Is it correct to say: Treating field or force as scalar and adding magnitudes without direction?

E and F are vectors. Always resolve into components; use vector superposition.

Is it correct to say: Confusing the axial factor (2p) with the equatorial factor (p) in the dipole field?

Axial: E = 2kp/r³ parallel to p. Equatorial: E = kp/r³ antiparallel to p.

Is it correct to say: Using cm or mm directly in Coulomb's law and capacitor formulas?

Convert to metres before substituting. Board answers are cross-checked in SI.

Is it correct to say: Applying the dielectric-slab formula C = Kε₀A/d when the slab does NOT fully fill the gap?

Use C = ε₀A / [d − t(1 − 1/K)] when t < d. It reduces to Kε₀A/d only when t = d.

Is it correct to say: Writing Gauss's theorem as Φ = q_enc without ε₀?

Correct statement: Φ = ∮E·dS = q_enc/ε₀. Missing ε₀ is an automatic mark cut.

Is it correct to say: Forgetting the negative sign in U = −pE cos θ for dipole potential energy?

The negative sign gives U_min at θ = 0 (stable) and U_max at θ = π (unstable). Include it always.

Formula sheet (PDF) →

Physics · Chapter 8

Electrostatics

Q: Is it correct to say: Writing Gauss's theorem as Φ = q_enc without ε₀?

Correct statement: Φ = ∮E·dS = q_enc/ε₀. Missing ε₀ is an automatic mark cut.

Q: Is it correct to say: Forgetting the negative sign in U = −pE cos θ for dipole potential energy?

The negative sign gives U_min at θ = 0 (stable) and U_max at θ = π (unstable). Include it always.

Coulomb's law, electric field, dipole, Gauss's law, capacitance — Maharashtra HSC Physics Ch 8

13 formulas · 3 solved examples · 5 practice questions · Board weight: 7 marks (theory + numerical) · Maharashtra State Board Class 12 · 2025–26 syllabus

Board Exam Tips

→HSC Board almost always asks the derivation of electric field on the axial or equatorial line of a short dipole for 3 marks — practise the diagram.
→Gauss's theorem statement + application (line charge / infinite sheet / spherical shell) is a favourite 4-mark question in Maharashtra Board.
→Write the SI unit of every quantity (C, N/C, V, F, C·m). Marks are cut for missing units — this is stricter than CBSE.
→In numericals convert cm to m before substituting. Substitution with wrong units is the top reason for zero marks in Section D.
→Maharashtra syllabus uses the symbol E₀ for permittivity of free space in some textbook editions — always define ε₀ before use to avoid confusion.

Diagram

Parallel-plate capacitor with dielectric slab (Maharashtra Board figure)

Formulas(13)

Coulomb's Law (vector form)Board favourite

F_{12} = \frac{1}{4 π ε _{0}} \frac{q _{1} q _{2}}{r ^{2}} r^_{12}

Symbol	Meaning
$F_{12}$	Force on q₁ due to q₂ (N)
$ε_{0}$	Permittivity of free space = 8.854×10⁻¹² C²·N⁻¹·m⁻²
$q_{1}, q_{2}$	Interacting point charges (C)
$r$	Separation between charges (m)

Electric Field IntensityBoard favourite

E = lim_{q_{0} \to 0} \frac{F}{q _{0}} = \frac{1}{4 π ε _{0}} \frac{q}{r ^{2}} r^

Electric Dipole Moment

p = q \cdot 2 l

Symbol	Meaning
$2 l$	Length of the dipole (m)
$q$	Magnitude of either charge (C)

Field on Axial Line of a Short DipoleBoard favourite

E_{a} = \frac{1}{4 π ε _{0}} \frac{2 p}{r ^{3}} (r ≫ l)

Field on Equatorial Line of a Short DipoleBoard favourite

E_{e q} = \frac{1}{4 π ε _{0}} \frac{p}{r ^{3}} (r ≫ l)

Torque on a Dipole in Uniform Field

τ = p \times E, τ = pE sin θ

Gauss's TheoremBoard favourite

\oint_{S} E \cdot d S = \frac{Q _{e n c}}{ε _{0}}

Field due to an Infinite Line of Charge

E = \frac{λ}{2 π ε _{0} r}

Field due to an Infinite Plane Sheet

E = \frac{σ}{2 ε _{0}}

Capacitance of a Parallel-Plate CapacitorBoard favourite

C = \frac{ε _{0} A}{d}

Symbol	Meaning
$A$	Common plate area (m²)
$d$	Plate separation (m)

Capacitor with Dielectric SlabBoard favourite

C = \frac{ε _{0} A}{d - t ( 1 - \frac{1}{K} )}

Energy Stored in a Capacitor

U = \frac{1}{2} C V^{2} = \frac{Q ^{2}}{2 C} = \frac{1}{2} Q V

Energy Density of Electric Field

u = \frac{1}{2} ε_{0} E^{2}

Solved Examples

1Solved Exampleeasy5 steps

Two identical charges of +5 µC are placed at the corners A and B of a square of side 0.2 m. Find the electric field at the corner C which is diagonally opposite to A.

AC is the diagonal, so distance AC = 0.2√2 m; BC = 0.2 m.

A C = 0.2 2 m, B C = 0.2 m

2Solved Exampleboard4 steps

A parallel-plate capacitor has plates of area 150 cm² separated by 2 mm. A dielectric slab (K = 4) of thickness 1 mm is introduced between the plates. Calculate its capacitance.

Convert to SI units.

A = 150 \times 1 0^{- 4} m^{2}, d = 2 \times 1 0^{- 3} m, t = 1 \times 1 0^{- 3} m, K = 4

3Solved ExampleHOTS4 steps

A dipole of moment 6×10⁻⁹ C·m is placed at 30° to a uniform electric field of 10⁵ N/C. Find (i) the torque, (ii) the work done in rotating the dipole from θ = 30° to θ = 90°.

Torque on dipole:

τ = pE sin θ = 6 \times 1 0^{- 9} \times 1 0^{5} \times sin 30° = 3 \times 1 0^{- 4} N\cdotm

Traps & Common Mistakes

Common Mistakes6

1
Treating field or force as scalar and adding magnitudes without direction
✓E and F are vectors. Always resolve into components; use vector superposition.
2
Confusing the axial factor (2p) with the equatorial factor (p) in the dipole field
✓Axial: E = 2kp/r³ parallel to p. Equatorial: E = kp/r³ antiparallel to p.
3
Using cm or mm directly in Coulomb's law and capacitor formulas
✓Convert to metres before substituting. Board answers are cross-checked in SI.
4
Applying the dielectric-slab formula C = Kε₀A/d when the slab does NOT fully fill the gap
✓Use C = ε₀A / [d − t(1 − 1/K)] when t < d. It reduces to Kε₀A/d only when t = d.
5
Writing Gauss's theorem as Φ = q_enc without ε₀
✓Correct statement: Φ = ∮E·dS = q_enc/ε₀. Missing ε₀ is an automatic mark cut.
6
Forgetting the negative sign in U = −pE cos θ for dipole potential energy
✓The negative sign gives U_min at θ = 0 (stable) and U_max at θ = π (unstable). Include it always.

Practice Yourself

Practice Yourself5 questions

Q1
Two point charges +3 µC and −3 µC are 8 cm apart. Find the electric dipole moment and its direction.
Q2
Calculate the electric field 30 cm from an infinite line charge of density 5×10⁻⁶ C/m.
Q3
A capacitor of 10 µF is charged to 200 V. Find the energy stored.
$U = \frac{1}{2} C V^{2}$
Q4
Three capacitors 2 µF, 3 µF, 6 µF are in parallel across a 10 V supply. Find charge on the 3 µF capacitor.
Q5
State Gauss's theorem and use it to derive the electric field due to an infinite plane sheet of charge.

Notes

Electrostatics — Maharashtra HSC Overview

Electrostatics, including capacitors and dielectrics, is Chapter 8 (Electrostatics) of the Class 12 Physics textbook (Balbharati).

What Maharashtra Board emphasises differently

The MSBSHSE textbook derives dipole fields using the diagram-based component method, whereas NCERT prefers direct scalar reasoning. Learn to sketch the field vectors and resolve components — the marking scheme rewards this.
The dielectric-slab formula with slab thickness t < d is a Maharashtra Board favourite. NCERT stays with K·ε₀A/d for a fully filled dielectric; Maharashtra pushes further.
Maharashtra introduces polar and non-polar dielectrics, dielectric strength, and dielectric constant within this chapter. Expect short theory questions on them.
The word electrical energy density appears explicitly in the Maharashtra syllabus; NCERT treats it in the exercises only.

Practical HSC tips

Always use ε₀ (not the numerical k) in derivation questions — the examiner needs to see the constant appear naturally.
Draw a labelled diagram for every derivation. In Maharashtra Board, a derivation without a diagram loses 1 mark.
Answer numericals in the sequence: Given → Formula → Substitution → Answer (with unit). This is the marking rubric.