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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. Total flux remains same
Explanation: Step 1: In Gauss's law, total flux depends only on enclosed net charge. Step 2: The charge is still inside, so enclosed charge is unchanged. Step 3: Distribution may change but total flux does not, so do not judge only from position.
Correct answer: A. Net flux is zero but local field may exist
Explanation: Step 1: Net flux is the sum of all contributions over the closed surface. Step 2: Outgoing and incoming contributions can cancel. Step 3: Zero net flux should not be confused with zero field everywhere.
Correct answer: A. One sixth
Explanation: Step 1: A cube has six identical faces. Step 2: With charge at the centre, symmetry is same for all faces. Step 3: Total flux is equally divided into six parts, so one face gets one sixth.
Correct answer: A. One eighth
Explanation: Step 1: Eight such identical cubes can be joined so that the charge becomes the centre of a larger cube. Step 2: The full flux is shared equally among the eight small cubes. Step 3: Therefore 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 changing radius does not change it. Step 2: Field of a point charge varies inversely as square of distance. Step 3: Doubling distance makes the field one fourth.
Correct answer: A. Field is not uniform or simply directed on the surface
Explanation: Step 1: Gauss's law always relates net flux to enclosed charge. Step 2: To find field, symmetry is needed so the field can be treated simply. Step 3: Without symmetry, total flux does not give field at each point.
Correct answer: A. Electric field is parallel to those ends
Explanation: Step 1: Field of a long line charge is radial. Step 2: At the flat ends of the cylinder, this field runs along the surface. Step 3: A field parallel to a surface gives no flux through that part.
Correct answer: A. One fourth
Explanation: Step 1: Electric field of a long line charge is inversely proportional to distance. Step 2: Making distance four times makes field one fourth. Step 3: Do not apply inverse square rule to a line charge.
Correct answer: A. It remains same
Explanation: Step 1: Field of an infinite plane sheet does not depend on distance. Step 2: By Gauss's law and symmetry, the field is uniform on both sides. Step 3: Changing distance does not change the ideal infinite-sheet field.
Correct answer: A. Field is parallel to the curved part
Explanation: Step 1: Field of an infinite sheet is normal to the sheet. Step 2: In the pillbox cylinder, this field runs parallel to the curved surface. Step 3: Field parallel to a surface gives zero flux.
Correct answer: A. Charges are at different positions so fields do not cancel everywhere
Explanation: Step 1: Equal opposite charges have zero net charge. Step 2: Since the charges are at different positions, local field can remain. Step 3: Keep net flux and local field separate.
Correct answer: A. Electric field inside the conductor is zero
Explanation: Step 1: Net electric field inside a conductor in electrostatic equilibrium is zero. Step 2: Hence flux through an internal Gaussian surface is zero. Step 3: By Gauss's law, excess charge enclosed by it is zero.
Correct answer: A. Free charges will not remain at rest
Explanation: Step 1: Free charges in a conductor can move on the surface. Step 2: A tangential field would make them move along the surface. Step 3: Electrostatic equilibrium is possible only when the field is normal to the surface.
Correct answer: A. Zero
Explanation: Step 1: Free charges in the conductor rearrange under the influence of external field. Step 2: In equilibrium, field inside conducting material must be zero. Step 3: The empty closed cavity remains field-free due to electrostatic shielding.
Correct answer: A. Zero
Explanation: Step 1: Charge inside the cavity induces charges on the surfaces. Step 2: Still, in electrostatic equilibrium, net field inside conducting material must be zero. Step 3: Think separately about conducting material and the cavity region.
Correct answer: A. The Gaussian surface encloses no charge
Explanation: Step 1: Charge of the shell is on its surface. Step 2: A Gaussian surface chosen inside the shell encloses no charge. Step 3: Therefore net flux is zero and by symmetry the inside field is zero.
Correct answer: A. Due to spherical symmetry, field on outside Gaussian surface is uniform and radial
Explanation: Step 1: An outside spherical Gaussian surface encloses the full charge. Step 2: Spherical symmetry makes the field equal in magnitude and radial on the surface. Step 3: The result is like a point charge at the centre.
Correct answer: A. Double
Explanation: Step 1: Inside a uniformly charged solid non-conducting sphere, field is proportional to distance from the centre. Step 2: Doubling distance doubles the field as long as the point remains inside. Step 3: Do not confuse it with zero field inside a conductor.
Correct answer: A. Effects from all directions cancel by symmetry
Explanation: Step 1: At the centre all directions are equivalent. Step 2: Effect from every direction is balanced by the opposite direction. Step 3: Hence resultant electric field at the centre is zero.
Correct answer: A. One ninth
Explanation: Step 1: Outside the sphere, field behaves like total charge at the centre. Step 2: Field of a point charge varies inversely as square of distance. Step 3: Tripling distance makes field one ninth.
Correct answer: A. Because only normal component contributes to flux
Explanation: Step 1: Flux comes from the component crossing the surface. Step 2: In an oblique field, the full field does not cross the surface. Step 3: Use simple multiplication only when field is uniform and normal to the surface.
Correct answer: A. Negative
Explanation: Step 1: In Gauss's law, net flux is decided only by enclosed net charge. Step 2: External positive charges can change field on the surface but not the sign of net flux. Step 3: Since enclosed net charge is negative, flux remains negative.
Correct answer: A. From positive to zero
Explanation: Step 1: Initially enclosed net charge was positive, so flux was positive. Step 2: Including an equal negative charge makes enclosed net charge zero. Step 3: By Gauss's law, net flux becomes zero.
Correct answer: A. Every point of the surface is at the same distance from the charge
Explanation: Step 1: A spherical Gaussian surface is chosen with the point charge at its centre. Step 2: Every point on the sphere is at equal distance from the charge. Step 3: Therefore field magnitude is same everywhere.
Correct answer: A. Field exists equally on both sides of the sheet
Explanation: Step 1: Field of an infinite sheet has equal magnitude on both sides. Step 2: The two flat faces of the pillbox lie on these two sides. Step 3: Therefore contributions from both faces are added.
