MIND GROW Magazine

Electrostatics – Stage 2

Electric Charges & Fields – Page 4

1. Multi-Charge System – Net Force Zero but Field ≠ 0

JEE Advanced often asks situations where:

Net force on a charge is zero
But electric field at that point is non-zero

Core Idea:

Force depends on test charge
Field exists independent of test charge

Trap:
Students wrongly conclude “force zero ⇒ field zero”

2. Equilibrium of Charges on a Line (JEE Favorite)

Problem Pattern:

Two fixed charges
Find point where third charge can stay at rest

Correct Method:

Equate electric fields (not forces)
Consider direction carefully

|E₁| = |E₂|

Advanced Trap:
For unlike charges, equilibrium lies outside the segment

3. Charge on a Ring + Point Charge Motion

Classic JEE Advanced Model:

Uniformly charged ring
Point charge released on axis

Key Observation:

Field varies with distance
Restoring force near centre

Motion ≈ SHM for small displacement

Insight:
Electrostatics + SHM crossover is highly tested

4. Gauss Law + Piecewise Field

Pattern:

Charge density changes with radius
Find E in different regions

Strategy:

Split space into regions
Apply Gauss law separately

Reminder:
Gaussian surface must respect symmetry, not charge distribution

5. Work Done in Complex Electric Fields

JEE Trick:

Particle moves along curved path
Ask work done by electric force

Golden Rule:

Work is path independent
Depends only on potential difference

W = qΔV

Trap:
Long curved paths are psychological traps

6. Flux Through Open vs Closed Surfaces

Closed surface → Gauss law valid
Open surface → no direct formula

Advanced Insight:

Flux can be zero even if field ≠ 0
Flux depends on angle + area

JEE Favorite:
Flux through half-sphere due to central charge

7. Stability of Electrostatic Equilibrium

JEE Advanced sometimes tests:

Whether equilibrium is stable or unstable

Rule:

Electrostatic equilibrium is generally unstable
No local minimum of potential energy

Reason:
Earnshaw’s theorem

Stage 2 – Page 4 Final Takeaway

Separate force and field clearly
Equilibrium ≠ stability
Energy methods beat force methods
Always check direction and region

Next → Stage 2 – Page 5 (Super-Tricky PYQs & Examiner Psychology)