Investigation: Resistance of Wires
Aim
To investigate the factors that affect the resistance of a wire.
Background knowledge
The conductivity (the opposite of resistivity) of a solid depends on the density of free electrons and how easily they can move through the crystal structure of the solid. Metals have lots of freely moving electrons. Other substances, such as glass, have few. Although all metals are pretty good conductors of electricity, they vary in how many free electrons are available and how easily they may move through the crystal structure of the metal.
In a bit more detail, some factors that affect conductivity and resistivity in solids are:
- Movement of electrons: In a conductor there are free electrons that are moving about randomly in all directions. When an electrical potential is applied there is a tendency for the electrons to move towards the positive charge, so while the motion of the electrons is still random, the average of all the movements is in that direction.
- Electron energy levels: The electrons in a solid are at various energy levels. The energy of an electron may increase, but only by specific amounts, called quanta. An electron may be at one of several levels, but not in between. It’s like a staircase. As you climb up or down you can come to rest on each step, but not in between. When the atoms are close together in a crystal, as they are in a piece of metal or glass, there tend to be many possible energy levels close together. The ranges of allowed energy levels are called bands. The lower energy band is called the valence band. The higher energy band is called the conduction band. For electricity to flow through a solid electrons must move from the valence band, which is where they are usually found, to the conduction band. This happens with some introduction into the solid of energy in the form of heat, light, or an applied electrical field. Once in the conduction band, the electrons may move freely from atom to atom. This movement is electrical current. In metal conduction the band overlaps the valency band so there are lots of free electrons.
