⚛️Physics

Electrostatics

Coulomb's law, electric field, potential, capacitance, dielectrics, and Gauss's law.

Class 11Class 12

What You Will Learn in Electrostatics

Electrostatics studies electric charges at rest, the forces between them (Coulomb's law), and the associated electric fields and potentials.

Coulomb's law: F = kq₁q₂/r² where k = 9×10⁹ N·m²/C². Force is along line joining charges.
Electric field E = F/q₀ = kQ/r² (N/C or V/m); direction away from +, towards -.
Electric potential V = kQ/r (scalar); relationship: E = -dV/dr.
Gauss's law: ∮E·dA = Q_enc/ε₀ — total electric flux through closed surface equals enclosed charge/ε₀.
Capacitance C = Q/V; parallel plate: C = ε₀A/d; energy stored: U = ½CV² = Q²/2C.
Dielectric constant K reduces E and increases C: C = Kε₀A/d.

Key Formulas

F = kq₁q₂/r²E = kQ/r²V = kQ/rC = ε₀A/dU = ½CV²k = 1/(4πε₀) = 9×10⁹

Example

Two charges of +2μC and -2μC separated by 0.3 m attract each other with F = 9×10⁹×4×10⁻¹²/0.09 = 0.4 N.

Solved Numerical Example

Two point charges q₁=+4μC and q₂=+9μC are 0.3 m apart. Find where electric field is zero.
Let distance from q₁ be x. E₁=E₂: k×4/(x²) = k×9/(0.3-x)².
4(0.3-x)² = 9x² → 2(0.3-x) = 3x → 0.6-2x=3x → x=0.12 m from q₁.

Expected Exam Questions — Electrostatics

Q1.State Gauss's Law and give one application.

Answer: Gauss's law: Total electric flux through any closed surface = Q_enclosed/ε₀. Application: Electric field due to an infinite line charge — using cylindrical Gaussian surface: E = λ/(2πε₀r), where λ = linear charge density.

Q2.A 4μF capacitor is connected to 100V. Find charge and energy stored.

Answer: Q = CV = 4×10⁻⁶ × 100 = 400 μC = 4×10⁻⁴ C. Energy U = ½CV² = ½×4×10⁻⁶×10000 = 0.02 J = 20 mJ.

Q3.What is the effect of inserting a dielectric in a capacitor? Explain.

Answer: Dielectric reduces electric field between plates (E_new = E₀/K). For constant charge: V decreases, C increases by K. For constant voltage: C increases, Q increases by K. Energy stored changes. Dielectric is polarised, creating opposing electric field.

🔘 MCQ Practice — Electrostatics

MCQ 1.The SI unit of electric field intensity is:

A. V/m²

B. N/C or V/m ✓

C. C/m

D. N·m/C

✓ Correct Answer: N/C or V/m

MCQ 2.Equipotential surfaces are always:

A. Parallel to electric field

B. Perpendicular to electric field ✓

C. Along electric field lines

D. Curved only

✓ Correct Answer: Perpendicular to electric field

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Frequently Asked Questions — Electrostatics

What is Electrostatics in Physics?

Electrostatics studies electric charges at rest, the forces between them (Coulomb's law), and the associated electric fields and potentials.

State Gauss's Law and give one application.

Gauss's law: Total electric flux through any closed surface = Q_enclosed/ε₀. Application: Electric field due to an infinite line charge — using cylindrical Gaussian surface: E = λ/(2πε₀r), where λ = linear charge density.

A 4μF capacitor is connected to 100V. Find charge and energy stored.

Q = CV = 4×10⁻⁶ × 100 = 400 μC = 4×10⁻⁴ C. Energy U = ½CV² = ½×4×10⁻⁶×10000 = 0.02 J = 20 mJ.

What is the effect of inserting a dielectric in a capacitor? Explain.

Dielectric reduces electric field between plates (E_new = E₀/K). For constant charge: V decreases, C increases by K. For constant voltage: C increases, Q increases by K. Energy stored changes. Dielectric is polarised, creating opposing electric field.

How do I prepare Electrostatics for exams?

To master Electrostatics, start by reading the theory carefully, then go through solved examples step by step. Practice numericals (if applicable), revise key formulas, and attempt previous year questions. SII notes cover all these aspects in a structured manner.

Are these Electrostatics notes free?

Yes! SII provides free access to Electrostatics notes and introductory study materials. Enrolled students get full access to detailed notes, solved papers, and live doubt-clearing sessions.

Which exams ask questions from Electrostatics?

Electrostatics is an important topic tested in Class 11, Class 12 board exams, as well as JEE Main, JEE Advanced, NEET. It frequently appears in both short-answer and long-answer sections.