Electrostatics – Stage 1
Chapter 1: Electric Charges & Fields – Page 5
1. What are Electric Field Lines?
Electric field lines are imaginary lines drawn to represent the direction and nature of the electric field in space.
Definition: The tangent drawn to an electric field line at any point gives the direction of the electric field at that point.
2. Origin and Termination of Field Lines
- Field lines originate from positive charges.
- Field lines terminate on negative charges.
- For isolated charges, lines start or end at infinity.
JEE Tip: Field lines never start or end in free space (except at infinity).
3. Rules of Electric Field Lines (VERY IMPORTANT)
- Field lines never intersect.
- Closer field lines → stronger electric field.
- Field lines are continuous curves.
- No closed loops (electrostatic field is conservative).
- Number of lines ∝ magnitude of charge.
- Field lines are perpendicular to conductor surface.
4. Field Lines for Single Charge
- Positive charge: Lines radiate outward.
- Negative charge: Lines converge inward.
Concept Check: Direction of field line = direction of force on a positive test charge.
5. Field Lines for Two Charges
Case 1: Two Like Charges (+ + or − −)
- Field lines repel each other.
- No line connects the two charges.
- Neutral point may exist on the line joining them.
Case 2: Unlike Charges (+ −)
- Field lines start from + and end on −.
- Field lines are dense between charges.
- Represents electric dipole.
6. Electric Dipole – Field Line Pattern
An electric dipole consists of two equal and opposite charges separated by a small distance.
Dipole Moment: 𝐩⃗ = q · 2a
- Field lines are symmetric.
- Strong field near charges.
- Zero field at center.
7. Field Lines and Conductors
- Inside a conductor: E = 0.
- Field lines are perpendicular to surface.
- More crowded near sharp edges → high field.
JEE Application: Reason for lightning rods & electric discharge at sharp points.
8. Common Misconceptions (Exam Traps)
- Field lines are not real paths of charges.
- Number of lines does NOT depend on distance.
- Crossing field lines is impossible.
- Closed loops indicate magnetic field, not electric.
Stage 1 – Page 5 Takeaway
- Field lines visualize electric field.
- Density indicates field strength.
- Patterns help solve symmetry problems.
- Used heavily in conceptual MCQs.
Next → Stage 1, Page 6: Electric Flux – Concept, Area Vector & Physical Meaning
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