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The electrons of different types of atoms have different degrees of freedom to move around. With some types of materials, such as metals, the outermost electrons in the atoms are so loosely bound that they chaotically move in the space between the atoms of that material by nothing more than the in°uence of room-temperature heat energy. Because these virtually unbound electrons are free to leave their respective atoms and °oat around in the space between adjacent atoms, they are often called free electrons.

In other types of materials such as glass, the atoms' electrons have very little freedom to move around. While external forces such as physical rubbing can force some of these electrons to leave their respective atoms and transfer to the atoms of another material, they do not move between atoms within that material very easily.

This relative mobility of electrons within a material is known as electric conductivity. Conductivity is determined by the types of atoms in a material (the number of protons in each atom's nucleus, determining its chemical identity) and how the atoms are linked together with one another.

Materials with high electron mobility (many free electrons) are called conductors, while materials with low electron mobility (few or no free electrons) are called insulators. Here are a few common examples of conductors and insulators:

 Conductors:

  • silver
  • copper
  • gold
  • aluminum
  • iron
  • steel
  • brass
  • bronze
  • mercury
  • graphite
  • dirty water
  • concrete

 Insulators:

  • glass
  • rubber
  • oil
  • asphalt
  • berglass
  • porcelain
  • ceramic
  • quartz
  • (dry) cotton
  • (dry) paper
  • (dry) wood
  • plastic
  • air
  • diamond
  • pure water

It must be understood that not all conductive materials have the same level of conductivity, and not all insulators are equally resistant to electron motion. Electrical conductivity is analogous to the transparency of certain materials to light: materials that easily "conduct" light are called "transparent," while those that don't are called "opaque." However, not all transparent materials are equally conductive to light. Window glass is better than most plastics, and certainly better than "clear" ¯berglass. So it is with electrical conductors, some being better than others. For instance, silver is the best conductor in the "conductors" list, ordering easier passage for electrons than any other material cited. Dirty water and concrete are also listed as conductors, but these materials are substantially less conductive than any metal.

Physical dimension also impacts conductivity. For instance, if we take two strips of the same conductive material { one thin and the other thick { the thick strip will prove to be a better conductor than the thin for the same length. If we take another pair of strips { this time both with the same thickness but one shorter than the other { the shorter one will offer easier passage to electrons than the long one. This is analogous to water °ow in a pipe: a fat pipe offers easier passage than a skinny pipe, and a short pipe is easier for water to move through than a long pipe, all other dimensions being equal.

It should also be understood that some materials experience changes in their electrical properties under di®erent conditions. Glass, for instance, is a very good insulator at room temperature, but becomes a conductor when heated to a very high temperature. Gases such as air, normally insulating materials, also become conductive if heated to very high temperatures. Most metals become poorer conductors when heated, and better conductors when cooled. Many conductive materials become perfectly conductive (this is called superconductivity) at extremely low temperatures.

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