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Electric Current

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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:

  1. 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).
  2. 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.

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Last modified: April 17, 2010