⚡ Electrostatics – Electric Charges & Fields

Page 9: Electric Field Lines, Density & JEE Advanced Visual Reasoning

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1. What are Electric Field Lines?

Electric field lines are imaginary lines drawn to represent the direction and strength of an electric field.

Tangent to field line → Direction of electric field

• Conceptual tool (not physical)
• Introduced by Michael Faraday

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2. Rules of Electric Field Lines (VERY IMPORTANT)

• Start from positive charge
• End on negative charge or infinity
• Never intersect
• Never form closed loops
• Denser lines → stronger field

⚠ Intersection implies two directions of field at one point – impossible.

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3. Field Lines for Isolated Charges

Charge Type	Field Line Nature
Positive charge	Radially outward
Negative charge	Radially inward

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4. Field Lines for Multiple Charges

• Like charges repel → lines diverge
• Unlike charges attract → lines connect
• Number of lines ∝ magnitude of charge

Ratio of number of lines = ratio of charges

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5. Field Line Density & Electric Field

Electric field magnitude is proportional to field line density.

E ∝ Number of lines per unit area

• Closer lines → stronger field
• Uniform spacing → uniform field

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6. Uniform Electric Field

A uniform electric field has:

• Parallel field lines
• Equal spacing
• Same magnitude everywhere

Example: Field between parallel plate capacitor

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7. Field Lines vs Equipotential Surfaces

• Field lines ⟂ Equipotential surfaces
• No work done along equipotential surface
• Closer equipotentials → stronger field

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8. JEE Advanced Conceptual Traps

• Field can be zero but field lines exist elsewhere
• Zero net force ≠ zero field lines everywhere
• Field lines do not indicate particle motion

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9. Conceptual Example (Visual Reasoning)

Question:

Two equal positive charges are placed close together. Where is electric field zero?

Thinking:

• Fields due to each charge are opposite at midpoint
• Magnitudes equal

Result: Electric field is zero at midpoint

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10. Exam Strategy & Visualization Tips

• Always sketch field lines in mind
• Check symmetry first
• Count line density for field comparison
• Do not confuse force path with field lines

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🎯 Page 9 Summary

• Field lines represent direction & strength
• Density gives field magnitude
• Visualization is key for JEE Advanced