⚡ Electrostatics – Objective Questions

Page 2: Clearly Explained Solutions (JEE Level)

---

Q1. Electric field at the midpoint of +q and −q

Given:

• Two charges: +q and −q
• Distance between charges = d
• Midpoint is at d/2 from each charge

Concept Used:

Electric field due to a point charge:
E = (1 / 4πε₀) · (q / r²)

Step-by-Step Explanation:

• Field due to +q points away from +q
• Field due to −q points towards −q
• Both fields act in the same direction

Calculation:

E_net = 2 × (1 / 4πε₀) × q / (d/2)²
= (2q) / (4πε₀ d²)

Answer: Option C

JEE Trap: Fields do not cancel; directions matter.

---

Q2. Unit of Electric Flux

Concept:

Electric Flux Φ = E · A

Units:

• Electric Field (E) → N/C
• Area (A) → m²

Result: N·m²/C

Answer: Option B

---

Q3. Electric field inside a charged conducting shell

Concept: Electrostatic Equilibrium

• Electric field inside a conductor = 0
• Charges reside on the surface
• Any internal field would cause charge motion

Answer: Option C

Important: Result is independent of shell radius.

---

Q4. Law applicable only for symmetric charge distributions

Comparison:

• Coulomb’s Law → Always valid, complex
• Gauss’s Law → Powerful only for symmetry

Symmetry Types: Spherical, Cylindrical, Planar

Answer: Option B

---

Q5. Electric Flux – Multiple Correct

Using Gauss Law:

Φ = Q_enclosed / ε₀

• A. Scalar quantity ✔
• B. Depends on enclosed charge ✔
• C. Depends on shape ✘
• D. Independent of external charges ✔

Correct Options: A, B, D

---

Q6. Conductor in electrostatic equilibrium

• Electric field inside = 0 ✔
• Charge on surface ✔
• Potential constant ✔
• Electric field maximum inside ✘

Correct Options: A, B, C

---

Q7. Assertion–Reason

Assertion: Electric field inside a conductor is zero (True)

Reason: Charges redistribute to cancel internal field (True)

Conclusion: Reason correctly explains assertion

Answer: Option A

---

Q8. Integer Type – Electric Flux

Given:

• Charge Q = 5 μC = 5 × 10⁻⁶ C
• ε₀ = 8.85 × 10⁻¹²

Calculation:

Φ = Q / ε₀
= (5 × 10⁻⁶) / (8.85 × 10⁻¹²)
= 5.65 × 10⁵ N·m²/C

Answer: 5.65 × 10⁵

---

🎯 JEE Key Takeaways

• Direction of electric field is crucial
• Flux depends only on enclosed charge
• Field can be zero while potential is not
• Gauss law is always true but not always useful