Topic 10.311 min read
Electric Field Lines
Electric field lines are imaginary lines used to represent an electric field. The tangent to a line gives the field direction, while closer spacing indicates greater field intensity. Lines leave positive charges and enter negative charges, never intersect, and become straight, parallel and equally spaced in a uniform field between oppositely charged parallel plates, except near the edges where fringing occurs.
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The main idea
Electric fields cannot be seen directly, so physicists draw field lines. The arrows show the direction a small positive test charge would move, and the spacing shows where the field is strong or weak.
How to read a field-line diagram
- Arrows away from a charge mean the charge is positive.
- Arrows toward a charge mean the charge is negative.
- Closely spaced lines show a strong field.
- Widely spaced lines show a weak field.
- Parallel, equally spaced lines show a uniform field.
Important rule
Recommended for Class 11
Meaning of electric field lines
Michael Faraday introduced electric field lines as a way to visualize electric fields. They are imaginary lines that represent the direction and relative intensity of the field near charged bodies.
Electric fields exist in three dimensions. A diagram on a page shows only the field lines that lie in the selected two-dimensional plane, so the real pattern contains many more lines around the charge.
Direction at a point
At any point on a curved field line, draw a tangent. The tangent arrow gives the direction of E⃗ at that point and therefore the direction of force on a positive test charge.
- \vec F
- force on a test charge (N)
- q
- test charge (C)
- \vec E
- electric field intensity (N·C⁻¹)
For q>0, force and field have the same direction. For q<0, the force is opposite to the field arrows.
Isolated positive point charge
The field lines of an isolated positive point charge radiate outward in every direction. A flat diagram shows a wheel-like pattern, while the complete three-dimensional pattern is spherical.
The lines are closest near the charge and farther apart at greater distance, representing the inverse-square decrease of field intensity.
- Q
- source charge (C)
- r
- distance from the source (m)
Isolated negative point charge
The field lines of an isolated negative point charge point radially inward. They extend from far away and terminate on the negative charge.
| Source | Arrow direction | Force on a positive test charge |
|---|---|---|
| Positive point charge | Radially outward | Repelled away from the source |
| Negative point charge | Radially inward | Attracted toward the source |
Electric dipole field
Two equal-magnitude charges of opposite sign separated by a small distance form an electric dipole. Its field lines start on the positive charge and end on the negative charge.
The high density of lines between the charges represents a strong electric field in that region. The resultant field at each point is the vector sum of the fields produced by the positive and negative charges.
Field of two like charges
For two equal positive charges, field lines leave both charges and bend away from the region between them. No line joins one positive charge to the other.
At the midpoint, the two charges produce equal fields in opposite directions, so the resultant field is zero. This is a neutral or field-free point for the ideal equal-charge arrangement.
Two equal negative charges give a similar geometrical pattern, but every arrow direction is reversed.
Uniform field between parallel plates
Two oppositely charged parallel plates produce field lines that begin on the positive plate and end on the negative plate. Away from the plate edges, the lines are straight, parallel and equally spaced.
| Region | Field-line pattern | Nature of field |
|---|---|---|
| Central region between large plates | Straight, parallel and equally spaced | Approximately uniform |
| Near the ends of finite plates | Curved and changing separation | Non-uniform fringing field |
Fringing field
Real plates have finite length. Near their edges, electric field lines curve outward instead of remaining straight. This curved edge field is called the fringing field.
Because both direction and spacing change near the edges, the electric field is not uniform there. The uniform-field approximation works best well inside plates whose dimensions are much larger than their separation.
Rules for electric field lines
- Field lines never intersect.
- Lines originate on positive charge and terminate on negative charge.
- For an isolated charge, lines can extend to or arrive from infinity.
- The tangent at any point gives the direction of electric field intensity.
- The number of lines per unit area represents relative field intensity.
- Lines are closer where the field is strong and farther apart where it is weak.
- Field lines tend to contract in length, which visually represents attraction between opposite charges.
- N/A
- number of field lines crossing unit area normally (diagram convention)
When to use: Use qualitatively to compare field strength in different parts of the same field-line diagram.
Exam-ready comparison
| Pattern | Recognition feature |
|---|---|
| Positive point charge | Radial arrows outward |
| Negative point charge | Radial arrows inward |
| Electric dipole | Lines join positive charge to negative charge |
| Two like charges | Lines bend away; a cancellation point may occur between equal charges |
| Uniform plate field | Parallel and equally spaced lines |
| Fringing field | Lines curve near plate edges |
Exam Focus
Exam Focus
Important points
- Definition, tangent direction and density rule.
- Positive and negative point-charge patterns.
- Dipole and two-like-charge patterns.
- Zero-field midpoint for equal like charges.
- Uniform parallel-plate field and edge fringing.
- Complete field-line rule list.
Important formulas
- F=q E
- Enet=∑i Ei
- E∝NA
Common exam traps
- Field lines show E⃗ direction for a positive test charge.
- A zero resultant field can occur even when individual source fields are present.
Long question structure
Definition → tangent and density → isolated charges → dipole → like charges → parallel plates → fringing → rules
Common Mistakes
Common Mistakes
- Mistake
Drawing electric field lines from negative charge toward positive charge.
- Mistake
Allowing two field lines to intersect.
- Mistake
Using line direction instead of the tangent direction at a curved point.
- Mistake
Calling the field near plate edges uniform despite fringing.
- Mistake
Treating field lines as physical objects rather than a representation.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
Electric field lines are an imaginary visual representation of electric-field direction and intensity.
The tangent to a field line at any point gives the direction of E⃗ at that point.
Field lines point outward from positive charges and inward toward negative charges.
Field lines begin on positive charge and terminate on negative charge or extend to infinity for an isolated charge.
Electric field lines are imaginary lines whose tangent gives field direction and whose relative density represents field intensity.
F=q E, E∝NA
- Definition
- Electric field lines are imaginary lines whose tangent gives field direction and whose relative density represents field intensity.
- Formula
- F=q E, E∝NA
- Example
- Parallel, equally spaced lines between oppositely charged plates represent a uniform field; curved edge lines show fringing.
- Critical warning
- Field lines never intersect and always point from positive toward negative charge.
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Exam Practice
Important Question
Q1. Define electric field lines and list their important properties.
Diagram Explanation
Q2. Describe the field-line patterns of isolated positive and negative point charges.
Diagram Explanation
Q3. Describe the electric field lines of an electric dipole.
Reasoning
Q4. Why can electric field lines never intersect?
Comparison
Q5. Differentiate between a uniform field and a fringing field between parallel plates.