Chapter 4 – Forces

Explaining electricity

Electrons in an atom can occupy only certain energy levels. This gives discrete layers that make it difficult for electrons to cross. That looks like this…

When many atoms are brought together, the electron wave functions interact and these levels split into ranges of allowed energies called bands. The available bands starts to look like this:

Depending on the element and arrangement in the material, there may be a gap between bands where no electron states are allowed like this:

For electricity to flow, electrons must be able to move into nearby empty states. That gives three types of materials:

• Conductors: In metals, there is minimal gap so are plenty of nearby states available, so electrons move easily and the material conducts electricity
• Insulators: the lower-energy band is completely full and the next band is separated by a large energy gap, so electrons cannot move easily and the material does not conduct.
• Semiconductors: they have a smaller gap, allowing their conductivity to be controlled, which is what makes transistors possible

Transitors

Transistors can switch between being conductors and insulators. Applying a voltage to a transistor’s base connector creates a path that allows electrons to flow across the gap making it a conductor

Transistors are improved by doping which adds tiny amounts of other atoms to the semiconductor to control how easily electricity flows