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SubjectsPhysics
गाउस का नियम और इसके अनुप्रयोग
In this Class 12 Physics topic from Chapter 1, Electric Charges and Fields, students learn how electric flux is related to the net charge enclosed by a closed surface through Gauss’s law. The topic develops the idea of Gaussian surfaces, uses symmetry to simplify electric-field calculations, and applies the law to charged spherical shells, uniformly charged spheres, infinite line charges, and plane sheets. It also helps students understand the electric field inside conductors and choose suitable surfaces for solving electrostatic problems.
Correct answer: A. Because an oblique field has a tangential component that would move charges
Explanation: Step 1: In an electrostatic conductor, free charges on the surface are at rest. Step 2: A tangential component of an oblique field would move them. Step 3: Therefore the field outside must be perpendicular to the surface.
Correct answer: A. Because electric field inside conductor material is zero in electrostatic condition
Explanation: Step 1: Inside conductor material in electrostatic condition, electric field is zero. Step 2: Therefore total flux through an internal closed surface is zero. Step 3: By Gauss's law, net charge enclosed by that surface must be zero.
Correct answer: A. Because excess charge inside would not allow zero electric field inside
Explanation: Step 1: Electric field inside an electrostatic conductor must be zero. Step 2: If excess net charge were inside, Gauss's law would give non-zero flux. Step 3: Hence excess charge arranges itself on the surface.
Correct answer: A. Zero
Explanation: Step 1: Electric field inside conductor material is zero. Step 2: An empty cavity has no internal charge to start or end field lines. Step 3: In electrostatic condition, the conductor shields the cavity, so field inside it is zero.
Correct answer: A. Zero
Explanation: Step 1: Even with a charge in the cavity, conductor material maintains zero field in electrostatic condition. Step 2: Induced charges on surfaces arrange to ensure this. Step 3: Therefore electric field inside conductor material remains zero.
Correct answer: A. Equal magnitude negative charge
Explanation: Step 1: Electric field inside conductor material must be zero. Step 2: A Gaussian surface in conductor material around the cavity has zero flux. Step 3: Hence induced charge on the inner surface must be equal and opposite to the positive charge in the cavity.
Correct answer: A. Because outside field is directly related to surface charge density
Explanation: Step 1: Field just outside a conductor surface depends on surface charge density. Step 2: Denser charge gives denser field lines. Step 3: Therefore field can be stronger near sharp parts.
Correct answer: C. Positive
Explanation: Step 1: Outward field gives positive flux through a closed surface. Step 2: Non-zero outward field everywhere gives positive total flux. Step 3: By Gauss's law, net enclosed charge is positive.
Correct answer: B. Negative
Explanation: Step 1: Area vector of a closed surface is outward. Step 2: Inward field is opposite to the area vector. Step 3: Total flux is negative, so net enclosed charge is negative.
Correct answer: A. Such field gives positive total flux, inconsistent with zero net enclosed charge
Explanation: Step 1: If net enclosed charge is zero, total flux must be zero. Step 2: If field is outward everywhere, every part gives positive contribution. Step 3: That gives positive total flux, so the statement is inconsistent.
Correct answer: A. Because the idea of enclosed charge is clear only for a closed surface
Explanation: Step 1: Gauss's law relates total flux to a closed surface. Step 2: For an open surface, inside and outside are not clearly defined. Step 3: Therefore the Gaussian surface must be closed.
Correct answer: C. Zero
Explanation: Step 1: Flux comes from the component of field crossing the surface. Step 2: A parallel field does not pass through the surface. Step 3: Hence flux through every part and total flux are zero.
Correct answer: A. Both statements cannot be true together
Explanation: Step 1: In Gauss's law, total flux is proportional to net enclosed charge. Step 2: If net enclosed charge is zero, total flux must be zero. Step 3: Hence positive flux and zero enclosed charge are inconsistent.
Correct answer: A. It becomes difficult to treat field as uniform on the Gaussian surface
Explanation: Step 1: Gauss's law remains true even without symmetry. Step 2: But to find field, flux must be written in a simple form. Step 3: Without symmetry, field is not uniform or simply directed on the surface.
Correct answer: A. No, total flux is determined by net enclosed charge
Explanation: Step 1: External charge can change field direction and magnitude on the surface. Step 2: But in Gauss's law total flux is determined by net enclosed charge. Step 3: If enclosed charge is positive, total flux remains positive.
Correct answer: A. One fourth
Explanation: Step 1: Outside a conducting sphere, field behaves like that of a point charge at the centre. Step 2: Point charge field decreases inversely as square of distance. Step 3: Doubling distance makes field one fourth.
Correct answer: B. Half
Explanation: Step 1: Field of a long line charge is inversely proportional to distance. Step 2: Doubling distance halves the field. Step 3: Do not confuse it with inverse square relation of point charge.
Correct answer: C. Unchanged
Explanation: Step 1: For an infinite plane sheet, Gauss's law gives field independent of distance. Step 2: Doubling distance does not change the field in the ideal infinite sheet case. Step 3: This result depends on infinite sheet symmetry.
Correct answer: B. Point charge inverse square, line charge inverse distance, infinite sheet distance independent
Explanation: Step 1: For a point charge, spherical area grows as square of distance. Step 2: For a line charge, cylindrical curved area grows with distance. Step 3: For an infinite sheet, field is independent of distance.
Correct answer: A. Double
Explanation: Step 1: In this special case, total flux equals field times total area. Step 2: Field is unchanged and area is doubled. Step 3: Therefore total flux doubles, meaning enclosed charge must also be doubled.
Correct answer: B. Negative
Explanation: Step 1: Area vectors of a closed surface point outward. Step 2: Inward field is opposite to them. Step 3: Hence total flux is negative and net enclosed charge is negative.
Correct answer: A. Because one face can be outside in field and the other inside the conductor where field is zero
Explanation: Step 1: Inside a conductor in electrostatic condition, electric field is zero. Step 2: A small pillbox can be chosen crossing the surface. Step 3: This gives a simple relation between flux through the outer face and surface charge density.
Correct answer: B. No, positive and negative local flux may cancel
Explanation: Step 1: Total flux is an algebraic sum. Step 2: Contributions from different parts may cancel even when local field is non-zero. Step 3: Therefore zero total flux does not mean field is zero everywhere.
Correct answer: A. No, unless symmetry makes the field simple
Explanation: Step 1: Gauss's law gives total flux. Step 2: Total flux is a surface sum, not field at one point. Step 3: Strong symmetry is needed to extract the electric field simply.
Correct answer: A. Whether surface is closed, what net charge is enclosed, and whether symmetry is useful
Explanation: Step 1: Gauss's law for total flux requires a closed surface. Step 2: Total flux is determined by net enclosed charge. Step 3: To find field, symmetry is needed, so check these three things first.
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