Electric Charges And Fields
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1. ELECTRIC CHARGE
Charge is the property associated with matter due to which it produces and experiences electrical and
magnetic effects. The excess or deficiency of electrons in a body is the causes of net charge on a body.
Types of charge :
(i) Positive charge : It is the deficiency of electrons as compared to protons.
(ii) Negative charge : It is the excess of electrons as compared to protons.
SI unit of charge : ampere × second i.e. coulomb; Dimension : [A T]
Practical units of charge are ampere × hour (=3600 C) and faraday (=96500 C)
l Millikan determined quanta of charge and estimated it to be equal to charge of electron.
l 1 C = 3 ´ 109 stat coulomb, 1 absolute - coulomb =10 C, 1 Faraday =96500 C.
Note : Charge of fundamental particles (i.e. electron, proton etc.) is their internal characteristic while charge
on a body depends on the number of protons & electrons inside the body.
1.1 Specific Properties of Charge.
Charge is a scalar quantity : It represents excess or deficiency of electrons.
* Charge is transferable : If a charged body is put in contact with an another body, then charge can be
transferred to another body.
* Charge is always associated with mass
Charge cannot exist without mass though mass can exist without charge.
* So the presence of charge itself is a convincing proof of existence of mass.
* The mass of a body changes after being charged.
* When a body is given a positive charge, its mass decreases.
* When a body is given a negative charge, its mass increases.
Charge is quantised
The quantization of electric charge is the property by virtue of which all free charges are integral multiple of a
basic unit of charge represented by e. Thus charge q of a body is always given by
q = ne n = positive or negative integer
The quantum of charge is the charge that an electron or proton carries.
Note : Charge on a proton = (–) charge on an electron = 1.6 × 10–19 C
Charge is conserved
In an isolated system, total charge does not change with time though individual charge may change, i.e. charge
can neither be created nor destroyed. Conservation of charge is also found to hold good in all types of reactions
either chemical (atomic) or nuclear. No exceptions to the rule have ever been found.
charge is invarient
Charge is independent of frame of reference. i.e. charge on a body does not change whatever be its speed.
Conductors & Insulators
Conductors : Materials in which the outer electrons of each atom or molecule are weakly bound and these electrons
are almost free to move throughout the body of the material are known as conductors.
Insulators : Materials in which all the electrons are tightly bound to their respective atoms or molecules are known
as insulators. In insulators there are very few free electrons. Such materials are also called dielectrics.
Methods of Charging
l Friction
If we rub one body with another body, electrons are transfered from one body to the other.
Electrostatic induction
If a charged body is brought near a neutral body, the charged body will attract opposite charge and repel similar
charge present in the neutral body. As a result of this one side of the neutral body becomes negative while the other
positive, this process is called 'electrostatic induction'. Hence induction is a phenomena of redistribution of charge
on a body when any other charged body is brought near it.
Charging a body by induction (in four successive steps
In case of induction it is worth noting that :
(i) Inducing body neither gains nor loses charge.
(ii) The nature of induced charge is always opposite to that of inducing charge.
(iii) Induced charge can be lesser or equal to inducing charge (but never greater).
(iv) Induction takes place only in bodies (either conducting or non conducting) and not in particles
Conduction
The process of transfer of charge by contact of two conducting bodies is known as conduction.
If a charged conducting body is put in contact with uncharged conducting body, the uncharged body becomes
charged due to transfer of electrons from one body to the other.
The charged body loses some of its charge (which is equal to the charge gained by the uncharged body).
Charge differs from mass in the following aspects :
(i) In SI units, charge is a derived physical quantity while mass is a fundamental quantity.
(ii) Charge is always conserved but mass is not.
(iii) Charge cannot exist without mass but mass can exist without charge.
(iv) Charges are of two type (positive and negative) but mass is of only one type (positive).
(v) For a moving charged body, mass increases while charge remains constant.
* True test of electrification is repulsion and not attraction as attraction can take place between a charged and an
uncharged body or between two similarly charged bodies.
* For a non relativistic (i.e. v << c) charged particle, specific charge
q
m
=constant.
l Charge can be detected and/or measured with the help of gold-leaf electroscope, electrometer, voltameter etc.
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