Theory of Electromagnetics

History of the Electromagnetics

Originally electricity and magnetism were thought of as two separate forces. This view changed, however, with the publication of James Clerk Maxwell's 1873 Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be regulated by one force. There are four main effects resulting from these interactions, all of which have been clearly demonstrated by experiments:

1. Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel.

2. Magnetic poles (or states of polarization at individual points) attract or repel one another in a similar way and always come in pairs: every north pole is yoked to a south pole.

3. An electric current in a wire creates a circular magnetic field around the wire, its direction (clockwise or counter-clockwise) depending on that of the current.

4. A current is induced in a loop of wire when it is moved towards or away from a magnetic field, or a magnet is moved towards or away from it, the direction of current depending on that of the movement.

While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation. As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.

Electrical Energy and Electrical Potential

In order to bring two like charges near each other work must be done.   In order to separate two opposite charges, work must be done.  Remember that whenever work gets done, energy changes form.

Please visit below link for getting concept animation of Electrical Energy and Electrical Potential :

http://regentsprep.org/Regents/physics/phys03/apotdif/charwork.gif

As the monkey does work on the positive charge, he increases the energy of that charge.  The closer he brings it, the more electrical potential energy it has.   When he releases the charge, work gets done on the charge which changes its energy from electrical potential energy to kinetic energy.  Every time he brings the charge back, he does work on the charge.  If he brought the charge closer to the other object, it would have more electrical potential energy.  If he brought 2 or 3 charges instead of one, then he would have had to do more work so he would have created more electrical potential energy.  Electrical potential energy could be measured in Joules just like any other form of energy.

Since the electrical potential energy can change depending on the amount of charge you are moving, it is helpful to describe the electrical potential energy per unit of charge.  This is known as electrical potential. 

As a formula it is written like this: 

        

 The energy per unit of charge is often called voltage so it is symbolized with the capital letter V.   Work or energy can be measured in Joules and charge is measured in Coulombs so the electrical potential can be measured in Joules per Coulomb which has been defined as a volt.