrons off molecules. Once free, electrons are accelerated by the electric field. Moving electrons collide with air molecules, which is responsible for release of light we see as lightning.

Figure 32.1.1. Credits: NOAA.

The difference in the electrical behavior of conductors and insulators can be traced to the presence of a large number of freely moving charges in conductors and their absence in insulators. For instance, in copper, which is a good conductor, there is one electron per atom that can freely move in the metal.

On the other hand, in insulators such as a piece of plastic or glass, electrons are tightly bound to molecules, and cannot move macroscopic distances. Electrons in insulators, however, do move within a molecule, e.g. across a covalent bond. This causes molecules in an insulator to be polarized if atoms across a bond have different attraction for electrons, or a polarization can be induced if the material is placed in an external electric field. We shall study the effects of polarization of insulators in the next chapter. In the present chapter we shall concentrate our efforts at understanding conductors.

Subsection 32.1.1 Electric Field inside a Conductor

The only time charges do not flow in a conductor is when there is no potential difference between any two points of the conductor. This means that a conductor will be in an electrostatic equilibrium when it has same potential everywhere. We say that a conductor forms a constant potential body. Since gradient of potential difference is electric field, this implies that inside a conductor in electrostatic equilibrium, electric field is zero.

\begin{equation*} \text{Conductor in Electrostatic Equilibrium: }\ \ \ \ E_\text{in} = 0. \end{equation*}

When you place some charge on a condutor, such as in Figure 32.1.2, initially, charges flow inside the condutor. But, after some time, an electrostatic equilibrium sets in when no more flow occurs. The charges distribute around th conductor in such a manner that electric field by them cancel at all points of the body of the conductor! If the outside surface is a spherical surface, they will distribute evenly. But, if outside surface is not spherical, charges will not spread out evenly; charge density will be higher at places that are pointed.

Figure 32.1.2. Electric charges on a conductor migrate to the outside surface of a conductor no matter where you put them initially. The charges on an insulator stay put in the location you place them.

This observation is used to electrically isolate some space from outside using a Faraday cage. You can isolate a volume electrically by using metal chicken wires. In the Faraday cage to the right, there is anulus of space that is isolated.