Electric Charges and Fields is Chapter 1 of NCERT Class 12 Physics (Part I) and the first part of Unit 11, Electrostatics, in both the NEET UG and JEE Main syllabus. These notes cover properties of charge, Coulomb’s law, superposition, electric field and field lines, electric flux, dipoles, and Gauss’s law with its applications, with a free PDF.
Electric Charges and Fields Notes for NEET and JEE Main
Section-wise key points from NCERT Chapter 1, formula tables, high-yield facts and common mistakes for 2027 revision.
Electric Charges and Fields at a Glance
| Item | Details |
|---|---|
| Syllabus unit | Unit 11: Electrostatics (same unit number in NEET UG and JEE Main) |
| Exams that cover it | NEET UG and JEE Main Paper 1 (B.E./B.Tech.) |
| NCERT textbook | Physics Part I, Class 12, Chapter 1 (rationalised book) |
| Other chapter in the unit | Electrostatic Potential and Capacitance (Chapter 2) |
| Syllabus topics | Conservation of charge; Coulomb’s law; forces between multiple charges; superposition principle and continuous charge distribution; electric field due to a point charge; field lines; electric dipole and its field; torque on a dipole in a uniform field; electric flux; Gauss’s law and its applications to an infinitely long wire, an infinite plane sheet and a thin spherical shell |
| Official chapter | NCERT Chapter 1 |
| Notes PDF | View PDF |
The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus published by the National Testing Agency (NTA). Both stay the reference until the 2027 syllabi are out. The full lists are on the NEET syllabus and JEE Main syllabus pages. Electric potential, capacitors and dielectrics, the rest of Unit 11, are in NCERT Chapter 2.
Electric Charge and Its Basic Properties
There are two kinds of charge, positive and negative: like charges repel and unlike charges attract.
- Thales of Miletus (around 600 BC) noticed that rubbed amber attracts light objects; ‘electricity’ comes from the Greek elektron (amber).
- Benjamin Franklin named the charges positive and negative. By convention, a glass rod rubbed with silk is positive and a plastic rod rubbed with fur is negative.
- A gold-leaf electroscope detects charge: the leaves diverge when charge reaches them.
- Conductors allow charge to move (electrons in metals; positive and negative ions in electrolytes); insulators do not.
| Property | Meaning |
|---|---|
| Additivity | Total charge is the algebraic sum of individual charges (charge is a scalar) |
| Conservation | Total charge of an isolated system does not change; charging by friction only transfers charge |
| Quantisation | q = ne, n = 0, ±1, ±2, …; e = 1.602 × 10⁻¹⁹ C |
- Quantisation was suggested by Faraday’s laws of electrolysis and shown experimentally by Millikan in 1912.
- About 6 × 10¹⁸ electrons make up a charge of −1 C; smaller units are µC (10⁻⁶ C) and mC (10⁻³ C).
- For macroscopic charges n is very large, so quantisation can be ignored.
- Charge is invariant: it is the same in frames of reference in relative motion.
Coulomb’s Law
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.
- F = k q₁q₂/r², with k = 1/4πε₀ ≈ 9 × 10⁹ N m² C⁻².
- Permittivity of free space: ε₀ = 8.854 × 10⁻¹² C² N⁻¹ m⁻².
- Vector form: F₂₁ = k(q₁q₂/r₂₁²) r̂₂₁, the force on q₂ due to q₁; F₁₂ = −F₂₁, in line with Newton’s third law.
- 1 C = 1 A s; the coulomb is defined through the ampere, which is why k has a fixed value.
- Electric force between an electron and a proton is about 2.4 × 10³⁹ times the gravitational force; between two protons the ratio is about 1.3 × 10³⁶.
- Gravity is always attractive, while Coulomb forces can attract or repel and so can cancel; that is why gravity dominates on large scales.
Forces Between Multiple Charges: Superposition Principle
The force on any charge is the vector sum of the forces due to every other charge, each found from Coulomb’s law as if the others were absent.
- F₁ = F₁₂ + F₁₃ + … + F₁ₙ (vector sum).
- The force between two charges is not affected by the presence of other charges.
Electric Field
The electric field at a point is the force per unit positive test charge placed there: E = lim (q → 0) F/q.
- Field of a point charge Q: E = Q/(4πε₀r²) r̂; radially outward for positive Q, inward for negative Q.
- SI unit: N C⁻¹ (equivalently V m⁻¹).
- A charge q in a field E feels F = qE.
- The test charge is made negligibly small so that it does not disturb the source charge.
- The field of a system of charges is the vector sum of the fields of each charge (superposition).
- The field exists at every point whether or not a test charge is placed there; the concept of field was introduced by Faraday.
Electric Field Lines
A field line is a curve whose tangent at every point gives the direction of the electric field there.
- Field lines start on positive charges and end on negative charges (or at infinity).
- In a charge-free region they are continuous curves without breaks.
- Two field lines never cross, because the field has a single direction at each point.
- Electrostatic field lines do not form closed loops.
- Lines crowd together where the field is strong and spread apart where it is weak; a uniform field is shown by parallel, equally spaced lines.
Electric Flux
Electric flux through a small area element is Δφ = E·ΔS = E ΔS cos θ.
- ΔS is a vector along the normal to the area; for a closed surface the outward normal is used.
- Flux is proportional to the number of field lines crossing the area.
- SI unit: N m² C⁻¹ (equivalently V m).
Electric Dipole
An electric dipole is a pair of equal and opposite charges, q and −q, separated by a distance 2a.
- Dipole moment p = q × 2a, directed from −q to +q; SI unit C m.
- The total charge of a dipole is zero, but its field is not, because the two charges are separated.
- A dipole field falls as 1/r³, faster than the 1/r² field of a single charge.
| Position (r ≫ a) | Field magnitude | Direction |
|---|---|---|
| On the axis | E = 2p/(4πε₀r³) | Along p |
| On the equatorial plane | E = p/(4πε₀r³) | Opposite to p |
- Molecules such as CO₂ and CH₄ have zero dipole moment but develop one in an applied field; polar molecules such as H₂O have a permanent dipole moment.
- A point (ideal) dipole has 2a → 0 and q → ∞ with p finite.
Dipole in an External Field
In a uniform field a dipole feels no net force but experiences a torque that tends to align it with the field.
- Torque: τ = p × E, magnitude pE sin θ.
- Net force in a uniform field: zero, since qE and −qE cancel.
- In a non-uniform field there is a net force: towards increasing field when p is parallel to E, and towards decreasing field when p is antiparallel to E.
- This is why a comb run through dry hair attracts bits of paper: the comb polarises the paper and its field is non-uniform.
Continuous Charge Distribution
For charge spread over a line, surface or volume, charge densities replace individual charges, and the field is found by summing (integrating) over small elements.
| Density | Symbol | SI unit |
|---|---|---|
| Linear charge density | λ | C m⁻¹ |
| Surface charge density | σ | C m⁻² |
| Volume charge density | ρ | C m⁻³ |
Gauss’s Law and Its Applications
The electric flux through any closed surface equals the total charge enclosed divided by ε₀: φ = q/ε₀.
- Charges outside the surface make no net contribution to the flux, though they affect the field at the surface.
- The law holds for a closed surface of any shape and size; the chosen surface is called a Gaussian surface and should not pass through a discrete charge.
- It is most useful for finding E when the charge distribution has simple symmetry.
| Charge distribution | Field | Notes |
|---|---|---|
| Infinitely long straight wire (λ) | E = λ/(2πε₀r) | Radial; r is the perpendicular distance from the wire |
| Infinite thin plane sheet (σ) | E = σ/(2ε₀) | Normal to the sheet; independent of distance |
| Thin spherical shell (charge q, radius R), outside (r ≥ R) | E = q/(4πε₀r²) | As if all charge were at the centre |
| Thin spherical shell, inside (r < R) | E = 0 | Field is zero everywhere inside |
- NCERT adds that a solid sphere with uniform volume charge also gives E = q/(4πε₀r²) outside.
Most-asked Facts for NEET and JEE Main: Electric Charges and Fields
Revise these one-line facts from the NCERT chapter just before the exam.
- q = ne, e = 1.602 × 10⁻¹⁹ C; Millikan showed quantisation in 1912.
- k = 1/4πε₀ ≈ 9 × 10⁹ N m² C⁻²; ε₀ = 8.854 × 10⁻¹² C² N⁻¹ m⁻².
- Field of a point charge ∝ 1/r²; field of a dipole ∝ 1/r³.
- Axial dipole field is twice the equatorial field at the same large distance.
- Dipole moment points from −q to +q.
- τ = p × E; net force on a dipole in a uniform field is zero.
- Field lines never cross and never form closed loops in electrostatics.
- Gauss’s law: φ = q<sub>enclosed</sub>/ε₀.
- Infinite line: E = λ/(2πε₀r); infinite sheet: E = σ/(2ε₀).
- Field inside a uniformly charged thin spherical shell is zero.
- Electric force between electron and proton is about 10³⁹ times the gravitational force.
Common Mistakes to Avoid
Watch for these mix-ups when you answer questions on this chapter.
- Adding forces or fields from several charges as plain numbers. Superposition is a vector sum.
- Taking the dipole moment direction from + to −. It points from −q to +q.
- Saying a dipole in a uniform field feels a net force. It feels only a torque; a net force needs a non-uniform field.
- Thinking charges outside a Gaussian surface change the net flux. They change the field on the surface, not the net flux.
- Using E = σ/ε₀ for an infinite thin sheet of charge. Gauss’s law gives σ/(2ε₀).
- Making the field inside a charged thin shell non-zero. It is zero at every point inside.
- Forgetting that the 1/r³ dipole field formulas hold only for r ≫ a.
Electric Charges and Fields Notes PDF and Related Pages
Download the AglaSem Class 12 Physics notes for Electrostatics and Current Electricity (key notes and important questions with answers) free: View PDF. This PDF covers all of Electrostatics, including Electrostatic Potential and Capacitance, and Current Electricity.
- All chapters: Physics notes for NEET and JEE Main
- Full topic list: NEET syllabus and JEE Main syllabus
- Practice: NEET question papers and JEE Main question papers
- Source text: NCERT Chapter 1 (NCERT, Class 12 Physics Part I)
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