Electrostatics and electronics are scientifically a bit like the United Kingdom and the United States. At first sight they seem to share a common language, but once
you start looking at them in detail you realise they are using the same words to mean different things!
The idea of electric potential comes from electrostatics. We can understand what it means by considering what happens to an electron which is free to move
around near to an object which contains a mixture of protons and electrons.
Usually, a material is electrically neutral. This means that it contains the same number of electrons and protons, with these spread fairly uniformly
throughout the material. (If we look closely enough we will discover that the protons are bunched together in atomic nuclei and the electrons are whizzing around
them. However, once we are more than a tiny distance away this detail has no effect.) A ‘free’ electron is attracted by each of the protons in the piece of material.
It is also repelled by each electron inside the material. Since the number of electrons and protons are the same, and they are distributed in much the same way,
these two forces cancel. As a result, a free electron does not really notice the presence of the neutral lump of material.
We can charge up an object by adding or removing electrons. The object is then said to have an electrostatic potential. The units of this are Volts. A
positive charge corresponds to having some electrons missing, whereas a negative charge means the object has extra electrons.
The attracting and repelling forces on a free electron near a charged object don't cancel out, so the electron will be pulled toward a
positively charged object or repelled from a negatively charged one.
If we release a free electron some way away from a positively charged object it will be pulled toward the object. As a result, it accelerates toward the object,
gaining velocity - and kinetic energy - as it approaches. When it hits the object the moving electron will have picked up an energy equal to eV, where e is the
electron's charge (in Coulombs) and V is the object's potential (in Volts).
Since this energy is released, does this means that by smashing electrons into objects we could continually get energy and so have perpetual motion?
This relationship between the energy gained and the potential gives us a way to define and measure the electrostatic potential of an object. We can 'drop' an electron on to the object and
measure the energy released. Dividing this energy by the electron's charge gives us the value of the object's potential.
The explanation given above tells us what we need to know to understand how potentials appear, and how they are used in the situations described by electrostatics. In electronic engineering
we use the related idea of potential differences to explain the movement of electrons.
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Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)