A uniform electric field of \(2\times10^{3}\ \mathrm{N\,C^{-1}}\) acts along the x-axis. A flat square surface of side 20 cm is placed in this field. The normal to the surface makes an angle of \(60^\circ\) with the x-axis. The electric flux through the surface is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(40\ \mathrm{N\,m^{2}\,C^{-1}}\)
Electric flux tells us how many field lines pass through a surface. The flux of a uniform field through a flat surface is \( \phi=EA\cos\theta \). Here \( \theta \) is the angle between the field and the normal to the surface.
The area of the square of side 0.20 m is \( A=0.04\ \mathrm{m^{2}} \).
\( \begin{aligned} \phi &= EA\cos\theta \\ &= 2\times10^{3}\times0.04\times\cos60^\circ \\ &= 40\ \mathrm{N\,m^{2}\,C^{-1}} \end{aligned} \)
NCERT (2026-27):
Electric Flux – “We now come to the definition of electric flux. Electric flux Δφ through an area element ΔS is defined by Δφ = E.ΔS = E ΔS cosθ … which, as seen before, is proportional to the number of field lines cutting the area element. The angle θ here is the angle between E and ΔS. …”