In electronics, the idea of Potential Difference (p.d.) is more important than the concept of electrostatic potential, upon which it is based. To understand the meaning
of p.d. consider the situation shown in the diagram.
For the sake of example, we can take a situation where one object has a negative potential,
V1 = -4 Volts
and the other has a positive potential,
V2 = +3 Volts.
The total energy released when an electron escapes the first object and falls on to the second object is
E = e(V1 - V2)
= -1.6×10-19×(-7) = 1.1×10-18 Joules
The electron gained ‘4 volts worth’ of energy escaping from the negatively charged object, and another ‘3 volts worth’ when it fell on to the positively
charged object. In effect it fell through a total change in potential of 7 volts. The total energy gained came from the total potential difference between its
starting and finishing locations.
Now, if the destination object had been negatively charges to, say, -1 volts, the electron would have gained 4 volts worth of energy escaping the first object, but will have then lost 1 volts worth in joining the
second object. So the total energy it gained would have been just 3 volts. Once again, so far as the electron is concerned, the only thing that matters is the difference in voltage
between where it starts and where it ends up. The individual potentials taken separately do not really matter, what matters is the difference between them.
If the place where the electron started was at -110 V, and the place where it ended was at -100 V, it would have dropped through 10 V. If it started at +356 V
and finished at +366 V it would have dropped through 10 V. The energy it would have liberated would have been the same in both cases. However, if it started at
-95 V and moved to -105 V it would have had to overcome a 10 V potential difference. It would therefore have to be given 10 V worth of energy.
When an electron moves to a ‘more positive’ place than where it started some energy is released (given to the electron). To make an electron move to a ‘more negative’ place than where it starts, we have to give the electron some extra energy. This distinguishes movements of electrons (current) which ‘charge’ something (electrons moved to make their destination more negative) from movements which ‘discharge’ something (electrons move away from the more negative places).
We can use the potential difference between two places to provide the energy required to push electrons around, making them do useful things like light a room or produce sound from
a hi-fi. However, the energy stored will quickly be used up and the potentials everywhere will settle to the same value unless we have some way of 'pumping up' some fresh electrons
and maintaining a useful potential difference.
There are many ways to 'pump' electrons from place to place. The most common examples are batteries, used in torches and portable radios, and generators, used in cars and large
power stations. In each case some other energy source is used to move electrons from place to place, maintaining the imbalance and the potential difference which is then used to
drive electronic equipment. The correct name for the thing pumping the electrons from place to place is a Power Supply.
In the example illustrated above, the electron 'drop' from negative to positive through a light bulb. The flow of electrons represents an Electric Circuit through the bulb.
If we use a suitable power supply to return electrons to the negative reservoir as fast as they flow through the bulb, then we maintain a steady potential difference across the bulb.
If we multiply the current (number of electrons per second times their charge) by the potential difference we will get the value of the amount of electric energy per second going into the
bulb. This power (energy divided by time) appears as light and heat. The bulb is an example of an electrical load - something which consumes electric power. Provided the
power supply can maintain a steady voltage, the brightness of the bulb will remain constant.
In order to keep the system working we need a closed loop around which the electrons can move. This is why the term electric Circuit is used. In the case shown above you
can see that the electrons flow around a loop from the power supply (pump) to the negative reservoir, then through the lamp to the positive reservoir, and finally back to the power supply.
If the circuit is broken, then this flow cannot be maintained.
Electronics homepage
Course contents
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)