2015 Oct

Electricity and Magnetism

October 29, 2015

Physics of Electrostatics

We have acknowledged the presence of charges since antiquity. We knew then that when cat fur was rubbed on an amber it produced one kind of charge and rubbing silk on glass rod produced another kind of charge which was opposite in polarity. The latter was named “positive” charge and the former “negative” charge. Just like how we thought about heat as the flow of a fluid named “caloric”, electricity was also thought about as a the flow of a fluid called “aether”.

It was in the late eighteenth century Coulomb for the first time using his torsional pendulum made a quantitative measurement of the force between two charges and established the famous inverse square relation where, the force between two charges is directly proportional to the product of the two charges and is inversely proportional to the square of the distance between them. The direction depended on the nature of the charges. For example, like charges attracted and unlike charges repelled. From coulombs law and from the introduction of the idea of force fields, we were able to understand the force due to static charges on complex objects too.

At this point came the Gauss’s law which almost completely guided us towards understanding static charges in any formation. Through experiments we found that the force on a charge in the presence of an external electric field caused the charges to move. In some materials the charges could move more freely than in others. This was the birth of electricity. The materials in which charges could move more freely were called conductors and the materials in which the motion of charges was inhibited were called the insulators. Experiments showed the relation between the material and the amount of energy per unit charge needed to move a charged particle in the material- the Electric Voltage. In the later years electrostatics and electricity was completely understood, and that’s what we are studying in our A Level Physics tuition classes!

Just like in the case of electricity, from centuries ago we knew that lodestone would attract iron and that the magnetism was already being put to practical use by the Chinese in the form of a magnetic compass. Experiments performed in the early nineteenth century threw light on the force due to a magnetic field on freely moving charges and further on current carrying conductors. At this point we found a very significant phenomenon that would bring about a technological revolution in the power sector (which shall be discussed in the article “Introduction to electromagnetism”) that when a closed loop is placed in the presence of a uniform magnetic field rotates due to the coupled force or torque on the loop.

It was during the same period that a series of experiments conducted by Biot and Savart showed that electric current or flowing charges created tiny magnetic fields around them. This is the third of the famous inverse square laws after the Newton’s law of attraction and the Coulombs law. The magnetic field generated due flowing charges or electric current was directly proportional to the magnitude of the current and inversely proportional to the square of the distance from the wire. The direction of the magnetic field was established to be always perpendicular to the direction of the electric current. Therefore if the electric conductor were to be bent in the form of a loop, the magnetic field due to each of the current carrying elements would be in the same direction and hence what was earlier a weak magnetic field can now be a strong one. Now, if multiple loops were to be added to the single loops and current passed through them, the magnetic field would still be in the same direction but would be much stronger depending on the number of loops in what can be called the coil. This is the birth of Solenoid and Toroid whose range of applications are fascinating. They are used in electromagnets, printers, television sets, hydraulic machinery, automobiles including several other areas.

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