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Electric Charges and Fields opens the second half of JEE Main physics and it sets up everything that follows. Gauss’s law is the heart of it: once you can choose the right surface, the field of a line, a sheet and a sphere all drop out in one step each. The dipole results are pure memory and are asked almost every year. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 2 questions every year, which is 8 marks.
- Gauss’s law applied to a line, a sheet and a sphere is the regular one.
- The electric dipole, its torque and its field fill the rest.
What the syllabus says
In the syllabus (Unit 11, Electrostatics): Conservation of charge, Coulomb’s law for the force between two point charges and between several charges, the superposition principle, continuous charge distribution, electric field due to a point charge, electric field lines, the electric dipole and its field, torque on a dipole in a uniform field, electric flux, and Gauss’s law applied to an infinitely long charged wire, an infinite charged plane sheet and a uniformly charged thin spherical shell.
Never in the JEE Main syllabus: the Van de Graaff generator. They are in NCERT, so they still turn up in old question banks.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| Coulomb’s law | F = q1q2 / 4πε0r2 |
| In a medium | the force falls K times, K being the dielectric constant |
| Gauss’s law | Φ = qenclosed/ε0 |
| Infinite line charge | E = λ / 2πε0r |
| Infinite plane sheet | E = σ / 2ε0 |
| Charged conducting sphere | E = 0 inside, kQ/r2 outside |
| Dipole moment | p = q × 2a |
| Axial field of a short dipole | E = 2kp/r3 |
| Equatorial field | E = kp/r3 |
| Torque on a dipole | τ = pE sinθ, greatest at 90° |
| Net force on a dipole | zero in a uniform field |
Look at the field of the infinite sheet. There is no distance in it at all — the field is the same one centimetre away and one metre away. That is a direct consequence of the sheet being infinite, and it surprises students every year.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. When two charges are placed in a medium of dielectric constant K instead of vacuum, the force between them
- (a) becomes K times larger
- (b) becomes K times smaller
- (c) does not change
- (d) becomes K2 times larger
Q2. The electric field at a distance r from an infinitely long line charge of linear density λ is
- (a) λ/2πε0r
- (b) λ/4πε0r2
- (c) λ/ε0
- (d) λ/2ε0
Q3. The electric field due to an infinite charged plane sheet of surface charge density σ is
- (a) σ/ε0
- (b) σ/2ε0
- (c) σ/4ε0
- (d) 2σ/ε0
Q4. The electric field at a point inside a charged conducting sphere is
- (a) kQ/r2
- (b) zero
- (c) kQ/R2
- (d) infinite
Q5. The total electric flux through a closed surface enclosing a charge q is
- (a) q/ε0
- (b) qε0
- (c) q/4πε0
- (d) zero
Q6. The greatest torque on a dipole of moment p placed in a uniform field E is
- (a) pE
- (b) pE/2
- (c) 2pE
- (d) zero
Q7. The electric field on the axis of a short dipole at a distance r from its centre is
- (a) kp/r3
- (b) 2kp/r3
- (c) kp/r2
- (d) kp/r
Q8. The net force on an electric dipole placed in a uniform electric field is
- (a) pE
- (b) zero
- (c) 2pE
- (d) qE
Q9. Charges +q and −q are placed 2a apart. The electric field at the midpoint of the line joining them has magnitude
- (a) zero
- (b) 2kq/a2
- (c) kq/a2
- (d) 4kq/a2
Q10. The SI unit of electric flux is
- (a) N/C
- (b) N·m2/C
- (c) C/m2
- (d) V/m
Answer key, with a hint for each
| Q1 | (b) | The medium weakens the force by exactly the dielectric constant. |
| Q2 | (a) | Use a cylindrical Gaussian surface. The field falls as 1/r, not 1/r2. |
| Q3 | (b) | Use a pill-box Gaussian surface cutting the sheet. The 2 appears because the flux comes out of both faces. |
| Q4 | (b) | All the charge sits on the outer surface, and inside a conductor the field must be zero or charges would keep moving. |
| Q5 | (a) | Gauss’s law. Only the charge inside counts, and the shape of the surface makes no difference. |
| Q6 | (a) | τ = pE sinθ, greatest when the dipole sits across the field. |
| Q7 | (b) | Axial is 2kp/r3, exactly twice the equatorial value. |
| Q8 | (b) | The two equal and opposite charges feel equal and opposite forces, so only a torque is left. |
| Q9 | (b) | Both charges push the test charge the same way, so the two fields of kq/a2 add. |
| Q10 | (b) | Flux is field times area, so N/C multiplied by m2. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
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