NEET Physics Notes Electrostatics-Electric charge

NEET Physics Notes Electrostatics-Electric charge

Electric charge

Electric Charge

Electric charge is the property associated with matter due to which it produces and experiences electric and magnetic effects. Electric charge is a scalar and is additive in nature. Total charge of the system is
q=q₁+q₂+q₃ ……..q_n
Like charges repel but unlike charges attract each other.

Conservation of Charge

According to the principle of conservation of charge, we can neither create nor destroy electric charge. The charge can simply be transferred from one body,.to another.

Quantisation of Charge

Electric charge is quantised. The minimum unit of charge, which may reside independently is the electronic charge e having a value of 1.60 x 10^-19 C. Charge on any other body, i.e. Q = ±ne, where n is any integer. SI unit of electric charge is coulomb (C).
Charge is invariant, i.e. charge does not change with change in velocity.

Properties of Charges

•  Like charges repel while opposite charges attract each other.
•  As electron is a fundamental particle which cannot be sub-divided, hence smallest number of electron lost or gained as n = 1 and hence charge on a body, q = ne = 1 x e. Hence, smallest charge in nature is electronic charge. Charge smaller than e does not exist and any amount of charge is an integral multiple of e, q = ne, where n = an integer. Thus, charge is quantised.
•  Charge is invariant, i.e. charge does not change with change in velocity.
  According to theory of relativity, the mass, time and length change with a change in velocity but charge does not change.
•  A charged body attracts a lighter neutral body.
•  Electronic charge is additive, i.e. the total charge on a body is the algebraic sum of all the charges present in different parts of the body. For example, if a body has different charges as +2q, +4 q, -3 q, – q, then the total charge on the body is + 2 q.

Coulomb’s Law of Forces between Two Point Charges

If q₁ and q₂ be two stationary point charges in free space separated by a distance r, then the force of attraction repulsion between them is

The term ε₀ is called the electric permittivity of free space having a value of 8.85 x 10^-12 C²N^-1m^-2. Its dimensional formula is [M^-1L^_3T⁴ A²].
If some dielectric medium is completely filled between the given charges, then the Coulomb’s force between them becomes

Here, ε₀ 

=absolute electric permittivity of the given medium, K dielectric constant and e_r is the relative permittivity of the given medium.

Continuous Charge Distribution

When charge is distributed along a line, straight or curved, then it is called linear charge distribution and its charge per unit length is called linear charge density λ

Its unit is coulomb/metre(Cm ¹).
When charge is distributed over a surface area, then it is called surface charge distribution and charge over its per unit area is called surface charge density σ.

Its unit is coulomb-metre ² (Cm ²).
When charge is distributed over entire volume of a body, then it is called volume charge distribution and charge per unit volume is called volume charge density ρ.

Its unit is coulomb-metre ³(Cm ³).

Superposition Principle

If a number of discrete point charges are present, then the net electric force on any one charge is equal to the vector sum of the electric force due to all other charges on that charge. If charges q₀, q₁, q₂ are present, then

F₀ = F₀₁ + F₀₂ +…+ F_0n

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