# Coulomb and Electron Theory

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

joined.

Electron Theory

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

charged.

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

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.)

Electrical terminology

Having studied electricity at the atomic level we have met a number

of words, which need to be defined and explained before we move

on.

The laws governing the behaviour of the different units are dealt with

in the relevant section rather than including them in these definitions.

Potential Difference

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.’

Electro-Motive Force

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.