1. Concept Overview
Electric field lines are imaginary curves used to visually represent the electric field around charges. Instead of dealing only with vectors, field lines give an intuitive picture of how a charge influences the region around it. With a quick sketch, you can understand the strength, direction and overall pattern of an electric field.
2. Definition
Electric field lines are imaginary lines drawn in such a way that their direction at any point shows the direction of the electric field at that point. A small positive test charge placed in the field would move along these lines.
3. Purpose of Field Lines
Field lines help to:
- Visualise the direction of the electric field
- Identify regions of strong or weak field
- Understand the pattern created by single or multiple charges
- Interpret interactions between charges
4. Properties of Electric Field Lines
Field lines follow specific rules that reflect the behaviour of electric fields in nature.
4.1. Basic Properties
- Field lines start on positive charges and end on negative charges.
- They never form closed loops because electric fields originate and terminate on charges.
- They never intersect, since the field has only one direction at any point.
- The density of lines shows field strength: closely spaced lines indicate a strong field; widely spaced lines indicate a weak field.
4.2. Direction of Field Lines
The direction of an electric field line is the direction of force on a small positive test charge. So:
- Field lines point away from a positive charge.
- Field lines point towards a negative charge.
5. Field Line Patterns for Different Charge Arrangements
Different charge configurations produce different field line patterns that reveal how the field behaves.
5.1. Field Lines of a Single Positive Charge
Lines radiate outward uniformly in all directions. The pattern is spherically symmetric.
5.2. Field Lines of a Single Negative Charge
Lines point inward from all directions, showing attraction towards the charge.
5.3. Dipole Field Pattern
For a positive–negative pair, lines start from the positive charge and curve smoothly into the negative charge, forming a characteristic dipole shape.
5.4. Two Like Charges
Field lines repel each other between the charges, leaving a region with no lines. This shows the strong repulsion and zero field at the midpoint.
6. Field Line Density and Strength
Field strength is linked to how close the field lines are. The closer the lines, the stronger the electric field. This applies both to point charges and extended charge distributions.
6.1. Interpretation
- A region with tightly packed lines indicates a large value of \(E\).
- A region with sparse lines indicates a small value of \(E\).
7. Electric Field Lines and Conductors
Conductors influence the field around them in a predictable way because charges rearrange on their surfaces.
7.1. Key Points
- Field lines are always perpendicular to the surface of a conductor in electrostatic equilibrium.
- There are no field lines inside a conductor, meaning the electric field is zero.
- Excess charge on a conductor lives entirely on its surface.
8. Physical Interpretation
You can think of electric field lines as footprints showing how a tiny positive charge would move if released in the field. They show the invisible influence created by charges in a simple, visual form. Even though the lines are not real, they make it easy to understand electric interactions and the shape of electric fields in different setups.