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Electrostatics: The branch of physics which deals with the charges at rest and the resulting phenomenon. [Charge being microscopic particles can be at rest only at zero kelvin. We assume charge to be at rest in any body if they have speed but velocity is zero]

Electrodynamics; The branch of physics dealing the charges in motion is called electrodynamics.

Electric Charge:

Charge on a body or particle is the property due to which it produces and experiences electrical and magnetic effects. Some of the naturally charges particles are electron, proton etc. But a naturally uncharged body can also acquire charge through different processes.

Types Of Charge:

Charge can be of two types:

Positive Charge: it implies deficiency of electrons in the body

Negative Charge: it is excess of electrons in the body

Positive and negative are the ‘names’ given by Benjamin Franklin and has no mathematical significance.

Properties Of charge:

[a] Additive Nature of Charge: Charge is a scalar quantity and total charge on a system of bodies or charges is the vector sum of charge on individual bodies/particles.

[b] Charge Is relativistically invariant: Relativistically invariant implies that charge on the body doesn’t vary with the speed of charge particle even if the speed of charge particle is comparable to the speed of light. As mass increases with the increase in speed specific charge [i.e. q/m] decreases with increase in speed of the charge particle.

[c] Charge is conserved: In an isolated system, the total charge always remain constant. i.e. charge can be transferred from one body to another but sum of positive and negative charges in isolated system always remain constant.

[d] Charge is quantized: Quantization implies charge on any body is always integral multiple of certain minimum value ‘e’ i.e. Q= ±ne where e= 1.6×10-19C. Quantization implies that n’ can never be a rational number.

[e] Association of mass with charge: charge is always associated with mass which implies that charges body can never be massless whereas converse is not true as mass can exist without net charge on the body.

[f]Force between like charges is repulsive and force between unlike charges is attractive. But repulsion is sure test of electrification of both the bodies.

[g]Charge at rest produces electric field only, the charge in uniform motion produces both the electric and magnetic field, whereas accelerated charge produces radiating electric and magnetic field i.e. em waves]

Charging Of bodies

An object can be charged by adding or removing electrons from the body. If body is initially neutral than adding electron will make it negatively charged and removing electrons make it positively charged.

Charging of Conductors:

The methods for charging conductors are

  1. Frictional electricity or rubbing a conductor with an insulating body
  2. Charging by conduction and induction
  3. Thermionic Emission And Photoelectric Emission

Charging Of insulators:

As movement of charges is not possible inside insulators therefore insulators can’t be charged wither by conduction or induction. When insulator is rubbed with another insulator or conductor work is done against the frictional force. This work is used to transfer electrons from one body to another , the body which gains electrons becoming negatively charged and other body gets an equal positive charge.

Change in mass on electrification:

Whichever method is used to charge a body it always involves transfer of electron between two bodies. Whenever a body is negatively charged it implies the body has gained electrons therefore mass of the body will increase and whenever body is positively charged the mass of body decreases as the body has lost electrons . For every 1C of charge, the change in mass of the body is 5.68×10-12kg.

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