Q.A positive point charge $+q$ is brought near an isolated (uncharged) conducting sphere. Four candidate electric-field-line patterns are proposed for this situation. In one pattern the field lines leave $+q$, reach the near side of the sphere and continue from its far side, and everywhere they touch the sphere they meet its surface at right angles (perpendicular), with a pattern symmetric about the line joining the charge and the sphere's centre. In a second pattern the lines on the far side of the sphere bend back and point toward the sphere and do not meet the surface perpendicularly. In a third pattern the lines continue past the sphere pointing outward but strike the sphere's surface obliquely rather than at right angles. In a fourth pattern the field lines on the far side of the sphere point inward, toward the sphere (arrows reversed). Which pattern correctly represents the electric field?

What Are Electric Field Lines?

Imagine you're standing in a field of invisible forces. Every positive charge pushes other positive charges away, and every negative charge pulls them in. If you could release a tiny positive test charge anywhere in space, it would instantly feel a push in some direction — that direction is the electric field at that point.

Now, if you let that test charge move freely, it would trace out a path through space. That path is an electric field line. It's not a real physical line — it's a visual tool, like contour lines on a map, that shows you which way the electric force points at every location.

The Four Rules, Built from Intuition

1. Field lines start on positive charges and end on negative charges

Think of a positive charge as a source that "emits" field lines outward in all directions. A negative charge is a sink — lines "drain" into it. If you have a single isolated positive charge, its field lines radiate outward to infinity. A single negative charge has lines coming in from infinity.

Why? Because the electric field points away from a positive charge (repelling a test positive charge) and toward a negative charge (attracting it). The line simply follows that direction from start to finish.

2. Field lines never intersect

At any point in space, the electric field has one unique direction. If two field lines crossed, that point would have two different directions for the field — which is impossible. The field can't point both left and right at the same spot.

3. The density of field lines tells you the field strength

Where field lines are packed closely together, the electric field is strong. Where they are spread far apart, the field is weak. This is a visual convention: we draw more lines per unit area in regions of stronger field.

For a point charge, lines spread out as you move away — the same number of lines passes through larger and larger spheres, so the density drops as $1/r^2$, exactly matching Coulomb's law.

4. Field lines are perpendicular to the surface of a conductor

When you place a conductor in an electric field, charges inside rearrange until the field inside becomes zero. At the surface, the field must be perpendicular — if it had a component parallel to the surface, charges would keep moving along the surface. So field lines always meet a conductor's surface at a right angle.

Putting It All Together

Consider two equal positive charges placed a distance apart. The field lines bulge outward from each charge, and in the region exactly midway between them, the fields from both charges cancel — the field is zero there. No line passes through that point. The lines curve away from the midpoint, never crossing, and eventually go to infinity.

For a positive and a negative charge (a dipole), lines start on the positive, curve smoothly through space, and end on the negative. The densest crowding occurs near the charges themselves, where the field is strongest.

Why These Rules Matter

These properties let you sketch the electric field for any charge distribution without doing a single calculation. You can see where the field is strong, where it's zero, and how charges interact — all from a simple drawing. In exams, these rules are the foundation for solving problems on field patterns, conductors, and equilibrium of charges.

The key insight: field lines are not real, but they obey real physics. Every rule follows directly from the definition of the electric field itself.

Properties of electric field lines are introduced early in the NCERT Class 12 Physics Electrostatics chapter, and 'properties of electric field lines class 12 physics' or 'electric field lines important questions' are common searches among students revising for boards. These four rules are also the basis for several JEE Main and NEET conceptual questions on field patterns around conductors and dipoles.