Physics Tutor in Sector 62 Noida | Kumar Physics Classes
+91-9958461445
If you are living in Sector 62 Noida and Physics is becoming difficult for you, then you are not alone. Many students feel that Physics is the most challenging subject, especially when they prepare for NEET, JEE Main, JEE Advanced, CBSE, ICSE, AP Physics, IB Physics or A-Level Physics.
NEET Physics is often considered one of the most scoring but also one of the most dangerous sections because a small conceptual mistake can change the answer completely. JEE Physics is also highly challenging because it requires deep conceptual clarity, mathematical application, logical thinking and problem-solving speed.
At Kumar Physics Classes, Mr. Kumar teaches Physics with strong conceptual clarity, proper numerical practice and exam-oriented preparation. Students are guided through standard books like H.C. Verma, NCERT, previous year questions, NEET-level problems and JEE-level conceptual questions.
Website: https://kumarphysicsclasses.com/
Contact: +91-9958461445
Email: kumarsirphysics@gmail.com
Why NEET and JEE Physics Need Strong Concept Clarity
Physics is not just about memorising formulas. In NEET Physics, students need speed, accuracy and concept clarity. In JEE Physics, students need deeper thinking, multi-concept application and strong mathematical handling.
For example, in Electrostatics, many students remember Gauss Law but do not understand Electric Flux properly. Without understanding Electric Flux, students cannot solve cube-based flux questions, symmetry-based questions and closed surface problems.
Mr. Kumar focuses on:
Concept clarity
Numerical practice
NCERT-based understanding
H.C. Verma-based explanation
NEET previous year questions
JEE Main and Advanced level problems
Board exam preparation
Step-by-step doubt clearing
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Electric Flux Through a Cube: Important Conceptual Cases
Gauss Law:
Net Electric Flux through closed surface = q enclosed / epsilon_0
Copy-paste friendly formula:
Phi = q / epsilon_0
For a cube, if charge q is placed symmetrically, total flux is divided equally among equivalent faces.
Case 1: Charge q at the Centre of the Cube
If a point charge q is placed at the centre of a cube, total flux through the cube is:
Total flux = q / epsilon_0
Since the cube has 6 identical faces, flux through each face is:
Flux through each face = q / 6 epsilon_0
Answer:
Each face gets flux = q / 6 epsilon_0
Case 2: Charge q at One Corner of the Cube
If charge q is placed at one corner of a cube, imagine 8 identical cubes joined together to make a bigger cube. The charge becomes at the centre of the bigger cube.
Total flux through bigger cube = q / epsilon_0
Flux through one small cube = q / 8 epsilon_0
Answer:
Flux through the given cube = q / 8 epsilon_0
Important note:
The three faces passing through the charge have zero flux because electric field is parallel to those faces.
Flux comes out through the three opposite faces.
By symmetry:
Flux through each opposite face = q / 24 epsilon_0
Case 3: Charge q at the Centre of One Face of the Cube
If charge q is placed at the centre of one face of the cube, imagine 2 identical cubes joined face to face. The charge becomes inside the combined closed surface.
Total flux through both cubes = q / epsilon_0
Flux through one cube = q / 2 epsilon_0
Answer:
Flux through the given cube = q / 2 epsilon_0
Important note:
Flux through the face on which charge is placed is zero because that face passes through the charge and field is along the plane.
Case 4: Charge q at the Centre of One Edge of the Cube
If charge q is placed at the centre of one edge of the cube, imagine 4 identical cubes arranged around that edge. The charge becomes inside the combined closed surface.
Total flux = q / epsilon_0
Flux through one cube = q / 4 epsilon_0
Answer:
Flux through the given cube = q / 4 epsilon_0
Case 5: Charge q Inside the Cube but Not at Centre
If charge q is anywhere inside the cube, total flux through the cube is still:
Total flux = q / epsilon_0
Answer:
Total flux through cube = q / epsilon_0
But flux through each face will not be equal because the charge is not symmetrically placed.
Case 6: Charge q Outside the Cube
If charge q is outside the cube, no charge is enclosed inside the cube.
Therefore:
Net flux = 0
Answer:
Net flux through cube = 0
Important note:
Electric field may pass through the cube, but net flux is zero because field lines entering and leaving are equal.
Case 7: Equal Positive and Negative Charges Inside the Cube
Suppose +q and -q are both inside the cube.
Net charge enclosed = +q – q = 0
Therefore:
Net flux = 0
Answer:
Net flux through cube = 0
Case 8: Two Charges q1 and q2 Inside the Cube
If two charges q1 and q2 are inside the cube:
Net enclosed charge = q1 + q2
Therefore:
Net flux = (q1 + q2) / epsilon_0
Answer:
Flux through cube = (q1 + q2) / epsilon_0
Case 9: Charge q at the Centre of a Face, Asked Flux Through Opposite Face
This is a common tricky question.
Total flux through one cube = q / 2 epsilon_0
But flux through individual faces is not always equally divided unless symmetry is clear.
For NEET and JEE level, remember:
Total flux through the cube = q / 2 epsilon_0
Do not blindly divide by 5 unless symmetry is clearly given.
Case 10: Charge q at Corner, Flux Through Three Opposite Faces
For charge at one corner:
Total flux through cube = q / 8 epsilon_0
Flux through three faces passing through charge = 0
Remaining flux comes through three opposite faces equally.
Therefore:
Flux through each opposite face = q / 24 epsilon_0
Quick Summary Table
Charge at centre of cube: Total flux = q / epsilon_0, flux per face = q / 6 epsilon_0
Charge at corner of cube: Total flux = q / 8 epsilon_0, flux through each opposite face = q / 24 epsilon_0
Charge at centre of face: Total flux through cube = q / 2 epsilon_0
Charge at centre of edge: Total flux through cube = q / 4 epsilon_0
Charge inside cube but not at centre: Total flux = q / epsilon_0, face-wise flux unequal
Charge outside cube: Net flux = 0
Equal +q and -q inside cube: Net flux = 0
Charges q1 and q2 inside cube: Net flux = (q1 + q2) / epsilon_0
Final Note
Physics becomes easy when concepts are explained properly. Electric Flux and Gauss Law are very important for NEET, JEE, CBSE and international Physics exams. At Kumar Physics Classes, Mr. Kumar teaches Physics in a clear, logical and exam-oriented way so that students can understand concepts and solve numerical problems confidently.
For Physics Tutor in Sector 62 Noida, contact Kumar Physics Classes.
Website: https://kumarphysicsclasses.com/
Contact: +91-9958461445
Email: kumarsirphysics@gmail.com
40 Conceptual Questions on Electric Flux Through a Cube
With Answers | Kumar Sir Style
Below are 40 conceptual questions on Electric Flux through a cube. These questions are useful for CBSE, NEET, JEE Main, JEE Advanced, AP Physics, IB Physics and A-Level Physics students.
All formulas are written in simple copy-paste friendly format.
Basic Formula
According to Gauss Law:
Net electric flux through a closed surface = charge enclosed / epsilon_0
Formula:
Phi = q / epsilon_0
For a cube:
If charge is placed symmetrically, flux is divided equally among equivalent faces.
1. A charge q is placed at the centre of a cube. What is the total flux through the cube?
Answer:
Total charge enclosed is q.
Therefore:
Total flux = q / epsilon_0
2. A charge q is placed at the centre of a cube. What is the flux through each face?
Answer:
A cube has 6 identical faces.
Total flux = q / epsilon_0
Flux through each face = q / 6 epsilon_0
3. Why is flux equally divided among all faces when charge is at the centre?
Answer:
Because the charge is symmetrically placed at the centre and all 6 faces are equivalent.
4. If charge q is inside the cube but not at the centre, what is the total flux?
Answer:
Total flux depends only on enclosed charge.
Total flux = q / epsilon_0
5. If charge q is inside the cube but not at the centre, is flux through each face equal?
Answer:
No. Total flux is q / epsilon_0, but face-wise flux is not equal because symmetry is lost.
6. A charge q is placed outside the cube. What is net flux through the cube?
Answer:
No charge is enclosed inside the cube.
Net flux = 0
7. If electric field lines enter and leave the cube due to an outside charge, why is net flux zero?
Answer:
Because the number of field lines entering the cube is equal to the number of field lines leaving the cube.
8. A charge q is placed at one corner of a cube. What is total flux through the cube?
Answer:
Imagine 8 identical cubes joined together. The charge becomes at the centre of the bigger cube.
Total flux through one small cube = q / 8 epsilon_0
9. Why do we use 8 cubes for a charge at the corner?
Answer:
Because 8 identical cubes can be arranged around that corner to make the charge lie at the centre of a bigger cube.
10. Charge q is at one corner of a cube. What is flux through the three faces meeting at that corner?
Answer:
Flux through the three faces passing through the charge is zero because electric field is parallel to those faces.
11. Charge q is at one corner. What is flux through each of the three opposite faces?
Answer:
Total flux through the cube = q / 8 epsilon_0
This flux passes through three opposite faces equally.
Flux through each opposite face = q / 24 epsilon_0
12. Charge q is at the centre of one face of the cube. What is total flux through the cube?
Answer:
Imagine 2 identical cubes joined face to face. The charge becomes inside the combined surface.
Total flux through one cube = q / 2 epsilon_0
13. Why do we use 2 cubes for charge at the centre of a face?
Answer:
Because two identical cubes joined through that face make the charge lie inside the combined closed surface symmetrically.
14. Charge q is at the centre of one face. What is flux through the face containing the charge?
Answer:
Flux through that face is zero because the electric field is along the plane of that face.
15. Charge q is at the centre of one edge of a cube. What is total flux through the cube?
Answer:
Imagine 4 identical cubes arranged around that edge.
Total flux through one cube = q / 4 epsilon_0
16. Why do we use 4 cubes for charge at the centre of an edge?
Answer:
Because 4 cubes can be arranged around that edge so that the charge becomes symmetrically enclosed in the combined surface.
17. Charge q is at the centre of cube. If q is doubled, what happens to total flux?
Answer:
Flux is directly proportional to charge.
New flux = 2q / epsilon_0
18. Charge q is at the centre. If side length of cube is doubled, what happens to total flux?
Answer:
Total flux does not change because flux depends only on enclosed charge.
Total flux = q / epsilon_0
19. Charge q is at the centre. If side length is doubled, what happens to flux through each face?
Answer:
Flux through each face remains the same.
Flux through each face = q / 6 epsilon_0
20. Why does flux not depend on size of the cube?
Answer:
Because Gauss Law says total flux depends only on enclosed charge, not on size or shape of the closed surface.
21. If +q and -q are both inside a cube, what is net flux?
Answer:
Net enclosed charge = +q – q = 0
Net flux = 0
22. If +2q and -q are inside a cube, what is net flux?
Answer:
Net charge enclosed = +2q – q = q
Net flux = q / epsilon_0
23. If q1 and q2 are inside a cube, what is net flux?
Answer:
Net enclosed charge = q1 + q2
Net flux = (q1 + q2) / epsilon_0
24. If q is inside the cube and another q is outside the cube, what is net flux?
Answer:
Only inside charge contributes to net flux.
Net flux = q / epsilon_0
25. Does outside charge affect electric field on the cube surface?
Answer:
Yes, outside charge can affect electric field at the surface, but it does not affect net flux through the closed cube.
26. A charge q is placed exactly on a face but not at the centre. What can we say about total flux through the cube?
Answer:
If charge lies on the surface, the standard simple answer depends on how the surface is treated. For ideal symmetric face-centre case, flux through one cube is:
Flux = q / 2 epsilon_0
But if it is not at the centre, face-wise distribution becomes complicated.
27. A charge q is placed exactly on an edge but not at the centre. What is the idea?
Answer:
If charge lies on an edge, multiple cubes may be imagined around that edge. For the symmetric edge-centre case:
Flux through one cube = q / 4 epsilon_0
If not at centre, distribution may not be simple.
28. A charge q is placed exactly at a vertex. Does the cube enclose the full charge?
Answer:
No. By symmetry, only one-eighth contribution belongs to one cube.
Flux through cube = q / 8 epsilon_0
29. Why is charge at corner treated as q / 8 for one cube?
Answer:
Because 8 cubes share that corner equally.
30. Why is charge at face centre treated as q / 2 for one cube?
Answer:
Because 2 cubes share that face equally.
31. Why is charge at edge centre treated as q / 4 for one cube?
Answer:
Because 4 cubes share that edge equally.
32. If a charge is at the centre of a cube, can flux through one face be negative?
Answer:
No. For a positive charge at the centre, field lines go outward through all faces, so flux through each face is positive.
33. If a negative charge is at the centre of a cube, what is total flux?
Answer:
For charge -q:
Total flux = -q / epsilon_0
Flux is negative because field lines enter the cube.
34. If a negative charge -q is at the centre, what is flux through each face?
Answer:
Flux through each face = -q / 6 epsilon_0
35. If a positive charge is at the centre and a negative charge of equal magnitude is outside, what is net flux?
Answer:
Outside charge does not contribute to net flux.
Net flux = q / epsilon_0
36. If no charge is inside the cube but electric field exists everywhere, what is net flux?
Answer:
Net flux through the cube is zero if no charge is enclosed.
37. If uniform electric field passes through a cube, what is net flux?
Answer:
Net flux is zero because flux entering one face is equal to flux leaving the opposite face.
38. In a uniform electric field, is flux through one face zero?
Answer:
Not necessarily. Flux through one face may be positive or negative, but total flux through the cube is zero.
39. What is the biggest mistake students make in cube flux questions?
Answer:
The biggest mistake is dividing flux equally without checking symmetry. Equal division is allowed only when faces are equivalent by symmetry.
40. What is the final rule for cube flux questions?
Answer:
Use Gauss Law first.
If charge is inside:
Total flux = q / epsilon_0
If charge is at centre:
Flux per face = q / 6 epsilon_0
If charge is at corner:
Flux through cube = q / 8 epsilon_0
If charge is at face centre:
Flux through cube = q / 2 epsilon_0
If charge is at edge centre:
Flux through cube = q / 4 epsilon_0
If charge is outside:
Net flux = 0
Kumar Sir Style Final Note
Electric Flux through a cube is not difficult if the student understands symmetry and Gauss Law properly. Do not blindly apply formulas. First check where the charge is placed, then decide whether symmetry is present or not.
At Kumar Physics Classes, Mr. Kumar teaches such concepts in a simple and logical way so that students can handle NEET, JEE Main, JEE Advanced, CBSE, AP Physics, IB Physics and A-Level Physics questions confidently.
Website: https://kumarphysicsclasses.com/
Contact: +91-9958461445
Email: kumarsirphysics@gmail.com