Quick Revision
The night before exam. 30 minutes to recall everything. One-page summary, flashcards, formula dump, memory tricks.
3 hours before exam: Flashcards + Formula dump
30 min before exam: Memory tricks + Common mistakes
In exam hall (waiting): Key formulas glance
One-Page Summary
Core Concepts (30 seconds)
Electric Charge
- Quantized: q = ±ne
- Conserved: Q_total = constant
- Additive: Q = Σq_i
- Two types: + and −
Properties
- Like charges repel
- Unlike charges attract
- Charge is invariant
- Unit: Coulomb (C)
Laws & Principles (60 seconds)
Dipole Formulas (Critical for JEE)
Ratio: E_axial/E_equatorial = 2
Constants (Memorize Exact Values)
- k = 9 × 10⁹ N·m²/C²
- ε₀ = 8.85 × 10⁻¹² C²/(N·m²)
- e = 1.6 × 10⁻¹⁹ C
- Relation: k = 1/(4πε₀)
Flashcards (Click to Flip)
Test your instant recall. If you hesitate for >3 seconds, you need more practice.
What is charge quantization?
Charge exists only in integer multiples of elementary charge e.
q = ±ne
Coulomb's Law (Vector Form)
F⃗₁₂ = (kq₁q₂/r²) r̂₁₂
Force on q₂ due to q₁
Dimension of Electric Field?
[MLT⁻³A⁻¹]
Unit: N/C or V/m
Relation between E and V?
E = -dV/dr
(for radial field)
Field points from high to low potential
Dipole moment direction?
From -q to +q
Magnitude: p = q × 2a
Stable equilibrium possible with electrostatic forces alone?
NO
Earnshaw's Theorem
Like charges cannot be in stable equilibrium
Work done moving charge along equipotential?
ZERO
Because ΔV = 0
W = q × ΔV = 0
Dipole in uniform field: Net force?
ZERO
But torque exists!
τ = pE sin θ
Linear charge density symbol and unit?
λ (lambda)
Unit: C/m
λ = Q/L
Formula Dump (Print This!)
Basic Formulas
F = k|q₁q₂|/r²
E = F/q = kQ/r²
V = kQ/r
U = kq₁q₂/r
W = q(V_B - V_A)
E = -dV/dr
Dipole Formulas
p = q × 2a
E_axial = 2kp/r³
E_equatorial = kp/r³
τ = pE sin θ
τ⃗ = p⃗ × E⃗
U = -pE cos θ = -p⃗·E⃗
Charge Density
λ = Q/L (linear, C/m)
σ = Q/A (surface, C/m²)
ρ = Q/V (volume, C/m³)
Constants
k = 9 × 10⁹ N·m²/C²
ε₀ = 8.85 × 10⁻¹² C²/(N·m²)
e = 1.6 × 10⁻¹⁹ C
k = 1/(4πε₀)
Memory Tricks & Mnemonics
Remember: E vs V
E is VECTOR (direction matters)
V is SCALAR (only magnitude)
Trick: "E for Electric arrows, V for Value (scalar)"
Dipole Orientation
Minimum Energy: θ = 0° (aligned with field)
Maximum Energy: θ = 180° (anti-aligned)
Trick: "Same direction = Minimum drama (energy)"
Force vs Field
Force: Depends on BOTH charges (q₁q₂)
Field: Depends on SOURCE charge only (Q)
Trick: "Field is property of space, Force needs two to tango"
Power of r
Force/Field: 1/r²
Potential/Energy: 1/r
Dipole Field: 1/r³
Pattern: Each integration adds 1 to power
Common Mistakes (DON'T DO THESE!)
WRONG: F_total = F₁ + F₂ + F₃
RIGHT: F⃗_total = F⃗₁ + F⃗₂ + F⃗₃ (vector addition)
Use components unless forces are collinear!
For opposite charges: U is NEGATIVE
For same charges: U is POSITIVE
U = kq₁q₂/r keeps the SIGN of charge product!
μC → C: multiply by 10⁻⁶
nC → C: multiply by 10⁻⁹
cm → m: multiply by 10⁻²
ALWAYS convert to SI before substituting in formula!
Force MAGNITUDE is always positive
Use F = k|q₁q₂|/r²
Direction is separate (attractive/repulsive)
E = kQ/r² (field, vector, N/C)
V = kQ/r (potential, scalar, V)
Field has r², potential has r. Don't mix!
Last-Minute Exam Tips
30 Minutes Before Exam
- Glance at all formulas once
- Review common mistakes
- Deep breath - you've prepared well
- Don't discuss problems with others
In the Exam Hall
- Read question twice before solving
- Write units in every step
- Check dimensional consistency
- Verify sign in vector problems
✅ All core concepts
✅ Problem types practice
✅ PYQ patterns
✅ This revision page
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