Electric Current
Normally, the free electrons in
a conductor are moving in random directions. If an appropriate electrical force
(called an electromotive force or EMF) is applied to the conductor, the free
electrons can be induced to move (drift) generally in one direction. This
movement of electrons is called current electricity and an electric current is
said to flow.
The rate at which the electrons
appear to drift through the conductor is called the drift velocity. The number
of electrons per second appearing to move past any point of the conductor gives
a measure of the electric current. Increasing the magnitude of the EMF applied
to the conductor will increase the drift velocity of the electrons in the
conductor. This increase in drift velocity would manifest itself as an increase
in the electric current passing through the conductor.
The source of EMF can be a
battery or a generator or a photoelectric cell. For an electric current to flow
through a conductor, the EMF source must apply an electric charge to one end of
the conductor and an opposite electric charge to the other end. A simple example
of this is the electric current flowing in a metal wire (the conductor)
connected between the "negative" terminal and the "positive"
terminal of a battery (the EMF source).
All materials offer some
resistance to the flow of electrons and hence work has to be done in forcing the
electrons through the material. Materials with low resistance are the
"conductors", the "insulators" having high resistance. The
degree of resistance ranges from almost zero (for special materials called
"super conductors") to very high (for the materials used to insulate power lines).
When an electric current flows
through a conductor, two effects are produced:
- the electrical energy used
to overcome the electrical resistance in the conductor is converted to
thermal energy which increases the temperature of the conductor.
Examples:
- heaters, stoves and
electric kettles use the heating effects (conversion of electrical
energy to thermal (heat) energy); and
- incandescent light bulbs
emit light because their elements are raised to a high temperature
(electrical to thermal energy conversion).
- a magnetic field forms
around the conductor.
for example:
- When a current carrying
conductor is placed in a magnetic field, the interaction between its
magnetic field and the other magnetic field exerts a force on the
conductor. In an electric motor, this interaction forces the shaft to
rotate (conversion of electrical energy to mechanical energy).
Most of the ways in which
electricity is used can be traced back to these two effects.
Electrical energy can therefore
be easily converted to other forms of energy.
Conversely, most of the
electricity in a large electricity supply system is generated by the use of
magnetic fields in machines called, appropriately enough, "generators"
(which can be thought of as being electrical motors driven backwards). Other
forms of energy are used to produce the mechanical energy used to rotate the
shafts of the generators. There are other ways in which electricity can be
generated, but they all involve the conversion of a source of energy into
electrical energy.
Electrical energy can therefore
be easily produced by the conversion of other forms of energy.
A law of physics formulated by
Isaac Newton notes that 'Energy cannot be created or destroyed but can be
transferred from one form into another'. The usefulness of electricity therefore
lies in its unique ability to be a convenient and easily controlled means to
transport energy from one location to another location and to convert energy
from one form into another form.
|