Q.A hemisphere is uniformly charged positively. The electric field at a point on a diameter away from the centre is directed
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Gauss Law
Gauss's Law is a shortcut. Instead of adding up the Coulomb contribution of every charge — a nightmare of vectors and integrals — it lets you read the field straight off the symmetry of a problem. The whole idea rests on one quantity: electric flux.
Flux — field-lines counted through a surface. For a uniform field E crossing a flat area A, the flux is Φ = E·A = EA cosθ, where θ is the angle between the field and the normal to the surface. Picture the field as a bundle of lines; flux is how many pierce the surface. When E is edge-on (θ = 90°) nothing passes through and Φ = 0; when E is ⊥ to the surface (θ = 0) the count is maximal. For a closed surface, Φ is the net number of lines leaving it — lines that enter and exit cancel.
The law itself. Gauss's law states that the net flux out of any closed surface depends only on the charge trapped inside:
Φ = q_enclosed / ε₀.
Two consequences do most of the work. First, only enclosed charge counts — a charge outside sends as many lines in as out, so its net contribution is exactly zero. Second, the surface's shape is irrelevant; move the charge around inside or deform the surface, and Φ never changes.
Why symmetry makes it powerful. By itself Φ = q/ε₀ has E buried in an integral. It becomes a tool only when you pick a Gaussian surface matched to the symmetry — one where E is constant and everywhere either ⊥ to the surface (so Φ = EA) or ∥ to it (contributing nothing). Then E slides out and you solve in one line. This works for exactly three geometries:
1 — Infinite line charge (linear density λ). Use a coaxial cylinder: E = λ / 2πε₀r, falling off as ∝ 1/r.
2 — Infinite sheet (surface density σ). Use a pillbox pierced through the sheet: E = σ / 2ε₀ — uniform and completely independent of distance. The field near a large charged plane simply doesn't weaken as you step back. …
Why this formula?
Gauss's Law: Why It Holds
Gauss's Law is one of the four Maxwell's equations and a cornerstone of electromagnetism. Let's build the understanding from the ground up — not just the formula, but the why.
1. The Core Idea: Flux as "Flow" of Field
Imagine an electric field E passing through a small patch of area dA. The electric flux through that patch is:
dΦE=E⋅dA=EdAcosθ
where θ is the angle between E and the outward normal to the surface.
Why this definition?
- If E is perpendicular to the surface (θ=0), maximum field "flows through".
- If E is parallel (θ=90∘), no flux — the field just slides along the surface.
Total flux through a closed surface S is:
ΦE=∮SE⋅dA
2. The Key Insight: Flux Depends Only on Enclosed Charge
Consider a single point charge +q at the centre of a spherical surface of radius r.
- By Coulomb's law, at every point on the sphere: E=4πε01r2q, radially outward.
- The area vector dA is also radially outward.
- So E⋅dA=EdA everywhere.
The total flux:
ΦE=∮EdA=E∮dA=(4πε01r2q)(4πr2)=ε0q
Notice: The r2 cancels! The flux is independent of the radius.
3. Why Shape Doesn't Matter
Now, what if the surface is not spherical but any closed shape enclosing the charge?
- Draw a small cone from the charge to the surface.
- The flux through a patch dA at distance r is dΦ=4πε01r2qcosθdA.
- But r2cosθdA is exactly the solid angle dΩ subtended by dA at the charge.
So dΦ=4πε0qdΩ.
Integrating over the entire closed surface: ∮dΩ=4π (total solid angle around a point).
Hence:
ΦE=4πε0q⋅4π=ε0q
Result: For any closed surface enclosing q, the flux is ε0q.
4. Multiple Charges: Superposition
If there are many charges q1,q2,…,qn inside the surface, the total electric field is the vector sum of individual fields:
E=E1+E2+⋯+En
Flux is linear: …
Concept: field of a uniformly charged hemisphere at points on its base.
By mirror symmetry about the plane through the axis and the diameter, the field at any point P on the diameter has no component out of that plane. At the centre, full symmetry makes the field purely axial (perpendicular to every diameter). Moving to a point away from the centre, the nearby part of the curved shell (whose outward normal there is nearly along the diameter) contributes an increasingly la …
By mirror symmetry the field at P lies in the plane containing the axis and the diameter; it is purely axial only at the centre and becomes increasingly aligned with the diameter near the rim, so at a general point away from the centre it is tilted towards the diameter — option (c).
Setting up the symmetry
Model the hemisphere as a uniformly (positively) charged hemispherical shell of radius R, flat circular face in a plane, with a diameter of that face lying along, say, the x-axis through the centre O. Let P be a point on this diameter at distance d from O (0<d<R), still in the plane of the flat face.
Step 1 — Kill the out-of-plane component. The hemisphere is symmetric under reflection through the plane containing the axis (the z-axis, perpendicular to the base) and the chosen diameter (the xz-plane). Every charge element at y>0 has a mirror partner at −y contributing an equal and opposite y-component of field at P (which itself sits at y=0). So Ey(P)=0: the resultant field must lie in the xz-plane, i.e., in the plane of the axis and the diameter.
Step 2 — What happens exactly at the centre. At O (d=0), the hemisphere has full rotational symmetry about the axis, so by the same mirror argument applied to EVERY diameter through O, all horizontal components cancel and only the axial (z) component survives. This is the familiar result EO=4ε0σ, directed along the axis, away from the curved surface — i.e., perpendicular to every diameter.
Step 3 — What happens near the rim. As P moves out to d→R (near the edge of the flat face), it approaches the ring where the curved surface meets the base — the "equator." Right there, the nearby patch of the curved shell is almost tangent to a vertical cylinder, i.e., its outward normal is nearly horizontal, along the diameter direction itself. A point just inside that patch sits in the field of what looks locally like a charged sheet whose normal is along the diameter — so the dominant, nearby contribution to E at points close to the rim is along the diameter, not axial. …
Concept: Superposition & Symmetry in Electrostatics
For a uniformly charged hemisphere, the electric field at a point on the axis (the diameter line) is not simply radial — it has a net direction due to the broken spherical symmetry.
Method: Superposition of Two Half-Spheres
Why this method?
A full uniformly charged sphere produces zero net field at its centre (by symmetry). A hemisphere is exactly half of that sphere. So we can think:
Full sphere = Hemisphere A + Hemisphere B (identical, oppositely oriented)
At the centre of the full sphere, the field is zero. Therefore, the field due to one hemisphere must be equal in magnitude and opposite in direction to the field due to the other hemisphere.
Steps
-
Imagine a full sphere of radius R, uniformly charged with total charge +2Q (so each hemisphere has charge +Q).
-
At the centre O of the full sphere, by symmetry:
Efull=0
-
Let Ehemi be the field at O due to one hemisphere (say the upper half).
-
The other hemisphere (lower half) produces field −Ehemi at O, so that:
Ehemi+(−Ehemi)=0
- Key result: The field at the centre of a uniformly charged hemisphere is not zero — it points away from the flat face (if positively charged). …
Here are the common mistakes students make when analyzing the electric field direction for a uniformly charged hemisphere, along with how to avoid each.
Mistake 1: Assuming the field is radial (like a full sphere)
The error: Students treat the hemisphere like a full sphere and conclude the field at a point on the axis (the diameter) is directed radially outward (away from the centre in all directions).
Why it’s wrong: A full sphere has spherical symmetry — every bit of charge pulls equally in all directions, so the net field at the centre is zero, and outside it is radial. A hemisphere breaks that symmetry. There is no charge on the missing half, so the field cannot be purely radial.
How to avoid: Always check for symmetry first.
- Full sphere: Symmetric → radial field.
- Hemisphere: Only half the charge exists. The missing half means the field will point away from the centre but also away from the flat face (i.e., along the axis, away from the flat side).
Mistake 2: Thinking the field points toward the flat face
The error: Some students imagine the field lines “leaking” out of the flat circular face and conclude the field points toward that face.
Why it’s wrong: The hemisphere is positively charged. Electric field lines point away from positive charge. The flat face has no charge (it’s an imaginary surface), so the field cannot point toward it. The field must point away from the bulk of the positive charge.
How to avoid: Remember the fundamental rule:
- Positive charge → field lines radiate outward.
- The field at any point is the vector sum of contributions from all charge elements. For a point on the axis (the diameter), the net field points away from the centre and away from the flat face — i.e., along the axis, outward from the curved side.
Mistake 3: Forgetting to use vector addition (superposition)
The error: Students try to guess the direction intuitively without summing contributions from all parts of the hemisphere.
Why it’s wrong: The field at a point is the vector sum of fields from every infinitesimal charge element. Without superposition, you miss that horizontal components cancel (due to symmetry about the axis) but vertical components add.
How to avoid: Always break the problem into components:
- Choose a coordinate system (e.g., axis along the diameter).
- For each charge element, find the field direction.
- Cancel components perpendicular to the axis (they sum to zero).
- Add components along the axis — they all point in the same direction (away from the flat face).
Mistake 4: Confusing “on a diameter” with “at the centre”
The error: Students think “on a diameter” means exactly at the centre of the hemisphere.
Why it’s wrong: The question says “on a diameter away from the centre” — meaning a point on the axis outside the centre, not at it. At the exact centre, the field is not zero (unlike a full sphere), but the direction is still along the axis away from the flat face.
How to avoid: Read carefully:
- “On a diameter” = on the axis of symmetry.
- “Away from the centre” = not at the centre, but somewhere along that line. …
Showing the 12 most recent of 33 on this concept.
- AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQQ.A spherical rubber balloon has the electric charges uniformly distributed over its surface. If the balloon is inflated further, the electric intensity E on the surface (A) increases (B) decreases (C) remains same (D) is zero
›Reveal solutionSolution
This tests how surface field strength changes for a charged sphere whose radius grows while total charge stays constant; the field decreases.
Concept and Intuition
Inflating the balloon spreads the same total charge Q over a larger surface area, so the surface charge density σ=Q/4πR2 drops. The electric field just outside a uniformly charged sphere depends only on Q and R (by Gauss's law, exactly as if all charge were concentrated at the centre), so as R grows for fixed Q, the field at the surface must weaken.
Step-by-Step Solution
- By Gauss's law, the field at the surface of a uniformly charged sphere of radius R carrying charge Q is E=4πε01R2Q.
- Inflating the balloon increases R but does not add or remove any charge, so Q stays constant. …
- AP EAPCET 2026Set eng-2026-05-15-AN1 markMCQQ.A cylinder of radius 50 cm and length 2m is placed along x-axis as shown in the figure. If electric field is 70i^ along its axis, the electric flux through the left face of the cylinder is (A) 50 (B) 52 (C) 55 (D) 58
›Reveal solutionSolution
A straightforward flux calculation: since the uniform field is parallel to the cylinder's axis, the flux through each flat end face equals E times the face's circular area, giving a magnitude of about 55 (SI units).
Concept and Intuition
Electric flux through a flat surface in a uniform field is Φ=E⋅A=EAcosθ, where θ is the angle between the field and the surface's (outward) normal. For a cylinder with its axis along x and a uniform field E=70i^ also along x, the two flat circular end faces have their normals exactly along (or exactly against) the field direction — so cosθ=±1 for these faces, and the flux magnitude reduces to simply E times the face area. (The curved lateral surface, by contrast, has its normal everywhere perpendicular to the axis, so it carries zero flux — consistent with the field lines simply passing straight through the cylinder without net flux crossing the sides.)
Step-by-Step Solution
- The left face is a circular disk of radius r=0.5 m, with area A=πr2.
- Compute the area: A=π×(0.5)2=π×0.25≈722×0.25=0.7857 m2. …
- AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQQ.Two large circular metal plates each of radius 50 cm carrying equal and unlike charges are parallel to each other. If the electric field between the plates is 720 NC−1, then the magnitude of charge on any one plate is (A) 7.5 nC (B) 10 nC (C) 2.5 nC (D) 5 nC
›Reveal solutionSolution
This tests the field between oppositely-charged parallel plates, E=σ/ε0, to find the charge on each plate. Answer: 5 nC.
Concept and Intuition
For a single charged plate (infinite sheet), the field on each side is σ/2ε0. But for two large plates with equal and opposite charges facing each other (a parallel-plate-capacitor-like arrangement), the fields from both plates point in the same direction in the region between them, so they add: Ebetween=2ε0σ+2ε0σ=ε0σ.
Step-by-Step Solution
- Field between two oppositely charged plates: E=ε0σ.
- Solve for surface charge density: σ=Eε0=720×8.854×10−12=6.3749×10−9 C/m2.
- Area of each circular plate: A=πr2=π(0.5)2=π(0.25)≈0.7854 m2. …
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Electric flux through cube of side 'a' enclosing charge 'q' is (A) q/a2 (B) q/ϵ0 (C) Zero (D) aϵ02q
›Reveal solutionSolution
Tests the core statement of Gauss's law: flux through a closed surface depends only on enclosed charge, not on the surface's shape, size, or the charge's exact position inside.
Concept and Intuition
Gauss's law is a global statement — it doesn't care whether the enclosing surface is a sphere, a cube, or any irregular shape, and it doesn't care exactly where inside the charge sits. All that matters is how much charge is inside. This is precisely why Gauss's law is so powerful for symmetric charge distributions: you're free to choose whatever surface makes the calculation easiest.
Step-by-Step Solution
- Gauss's law states ∮E⋅dA=ϵ0qenc for any closed surface.
- Here the closed surface is a cube of side a that encloses a charge q.
- Regardless of the cube's side length a or the position of q inside it, the total flux is simply Φ=ϵ0q. …
- AP EAPCET 2026Set eng-2026-05-18-AN1 markMCQQ.The net outward flux through surface of a box is 8.0×103 Nm2C−1. The net charge inside the box is (approximately) (A) 70 nC (B) 42 nC (C) 21 nC (D) 60 nC
›Reveal solutionSolution
Gauss's law directly relates the flux through a closed surface to the enclosed charge: q=ε0Φ≈70nC.
Concept and Intuition
Gauss's law states that the net electric flux through any closed surface depends only on the charge enclosed, regardless of the shape of the surface or the position of the charge inside it: Φ=ε0qenc.
Step-by-Step Solution
- Given Φ=8.0×103Nm2C−1.
- Rearranging Gauss's law: qenc=ε0Φ.
- Substitute ε0=8.85×10−12C2N−1m−2: qenc=8.85×10−12×8.0×103=7.08×10−8C.
- Convert: 7.08×10−8C=70.8nC≈70nC. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.The electric field intensity on the surface of a charged sphere of radius R and volume charge density ρ is (A) R23ϵ0 (B) 4πϵ01ρR2 (C) 3ϵ0R2 (D) 3ϵ0ρR
›Reveal solutionSolution
This tests Gauss's law applied to a uniformly charged solid sphere — the field just outside the surface behaves as if all charge were concentrated at the centre.
Concept and Intuition
For any spherically symmetric charge distribution, Gauss's law lets us treat the enclosed charge as if it were a point charge at the centre when evaluating the field outside (or at) the surface. So we first find the total charge in terms of the given volume charge density, then apply the point-charge field formula at r=R.
Step-by-Step Solution
- Total charge enclosed by the sphere: Q=ρ×Volume=ρ⋅34πR3.
- By Gauss's law, the field at the surface (r=R) is E=4πϵ01R2Q. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Two conducting thin concentric shells of radii r and 2r are shown in figure. Outer shell carries a charge Q. Inner shell is neutral. The charge that will flow from inner shell to earth after closing the switch s is [FIGURE] (two concentric conducting spherical shells, inner of radius r and outer of radius 2r; the outer shell carries charge Q; the inner shell is connected through a switch s to earth/ground) (A) Q (B) 3Q (C) 2Q (D) 4Q
›Reveal solutionSolution
Grounding the inner shell of a concentric-shell system forces its potential to zero; solving for the resulting charge shows exactly Q/2 flows to earth.
Concept and Intuition
When a conductor is grounded, its potential is fixed at zero (earth's potential), and charge flows onto or off it until that condition is met. For concentric shells, we use the fact that inside a uniformly charged shell the potential is constant and equals kqshell/Rshell, and we superpose contributions from every shell of charge present.
Step-by-Step Solution
- Let the final charge on the inner shell (radius r) be q. The outer shell (radius 2r) has total charge Q; by electrostatic induction its inner surface carries −q and its outer surface carries Q+q.
- Potential of the inner shell = (its own contribution) + (contribution of outer shell's charges, since inner shell is inside the outer shell):
V=rkq+2rk(−q)+2rk(Q+q)=rkq+2rkQ
- Setting V=0 (grounded): rq=−2rQ⇒q=−2Q. …
- AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQQ.Two charged conducting spheres of radii 5 cm and 10 cm have equal surface charge densities. If the electric field on the surface of the smaller sphere is E, then the electric field on the surface of the larger sphere is (A) 2E (B) 4E (C) 0.5E (D) E
›Reveal solutionSolution
This tests a subtlety of conductors: the field just outside a charged conducting surface depends only on the local surface charge density, not on the sphere's radius. Equal σ means equal E, regardless of size — the answer is E (unchanged).
Concept and Intuition
It's tempting to reach for E=kQ/r2 and think a bigger sphere (different Q, different r) must have a different field. But the correct starting point for a conductor's surface is Gauss's law applied to a small pillbox straddling the surface, which gives the general boundary condition E=σ/ε0 for ANY conductor surface — this holds locally, independent of the conductor's overall size or shape. Since surface charge density is defined as σ=Q/A=Q/(4πr2), a sphere's total charge Q does scale with r2 for a given σ, but the field expressed in terms of σ directly doesn't care about r at all.
Step-by-Step Solution
- Field just outside any conductor's surface: E=ε0σ (a standard result from Gauss's law at a conducting boundary).
- This expression contains no explicit dependence on the sphere's radius.
- Both spheres (radii 5 cm and 10 cm) are given to have the SAME surface charge density σ. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.In a region, the electric field is given by Eˉ=(3i^+5j^+7k^) NC−1. The electric flux through a surface of area 3 m2 in yz plane is (in SI units) (A) 21 (B) 15 (C) 12 (D) 9
›Reveal solutionSolution
Electric flux through a surface only depends on the field component perpendicular to it; for the yz-plane that's Ex, giving Φ=9 (SI units).
Concept and Intuition
Electric flux is the dot product Φ=E⋅A, where A points along the surface's normal. A surface lying in the yz-plane has its normal along the x-axis, so components of E along j^ and k^ lie within the plane and contribute nothing to flux through it — only the i^ component matters.
Step-by-Step Solution
- Surface lies in the yz-plane ⇒ its area vector is A=3i^ m2 (magnitude 3, along x).
- Given E=(3i^+5j^+7k^) N/C. …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.The electric flux through the surface of a thin spherical shell of radius 6 cm, having a point charge 2 μC at its center is (A) 36π×105 N m2 C−1 (B) 72π×105 N m2 C−1 (C) 36π×108 N m2 C−1 (D) 72π×103 N m2 C−1
›Reveal solutionSolution
Gauss's law says the flux through a closed surface depends only on the enclosed charge, never on the surface's size or shape — giving 72π×103 N m2C−1 regardless of the 6 cm radius given.
Concept and Intuition
Gauss's law states ∮E⋅dA=ε0qenc. This is a purely topological statement — as long as the same charge is enclosed, the total flux out of any closed surface around it (sphere, cube, irregular blob) is identical. The radius of the shell given here is a distractor; it doesn't enter the flux calculation at all.
Step-by-Step Solution
- Enclosed charge: q=2 μC=2×10−6 C.
- Φ=ε0q. Using 4πε01=k=9×109, we get ε01=4πk. …
- AP EAPCET 2025Set ap-2025-05-20-FN1 markMCQQ.A sphere of radius R and charge 'Q' is placed inside an imaginary sphere of radius 2R such that the centres of two spheres coincide. The electric flux linked with the imaginary sphere is (A) ε04Q (B) ε02Q (C) ε0Q (D) 2ε0Q
›Reveal solutionSolution
Gauss's law says the flux through a closed surface depends only on the enclosed charge, not on the surface's size or shape; since the imaginary sphere of radius 2R encloses the same charge Q as the inner sphere, the flux is simply Q/ε0.
Concept and Intuition
Gauss's law, ∮E⋅dA=ε0Qenc, is a statement about total enclosed charge, completely independent of the geometry of the enclosing (Gaussian) surface or the exact charge distribution inside it. Doubling or tripling the radius of the imaginary sphere changes nothing about the flux, as long as no additional charge is included or excluded.
Step-by-Step Solution
- The imaginary sphere of radius 2R is concentric with, and completely encloses, the charged sphere of radius R and charge Q.
- Total charge enclosed by the imaginary sphere: Qenc=Q (no additional charge outside the inner sphere but inside the imaginary sphere). …
- AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQQ.As shown in the figure, a surface encloses an electric dipole with charge ±6×10−6 C. The total electric flux through the closed surface is [FIGURE] (an oval closed surface enclosing an electric dipole shown as a rod with +Q labelled at its left end and −Q labelled at its right end) (A) +12×10−6 Nm2C−1 (B) −12×10−6 Nm2C−1 (C) Zero (D) +6×10−6 Nm2C−1
›Reveal solutionSolution
Gauss’s law says the net electric flux through a closed surface equals the net charge enclosed divided by ε₀. Since a dipole has equal and opposite charges, the net enclosed charge is zero, so the total flux is zero.
The key insight is that Gauss’s law relates the total electric flux through a closed surface to the net charge inside that surface — not to the arrangement or separation of the charges. A dipole consists of two equal-magnitude, opposite-sign charges. When both are fully inside the closed surface, their contributions to the net enclosed charge cancel exactly.
- State Gauss’s law For any closed surface, the total electric flux Φₑ is given by
Φe=ε0Qenc
where Qenc is the algebraic sum of all charges inside the surface.
- Identify the enclosed charges The dipole has +Q=+6×10−6 C at one end and −Q=−6×10−6 C at the other. Both charges lie completely inside the oval surface.
Qenc=(+6×10−6)+(−6×10−6)=0
- Apply Gauss’s law With Qenc=0,
Φe=ε00=0
The flux is zero regardless of the shape of the surface or the positions of the charges inside, as long as both are enclosed. …
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.