Question:medium

For the given diagram, E=ϕxN/C, the net flux through the cube of side x=1 cm, placed at x=1 cm from the origin is:
Problem fig

Updated On: Mar 27, 2026
  • \(2 \times 10^{-6} \, \text{Wb} \\\)
  • \(1 \times 10^{-6} \, \text{Wb} \\\)
  • \(3 \times 10^{-6} \, \text{Wb} \\\)
  • \(4 \times 10^{-6} \, \text{Wb} \\\)
Show Solution

The Correct Option is B

Solution and Explanation

Provided Information:

  • Electric field \( E = \phi x \) N/C, with \( \phi = 1 \).
  • Cube side length is \( x = 1 \) cm, which is \( 0.01 \) m.
  • The cube is positioned at a distance of \( x = 1 \) cm (\( 0.01 \) m) from the origin.
  • The electric field is oriented along the positive \( x \)-axis.

Step 1: Determine the electric field at each relevant face

The cube possesses two faces that are perpendicular to the \( x \)-axis:

  • The left face is located at \( x = 0.01 \) m, as the cube starts 1 cm from the origin.
  • The right face is located at \( x = 0.02 \) m, due to the cube's 1 cm side length.

Calculated electric field values:

  • At the left face: \( E_{\text{left}} = \phi \times 0.01 = 1 \times 0.01 = 0.01 \) N/C.
  • At the right face: \( E_{\text{right}} = \phi \times 0.02 = 1 \times 0.02 = 0.02 \) N/C.

Step 2: Calculate the flux through each face

The formula for flux through a face is \( \Phi = E \cdot A \cdot \cos\theta \), where \( \theta \) represents the angle between the electric field and the face's normal vector.

For the left face:

  • The area \( A \) is \( (0.01)^2 = 1 \times 10^{-4} \) m².
  • The angle \( \theta \) is \( 180^\circ \) because the electric field points right, and the normal to the left face points left.
  • The flux \( \Phi_{\text{left}} \) is \( 0.01 \times 1 \times 10^{-4} \times \cos(180^\circ) = -1 \times 10^{-6} \) Wb.

For the right face:

  • The area \( A \) is \( (0.01)^2 = 1 \times 10^{-4} \) m².
  • The angle \( \theta \) is \( 0^\circ \) because both the electric field and the normal vector point right.
  • The flux \( \Phi_{\text{right}} \) is \( 0.02 \times 1 \times 10^{-4} \times \cos(0^\circ) = 2 \times 10^{-6} \) Wb.

For the remaining four faces, which are parallel to the \( x \)-axis, the flux is zero because the electric field is perpendicular to their respective normal vectors.

Step 3: Calculate the net flux

The net flux \( \Phi_{\text{net}} \) is the sum of the fluxes through the left and right faces: \( \Phi_{\text{net}} = \Phi_{\text{left}} + \Phi_{\text{right}} \).

\( \Phi_{\text{net}} = -1 \times 10^{-6} + 2 \times 10^{-6} = 1 \times 10^{-6} \) Wb.

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