NEET Important Chapter - Electric Charges and Fields

Sl.No

Name of the Concept

Key Points

1.

Electric Charges and properties

• An invariant property which is related to matter producing and experiencing electric and magnetic effects.

• Charges are of two types :

• Positive charge

• Negative charge

2. 

Properties of Electric Charges

• Charge is a scalar quantity with value e = $1.6\times 10^{-19}$ and SI Unit = Coulomb.

• Opposite charges attract one other whereas similar charges repel against each other.

• Charge is conserved i.e. the charge can neither be created nor be destroyed but it can be transferred from one body to the other.

• Quantisation of charge -  This property means that any body having charge existing on it will have an integral multiple of charge.

q = ne where n is an integer.

• Charges are additive in nature which means charges follow algebraic law of addition.

3.

Conductors and Insulators

• Those materials which allow electricity to flow through them are known as conductors. Metals are very good conductors of electricity as they have free electrons associated with them which can move freely through the conductor.

• Insulators :- The materials which do not allow electric charge to flow through them are called insulators.

• For example Non metals, mica, wax, plastics, rubber all fall into the category of insulators.

4.

Methods of Charging

• Mainly there are three methods of charging.

• By friction : If we rub two suitable bodies such that one will be charged by +ve and another by –ve charge in equal amounts.

• By conduction : When a neutral body is brought in contact with a charged body then that process is called charging by conduction.

• By induction : If a neutral body without bringing it in contact with a charged body gets charged then that’s called charging by induction.

5.

Coulomb's Law

• Coulomb’s law states :-

• “The force of attraction or repulsion between any two point charges (q1 and q2) separated at a finite distance “r” is directly proportional to the product of charges and inversely proportional to the square of distance between the charges. The direction of force is along the line joining the two charges.

• $F = \dfrac{1}{4\pi \epsilon_{\circ}}(\dfrac{q_{1}q_{2}}{r^{2}})$

• where $\epsilon_{\circ}$ is the permittivity of free space.

6.

Dielectric

• A dielectric is an insulator which can be made a conductor when some kind of electric field is applied.

• It is given by the formula K = $\dfrac{\epsilon}{\epsilon_{\circ}}$

8.

Electric Field

• Electric Field is considered as the space around an electric charge, where it exerts a force on another charge. 

• An electric charge produces an electric field around it so that it interacts with any other charges present there.

• Unit of Electric Field is N/coulomb and is directed from positive charge to negative charge.

9.

Electric Lines Of Force

• These are imaginary lines of force around an electric region such that  the tangent at any point on the lines of force gives the direction of the electric field at that point.

10.

Properties of Electric Lines of Force

• The lines of force diverge from a positive charge and converge at a negative charge. 

• Two electric lines of force can never intersect each other as the intersection will give two different directions of force around a particular point which is not possible.

• The Electric field lines don’t make any loop like magnetic field lines.

• The strength of the electric field can be determined from the electric lines of forces.

• The close the field lines are to each other, the stronger the force is.

11.

Electric Flux

• Electric flux is a measurement of the number of electric field lines passing through the closed surface and that closed surface is called Gaussian surface.

•  If the surface is a closed one enclosing some net charge, then the net number of lines going through the surface is proportional to net charge within the surface.

• $\phi$= $\vec{E}.\vec{S}$

• The SI unit of Electric Flux is Nm2/C.

12.

Gauss’s Law

• Gauss’s Law states the net electric flux through a closed surface in vacuum is equal to 1/εo times the net charge enclosed within the surface.

• $\phi$ = $\vec{E}.\vec{S}$ = $\dfrac{q}{\varepsilon_{\circ}}$.

13. 

Electric Dipole

• An Electric Dipole is defined as a pair of equal and opposite charges separated by a distance 2a.

• The direction of the electric dipole is from negative potential to higher potential.

14. 

Physical Significance of Electric Dipole

• Dipole moment helps to identify whether the given molecule is Polar or Nonpolar.

• If the centres of positive and negative charges coincide or lie at the same place then they are said to be Nonpolar molecules with zero dipole moment.

• If the dipole moment of some molecules in non zero even in the absence of electric field and centres of positive and negative charges don’t coincide then they are called Polar Molecules.

Sl. No

Name of the Concept

Formulae

1. 

Coulomb's Law

$F = \dfrac{1}{4\pi \epsilon_{\circ}}(\dfrac{q_{1}q_{2}}{r^{2}})$

where $\epsilon_{\circ}$ is the permittivity of free space.

2.

Dielectric

K = $\dfrac{\epsilon}{\epsilon_{\circ}}$

3.

Vector form of Coulomb’s Law

$F_{12}$ = $\dfrac{1}{4\pi \epsilon_{\circ}}(\dfrac{q_{1}q_{2}}{r^{3}})\overrightarrow{r_{12}}$

4.

Superposition Principle For Discrete Charge Distribution: Force Between Multiple Charges.

$F_{12}$ = $\displaystyle\sum\limits_{i=0}^n \dfrac{1}{4\pi \epsilon_{\circ}}(\dfrac{q_{1}q_{2}}{r^{3}})\overrightarrow{r_{12}}$

5.

Linear Charge Distribution

$\lambda = \dfrac{Q}{l}$

Units of linear charge distribution = $\dfrac{C}{l}$

6.

Surface Charge Distribution

$\sigma = \dfrac{Q}{A}$

Units of surface charge distribution = $\dfrac{C}{m^{2}}$

7.

Volume Charge Distribution

$\rho = \dfrac{Q}{V}$

Units of volume charge distribution = $\dfrac{C}{m^{3}}$

8.

Electric Field

$\overrightarrow{E}= \dfrac{\overrightarrow{F}}{q}$

9.

Electric Field for Continuous Charge Distribution

$\overrightarrow{E} = \int\dfrac{1}{4\pi \epsilon_{\circ}}\dfrac{dq}{r^{3}}\overrightarrow{r}$

10.

A circular ring of radius R with uniformly distributed charge

E= $kQ\dfrac{x}{\sqrt{(x^{2}+R^{2})^{3/2}}}$

11.

A circular disc of radius R with uniformly distributed charge with surface charge density σ

E= $\dfrac{\sigma}{2\varepsilon_{\circ}}(1-\dfrac{x}{\sqrt{x^{2}+R^{2}}})$

12.

An infinite sheet of uniformly distributed charges with surface charge density σ

E= $\dfrac{\sigma}{2\varepsilon_{\circ}}$

13.

Due to a spherical shell of uniformly distributed charges with surface charge density σ

$E_{surface}$= $\dfrac{\sigma}{2\varepsilon_{\circ}}$  at (x=R)

$E_{out}$= $\dfrac{\sigma}{2\varepsilon_{\circ}}$

$E_{in}$ = 0 (x<R)

14.

Due to a solid non conducting sphere of uniformly distributed charges with charge density ρ

$E_{centre}$= 0

$E_{in}$= K$\dfrac{Qx}{R^3}$

$E_{surface}$= K$\dfrac{Q}{R^2}$

$E_{out}$= K$\dfrac{Q}{x^2}$

15.

Electric Flux

$\phi$= $\vec{E}.\vec{S}$

16.

Gauss’s Law

$\phi$=$\vec{E}.\vec{S}$= $\dfrac{q}{\varepsilon_{\circ}}$

17.

Electric Dipole

$p = q\times 2a$

18.

Electric Torque

$\tau= \vec{p}\times \vec{E}$

19.

Work done in rotating the Electric Dipole

$W= = \int_{\theta_{1}}^{\theta_{2}}pE\sin\theta d\theta$