# NEET Important Chapter - Electric Charges and Fields

Get plus subscription and access unlimited live and recorded courses

## Electric Charges and Fields - An Important Concept of Physics

The chapter Electric Charges and Fields notes down some of the most important concepts of electric charges and fields for NEET, JEE, and other competitive exams. In the Electric Charges and Fields chapter, students will study the basic properties of electric charge, conductors, and insulators, and some laws related to them.

The chapter on Electric Charges and Fields begins with the very basic definition of Coulomb’s law, Electrostatic Forces, Dielectric, Electric Field, and what are different charge distributions helpful in studying the chapter ahead. These concepts will lead us to a very important part of the chapter, i.eVector notations of Electric Field, Electric Lines of Forces, Energy Density, and Electric Dipole.

The concepts mentioned above will help us in studying the chapter deeply along with the hands-on practice of many important numerical problems that will help us in acing the exams.

In this article, we will cover the important concepts and topics with the Electric Charges and Fields formulas as well as Electric Charges and Fields solutions for numericals that will help students to revise and boost their preparations for NEET and JEE.

### Important Topics of Thermal Physics

• Electric Charges and properties

• Coulomb's Law

• Electric Field

• Conductors and Insulators

• Dielectric

• Electric Lines of Forces

• Electric Flux

• Electric Dipole

• Gauss's Law

### Important Concepts of Thermal Physics

 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 chargeNegative 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.

### List of Important Formulae of Electric Charges and Fields

 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

### Solved Examples of Electric Charges and Fields

1. An electric dipole with a dipole moment $4 \times 10^{-9}$ C/m aligned at $30^{\circ}$ with the direction of a uniform electric field of magnitude $5 \times 10^{4}$N/C. Calculate the magnitude of torque acting on the dipole.

Sol:

Given,

Dipole moment p = $4 \times 10^{-9}$ C/m

E = $5 \times 10^{4}$N/C and angle = $30^{\circ}$

Torque = $\tau= \vec{p}\times \vec{E}$ = pEsin$\theta$

=$4 \times 10^{-9}$ $5 \times 10^{4}$ sin$30^{\circ}$

=$\times 10^{-4}$Nm is the correct answer.

Key point: Here, the formula of torque due to an electric dipole can be used directly.

2.Consider a uniform electric field E = $3 \times 10^{3}\hat{i}$N/C.

(a) What is the flux of this field through a square of 10 cm on a side whose plane is parallel to the yz plane(b) What is the flux through the same square if the normal to its plane makes a 60° angle with the x-axis?

Sol:

Given,

(a) E = $3 \times 10^{3}\hat{i}$N/C, side of square = 10cm = 0.1m

Area of square = 0.01m2.

Since the plane of square is parallel to the yz plane and the area vector is also in this plane so the angle between E and area vector is zero.

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

= $\phi$= $\phi$= $\vec{E}\vec{S}\cos\theta$

= $3 \times 10^{3} \times 0.01 \cos 0^{\circ}$

=30 Nm2/C.

(b) $\phi$= $\vec{E}.\vec{S}$

= $\phi$= $\phi$= $\vec{E}\vec{S}\cos\theta$

=$3 \times 10^{3}\times 0.01 \cos 60^{\circ}$

= 15 Nm2/C

Key point: Here, the formula of electric flux  can be used directly.

### Previous Year Questions of Electric Charges and Fields

1. Two parallel infinite line charges with linear charge densities +$\lambda$ C/m and -$\lambda$ C/m are placed at a distance of 2R in free space. What is an electric charge field midway between the two line charges? (NEET 2019)

1. Zero

2. $\dfrac{2 \lambda}{\pi \varepsilon_{\circ}R}$ N/C

3. $\dfrac{\lambda}{\pi \varepsilon_{\circ}R}$ N/C

4. $\dfrac{\lambda}{2\pi \varepsilon_{\circ}R}$ N/C

Sol:

Given,

As one can clearly see the linear charge distribution of parallel plates and the direction of $\vec{E}$ by both the plates on each other so the electric field due to plate 1 having linear charge density -$\lambda$ is

$\vec{E_{1}}$ = $\dfrac{\lambda}{2\pi \varepsilon_{\circ}R}\hat{i}$ N/C

Similarly the electric field due to plate 2 having linear charge density +$\lambda$ is

$\vec{E_{2}}$ = $\dfrac{\lambda}{2\pi \varepsilon_{\circ}R}\hat{i}$ N/C

As the Electric Field follows the vector superposition principle so the net $\vec{E}$ = $\vec{E_{1}}$ +$\vec{E_{2}}$

= $\dfrac{\lambda}{2\pi \varepsilon_{\circ}R}\hat{i}$ + $\dfrac{\lambda}{2\pi \varepsilon_{\circ}R}\hat{i}$

=$\dfrac{\lambda}{\pi \varepsilon_{\circ}R}\hat{i}$

Which is option (c)

Trick: The relationship between the Electric Field having $\lambda$ as linear charge density is  $\vec{E}$ = $\dfrac{\lambda}{2\pi \varepsilon_{\circ}R}$.

2. Two point charges A and B, having charges Q and -Q respectively, are placed at a certain distance apart and the force acting between them is F. If 25% charge of A is transferred to B, then the force between charges becomes: (NEET 2019)

1. F

2. 9F/16

3. 16F/9

4. 4F/3

Sol:

Given,

We have been provided two point charges A and B having charges +Q and -Q separated by distance r having force F between them.

Hence F = $\dfrac{KQ^{2}}{r^{2}}$

Now 25% of the charge is transferred from A to B then the charge left on A
$Q_A^\bracevert$ = Q- $\dfrac{Q}{4}$ = $\dfrac{3Q}{4}$

Similarly the charge on B now is

$Q_B^\bracevert$ = - Q+ $\dfrac{Q}{4}$ = $\dfrac{-3Q}{4}$

So the new forces due to $Q_A^\bracevert$ and $Q_B^\bracevert$

$F^{\bracevert}$ = $\dfrac{\dfrac{(3Q)}{4}^{2}}{r^{2}}$

$F^{\bracevert}$  = 9/16 $\dfrac{KQ^{2}}{r^{2}}$

Which is option (b)

Trick: Here, the formula of electrostatic force between two charge particles can be  used directly.

### Practice Questions

1. A Conducting sphere of radius 10cm has an unknown Charge. If the Electric Field at a Point 20cm from the center of the sphere of magnitude 1.5×103 N/C is directed radially inwards, what is the net charge on the sphere?

(Ans: 6.67nC)

2. Four charges qA = 2 microcoulomb, qB = -5 microcoulomb,qC = 2 microcoulomb,qD = -5 microcoulomb  are located at the corners of a square ABCD of side 10cm. What is the force on a charge of 1micro microcoulomb placed at the centre of a square?

(Ans: zero)

### Conclusion

We conclude that the chapter electric charges and fields has many important concepts. In this article, we covered Electric Flux, Gauss’s Law and all other important concepts from the chapter along with solved and previous year questions. Students can test their knowledge with the help of practice questions.

See More

## NEET Important Dates

View All Dates
NEET 2022 exam date and revised schedule have been announced by the NTA. NEET 2022 will now be conducted on 17-July-2022, and the exam registration closes on 20-May-2022. You can check the complete schedule on our site. Furthermore, you can check NEET 2022 dates for application, admit card, exam, answer key, result, counselling, etc along with other relevant information.
See More
View All Dates

## NEET Information

Application Form
Eligibility Criteria
Reservation Policy
NTA has announced the NEET 2022 application form release date on the official website https://neet.nta.nic.in/. NEET 2022 Application Form is available on the official website for online registration. Besides NEET 2022 application form release date, learn about the application process, steps to fill the form, how to submit, exam date sheet etc online. Check our website for more details.

## NEET 2022 Study Material

View NEET Syllabus in Detail
View NEET Syllabus in Detail

## NEET 2022 Study Material

View all study material for NEET
All
Physics
Chemistry
Biology
See All

## NEET Question Papers

View all NEET Important Books
Biology
NCERT Book for Class 12 Biology
Physics
NCERT Book for Class 12 Physics
Chemistry
NCERT Book for Class 12 Chemistry
Physics
H. C. Verma Solutions
See All

## NEET Mock Tests

View all mock tests
"NEET 2022 free online mock test series for exam preparation are available on the Vedantu website for free download. Practising these mock test papers of Physics, Chemistry and Biology prepared by expert teachers at Vedantu will help you to boost your confidence to face the NEET 2022 examination without any worries. The NEET test series for Physics, Chemistry and Biology that is based on the latest syllabus of NEET and also the Previous Year Question Papers."
See More

## NEET 2022 Cut Off

NEET Cut Off
NTA is responsible for the release of the NEET 2022 cut off score. The qualifying percentile score might remain the same for different categories. According to the latest trends, the expected cut off mark for NEET 2022 is 50% for general category candidates, 45% for physically challenged candidates, and 40% for candidates from reserved categories. For the general category, NEET qualifying marks for 2021 ranged from 720-138 general category, while for OBC/SC/ST categories, they ranged from 137-108 for OBC, 137-108 for SC and 137-108 for ST category.
See More

## NEET 2022 Results

The NEET 2022 result is published by NTA on https://neet.nta.nic.in/ in the form of a scorecard on 7-Sep-2022. The scorecard will include the roll number, application number, candidate's personal details, and the percentile, marks, and rank of the candidate. Only those candidates who achieve the NEET cut-off will be considered qualified for the exam.
See More
Rank List
Counselling
Cutoff
NEET 2022 state rank lists will be released by the state counselling committees for admissions to the 85% state quota and to all seats in private medical and dental colleges. NEET 2022 state rank lists are based on the marks obtained in entrance exams. Candidates can check the NEET 2022 state rank list on the official website or on our site.

## NEET Top Colleges

View all NEET 2022 Top Colleges
Want to know which Engineering colleges in India accept the NEET 2022 scores for admission to Engineering? Find the list of Engineering colleges accepting NEET scores in India, compiled by Vedantu. There are 1622 Colleges that are accepting NEET. Also find more details on Fees, Ranking, Admission, and Placement.
See More

## FAQs on NEET Important Chapter - Electric Charges and Fields

FAQ

1. Is Electric Charges and Fields important for the JEE/NEET exam?

Yes, the electric charges and fields chapter is one of the important chapters for JEE and NEET exams. It carries around 1-2 questions in NEET which carry around 2% marks. Due to the formula related questions from this chapter it becomes easy to solve and secure marks in the exams.

2. Is the electric charges and fields chapter tough?

No, the electric charges and fields chapter is easy to understand. If you understand the concepts discussed in this chapter, and practice related questions then you can successfully attempt the questions asked from this chapter in both NEET and JEE exams.

3. Can I crack NEET by solving the previous years’ questions?

According to experts, applicants who practice previous year's papers for JEE and NEET have a good probability of passing the exam. Anyway, practice is the key to success. Students must focus on solving methods and study conceptually along with the PYQ’s.

News
Blog