We have seen that a current of electricity ( ) is a flow of electrons but
the electron itself is too small to be of use as the unit of electrical
quantity and therefore a more practical unit consisting of many
millions of electrons has been chosen. It is called the Coulomb (C)
and is 6.28 x 1018 electrons.
Note: This is a Quantity of electricity (Q) not a measure of current,
but it is used to define the unit of electrical current the
AMPERE (A). When a current of one ampere is flowing in a
conductor, 1 coulomb of electrons pass any point in the
conductor every second. In other words the size of an
electrical current is dependent upon the rate of flow of
electrons not a number of electrons.
We can write this in equation form.
Thus1 ampere of current flowing in a conductor for 1 hour is
equivalent to 3600 Coulombs and this is called an amperehour.
Now we have to look at what makes the electrons flow in a conductor
to form an electric current. Consider the diagram in which 2 bodies
with opposite charges on them are fixed in their position and not
If the bodies were free to move they would be attracted to one
another so clearly there is potential mechanical energy between
them. There is also electrical potential energy between them since
we know that if a conductor joins them, electrons will flow from the
negative body to the positive body until the bodies are equally
Therefore the oppositely charged bodies are producing the energy
required to move the electrons, i.e. to produce a current of electricity.
The oppositely charged bodies are said to have a potential difference
(PD) between them and the size of this PD is measured in the unit of
the Volt (V).
Conventional theory, also known as hole theory, states that current
flows from positive to negative. Protons or the lack of electrons (the
holes) move towards the negative. (Current flow direction in hole
theory is the opposite of that in Electron Theory.)
Having studied electricity at the atomic level we have met a number
of words, which need to be defined and explained before we move
The laws governing the behaviour of the different units are dealt with
in the relevant section rather than including them in these definitions.
Is the difference between charge values, which exists at the atomic
level in materials with free electrons.
The unit of potential difference (PD) is the Volt, which is defined as:
‘The difference of potential across a 1 ohm resistor carrying a
current of 1 ampere.’
This is the ability to cause current to flow in a complete circuit.
The unit of EMF is the Volt (V).
Note: It should be noted that both EMF and PD are measured in the
same units, they are, in fact, both differences in charge potential.
However, it is important to realize that an EMF is the force to do work,
i.e. cause current to flow, whereas PD is the volts drop as a result of
the current flow. Another way to look at it is that the EMF is off- load,
the PD is on-load.