Physics · Electric Charges And Fields · NEET
Electric flux is a SCALAR quantity, even though it is made from two vectors (E and the area vector S). It comes from the dot product E·S = E S cosθ, and a dot product always gives a scalar (a number with a sign, no direction). So flux has a value and a sign (positive or negative), but no direction of its own.
Only the part of the field that passes straight through the surface counts. We measure the angle θ between the field E and the normal (the line perpendicular to the surface). The 'through' part of E is E cosθ, so flux = E cosθ × S. When the field is perpendicular to the surface (θ = 0), cosθ = 1 and flux is maximum E S. When the field lies along the surface (θ = 90°), cosθ = 0 and flux is zero because no line pierces the surface.
The SI unit is N m²/C (newton metre squared per coulomb). This comes from Φ = E × S, where E is in N/C and S is in m². An equivalent unit is volt-metre (V m), since 1 N/C = 1 V/m.
Yes. Flux is negative when field lines enter a surface (θ between 90° and 180°, so cosθ is negative), and positive when lines leave it. For a closed surface we fix the outward normal as positive, so outgoing lines give positive flux and incoming lines give negative flux. Net flux is the sum of both.
Electric field E is a vector at a single point that tells the force per unit charge there. Electric flux Φ is a scalar for a whole surface that tells how many field lines cross that surface. Field is 'strength at a point'; flux is 'total lines through an area'. You need both E and the area to get flux.
The area vector S has magnitude equal to the area and direction along the normal (perpendicular to the surface). For a flat open surface either normal can be chosen; for a closed surface the outward normal is always taken as positive. Writing area as a vector lets us use the neat dot product Φ = E·S.
A sphere encloses an electric dipole with charges ±3 × 10⁻⁶ C. What is the total electric flux across the sphere?
According to Gauss's law, the electric flux through a closed surface depends on:
A charge Q µC is placed at the centre of a cube. The flux coming out from any one of its faces will be (in SI unit):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a count of how many electric field lines pass through a chosen surface. More lines through the surface means more flux.
For a uniform field, Φ = E·S = E S cosθ. For a general (varying) field, Φ = ∮ E·dS, adding up E·dS over every small piece of the surface.
Flux is maximum when the field is perpendicular to the surface (θ = 0, cosθ = 1). Flux is zero when the field is parallel to the surface (θ = 90°, cosθ = 0).
No. Zero net flux only means equal lines enter and leave (net enclosed charge is zero). The field on the surface can still be strong. This is a common NEET trap.
It is the base concept for Gauss's law, which NEET uses every year to find fields of spheres, sheets and wires quickly. Understanding flux, its cosθ factor, and its sign saves you time on those questions.
