Electric Field Lines

Definition

Electric field lines (or lines of force) are imaginary lines used to represent the electric field around a charged body. These lines depict the path a unit positive test Charge would follow when placed in the field of a source charge.

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Significance

• Electric field lines help visualize the electric field.
• They act as a “map” that conveys information about the field’s strength and direction at various points.

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Characteristics of Electric Field Lines

1. Direction of Field Lines:

   • Radially outward for a positive charge.
   • Radially inward for a negative charge.
     

2. Origination and Termination:

   • Field lines originate from a positive charge and terminate on a negative charge.
     

3. Indication of Strength:

   • The density of field lines indicates the field strength. It increases as we move radially inward and decreases outward.

4. No Intersecting Lines:

   • Field lines never intersect because a single point cannot have two directions of the field.

5. Interaction with Conductors:

   • Field lines cannot pass through a conductor, making the electric field zero inside a conductor.

6. Length and Tension:

   • Field lines tend to contract in length, explaining the attraction between oppositely charged bodies.

7. Field Line Density:

   • The number of field lines passing through a unit area perpendicular to the field is proportional to the magnitude of the electric field at that point.

8. Tangent to Field Lines:

   • The tangent to a field line at any point gives the direction of the electric field at that point.
     

9. Resultant Intensity:

   • At some points, the resultant intensity is equal to the sum of intensities due to positive and negative charges. The direction aligns with the tangent to the field.

10. Neutral Points:

    • Points where the resultant intensity of the electric field is zero are known as neutral points.

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Uniform and Non-Uniform Electric Fields

Uniform Electric Field

A field where the strength remains the same at all points is called a uniform electric field.

• Example: The field between two infinitely large parallel charged plates.

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Non-Uniform Electric Field

A field where the magnitude and direction of the electric intensity vary at different points is called a non-uniform electric field.

• Example: The field around a curved sheet of charges.

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Electric Field Between Two Parallel Plates

Consider two infinitely large parallel plates separated by a small distance:

• The upper plate has a uniform positive charge distribution, and the lower plate has a uniform negative charge distribution.
• Field lines start from the positive plate and terminate at the negative plate.
• At the ends of the plates, the field lines bulge out slightly, creating a fringing field, indicating the field is non-uniform near the edges.
  

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Electric Field on the Surface of a Metal Plate

If a charge $+q$ is placed near a metal plate:

1. The positive charge attracts the negative charges (electrons) in the metal plate.
2. These charges move until some reach the surface of the plate closest to $+q$ and come to rest.
3. The electric field lines originating from $+q$ terminate on the negative charges of the metal plate.
4. These lines are always perpendicular to the surface of the metal.

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Summary

Key Points

Concept	Details
Electric Field Lines	Represent the field’s strength and direction.
Characteristics	Radial direction, non-intersecting, proportional to field strength.
Uniform Field	Field strength is constant (e.g., between parallel plates).
Non-Uniform Field	Field strength varies (e.g., around a curved sheet of charge).
Field at Metal Plate Surface	Field lines are perpendicular to the surface due to charge redistribution.

Electric field lines provide an intuitive and visual representation of electric fields, making them essential in understanding field behavior around charges and conductors.

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References