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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 outward field everywhere would give positive total flux
Explanation: Step 1: If net enclosed charge is zero, total flux must be zero. Step 2: If field is outward everywhere, every contribution is positive. Step 3: That would give positive total flux, so the statement conflicts with Gauss's law.
Correct answer: A. No, as long as net enclosed charge is positive total flux cannot be zero
Explanation: Step 1: Net closed flux is determined by net enclosed charge, not by surface shape. Step 2: If enclosed charge is positive, total flux remains positive. Step 3: External changes may alter local field, not total flux.
Correct answer: A. Total flux changes from zero to positive
Explanation: Step 1: Initially net enclosed charge was zero, so total flux was zero. Step 2: Adding a positive charge makes net enclosed charge positive. Step 3: Therefore total flux becomes positive.
Correct answer: A. It becomes zero
Explanation: Step 1: Initially the net enclosed charge was positive. Step 2: Adding an equal magnitude negative charge makes the algebraic sum zero. Step 3: Therefore total closed flux becomes zero.
Correct answer: A. Positive
Explanation: Step 1: In Gauss's law, only enclosed charge is counted. Step 2: The outside negative charge is not included in total closed flux. Step 3: Since the inside charge is positive, total flux is positive.
Correct answer: A. Negative
Explanation: Step 1: The outside positive charge can change field on the surface. Step 2: But total closed flux is decided by net enclosed charge. Step 3: Since enclosed charge is negative, total flux remains negative.
Correct answer: A. Because all points on the curved surface are at the same distance from the line
Explanation: Step 1: An infinite line charge has cylindrical symmetry. Step 2: All points on the curved surface of the Gaussian cylinder are at the same radial distance from the line. Step 3: Therefore field magnitude is the same there.
Correct answer: A. Zero degree
Explanation: Step 1: Field of a line charge is radial. Step 2: Area vector of the curved cylindrical surface is also radially outward. Step 3: For a positive line, both are in the same direction, so the angle is zero degree.
Correct answer: A. Because symmetry is same on both sides of the sheet
Explanation: Step 1: An infinite plane sheet has identical symmetry on both sides. Step 2: Field magnitude is same on both sides. Step 3: Hence flux through the two flat faces of the pillbox is equal.
Correct answer: A. Because the field does not cross that part
Explanation: Step 1: Flux is related to the component of field crossing a surface. Step 2: A parallel field runs along the surface and does not pass through it. Step 3: Therefore flux through that part is zero.
Correct answer: A. Both statements cannot be correct together
Explanation: Step 1: According to 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: Therefore positive total flux and zero enclosed charge are inconsistent.
Correct answer: A. It doubles
Explanation: Step 1: In this special case, total flux is field multiplied by total area. Step 2: Field is same and area is doubled. Step 3: Therefore total flux doubles, implying enclosed charge must also be doubled.
Correct answer: A. Negative
Explanation: Step 1: Area vector of a closed surface is outward. Step 2: Inward electric field is opposite to the area vector. Step 3: Therefore total flux is negative and net enclosed charge is negative.
Correct answer: A. It can be possible if net charge is zero and field does not reach the surface
Explanation: Step 1: If field is zero everywhere on the surface, total flux is zero. Step 2: By Gauss's law, net enclosed charge is zero. Step 3: A complete dipole has zero net charge, so Gauss's law alone does not make it impossible.
Correct answer: A. No, zero total flux can occur with non-zero local field
Explanation: Step 1: Total flux is an algebraic sum. Step 2: Positive and negative contributions on different parts may cancel. Step 3: Therefore zero total flux does not always mean field is zero everywhere.
Correct answer: A. Point inverse square, line inverse distance, sheet distance independent
Explanation: Step 1: For a point charge, spherical area grows as square of distance. Step 2: For a long line, curved cylindrical area grows with distance. Step 3: For an infinite sheet, ideal field is independent of distance.
Correct answer: A. Whether the 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 decided by net enclosed charge. Step 3: To find field, symmetry is needed, so check these three points first.
Correct answer: A. It remains same
Explanation: Step 1: Gauss's law relates total flux to enclosed net charge. Step 2: Changing position of the charge does not change the enclosed charge. Step 3: Distribution may change but total flux remains same.
Correct answer: A. Net flux is zero
Explanation: Step 1: Net flux is decided only by enclosed net charge. Step 2: Since net enclosed charge is zero, net flux is zero. Step 3: Zero net flux does not mean zero field at every point.
Correct answer: B. One sixth
Explanation: Step 1: A cube has six equal faces. Step 2: With charge at the centre, symmetry divides total flux equally. Step 3: Each face gets one sixth of the total flux.
Correct answer: A. One eighth
Explanation: Step 1: Eight identical cubes can be combined to make a larger cube. Step 2: The charge becomes the centre of the larger cube and flux is shared equally. Step 3: Hence one small cube gets one eighth of the full flux.
Correct answer: A. Flux same and field one fourth
Explanation: Step 1: Total flux depends on enclosed charge, so it does not change. Step 2: Field of a point charge varies inversely as square of distance. Step 3: Doubling radius makes field one fourth.
Correct answer: A. Because field distribution on the surface is also needed
Explanation: Step 1: Gauss's law gives total flux. Step 2: To find field, the field must be uniform or simply directed on the surface. Step 3: Without symmetry, total flux does not directly give the field.
Correct answer: A. Field is parallel to the flat ends
Explanation: Step 1: Field of a long wire is radial from the wire. Step 2: At the flat ends of the cylinder, this field lies parallel to the surface. Step 3: A field parallel to a surface gives zero flux.
Correct answer: B. One third
Explanation: Step 1: Field of a long line charge is inversely proportional to distance. Step 2: If distance becomes three times, field becomes one third. Step 3: Do not use inverse square law for line charge.
