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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: C. It remains same
Explanation: Step 1: Field of an infinite plane sheet does not depend on distance. Step 2: Changing distance does not change its symmetry result. Step 3: Therefore electric field remains same.
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, this field runs along the curved surface. Step 3: Hence no flux passes through the curved part.
Correct answer: A. Because net charge is zero but charges are at different positions
Explanation: Step 1: Equal opposite charges have zero net charge. Step 2: But they are at different positions, so their fields do not cancel everywhere. Step 3: Zero net flux and zero local field are different ideas.
Correct answer: A. Because electric field inside conductor is zero
Explanation: Step 1: Inside a conductor in electrostatic equilibrium, electric field is zero. Step 2: Therefore net flux through an internal closed Gaussian surface is zero. Step 3: By Gauss's law, enclosed excess charge is zero.
Correct answer: A. Free charges would move along the surface
Explanation: Step 1: Free charges in a conductor can move. Step 2: If a field component exists along the surface, charges will not remain at rest. Step 3: Hence in equilibrium, field is normal to the surface.
Correct answer: A. Zero
Explanation: Step 1: A conductor in electrostatic equilibrium can shield an empty cavity from external influence. Step 2: Free charges rearrange so that field inside the cavity becomes zero. Step 3: Remember this as electrostatic shielding.
Correct answer: A. Zero
Explanation: Step 1: In conducting material at electrostatic equilibrium, electric field must be zero. Step 2: The charge in the cavity induces charges on surfaces. Step 3: Still the net field inside the conducting material remains zero.
Correct answer: A. Because it encloses no charge
Explanation: Step 1: Charge of a uniformly charged shell lies on its surface. Step 2: A Gaussian surface chosen inside the shell encloses no charge. Step 3: Hence total flux is zero and field is zero.
Correct answer: A. Because spherical symmetry makes the whole shell act like a central charge outside
Explanation: Step 1: An outside spherical Gaussian surface encloses the whole charge. Step 2: Spherical symmetry makes field same at equal distance. Step 3: The result is like total charge placed at the centre.
Correct answer: A. Half
Explanation: Step 1: Inside such a solid sphere, electric field is proportional to distance from centre. Step 2: Halving distance reduces the enclosed-charge effect accordingly. Step 3: Therefore field becomes half.
Correct answer: A. By symmetry effects from all directions cancel
Explanation: Step 1: At the centre, all directions are equivalent. Step 2: Electric effect from one direction balances that from the opposite direction. Step 3: Hence resultant electric field at the centre is zero.
Correct answer: A. One fourth
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: Doubling distance makes field one fourth.
Correct answer: A. No, angular component must be considered
Explanation: Step 1: Only the component crossing the surface contributes to flux. Step 2: If field is not normal everywhere, angle can vary. Step 3: Use simple multiplication only when field is uniform and normal.
Correct answer: B. Negative
Explanation: Step 1: Net flux is decided only by enclosed net charge. Step 2: External positive charges may change local field but not net flux. Step 3: With negative net charge inside, flux remains negative.
Correct answer: A. It becomes zero
Explanation: Step 1: Initially the surface enclosed positive net charge, so flux was positive. Step 2: Including an equal negative charge makes net enclosed charge zero. Step 3: Zero enclosed net charge gives zero net flux.
Correct answer: A. Because every point on the spherical surface is at the same distance
Explanation: Step 1: A spherical Gaussian surface centered on the point charge is chosen. Step 2: Every point of that surface is at the same distance from the charge. Step 3: Therefore field magnitude is same everywhere on it.
Correct answer: A. Because field exists on both sides of the sheet
Explanation: Step 1: Field of an infinite sheet has equal magnitude on both sides. Step 2: The pillbox has two flat faces on the two sides. Step 3: Thus both equal face contributions are added to get total flux.
Correct answer: A. Because field lines emerge from positive charge
Explanation: Step 1: Positive charge acts as a source of field lines. Step 2: Symmetry of an infinite sheet is same on both sides. Step 3: Therefore field points away from the sheet on both sides.
Correct answer: A. Toward the sheet
Explanation: Step 1: Negative charge is treated as a sink of field lines. Step 2: Symmetry of an infinite sheet is same on both sides. Step 3: Hence field points toward the sheet on both sides.
Correct answer: A. Radially outward from the wire
Explanation: Step 1: Field lines emerge from a positive line charge. Step 2: Cylindrical symmetry makes direction radial and perpendicular to the wire. Step 3: For a positive wire, remember outward direction.
Correct answer: A. Because without symmetry it is difficult to separate field from flux
Explanation: Step 1: Gauss's law correctly gives total flux. Step 2: To find electric field, simple field behaviour on the surface is needed. Step 3: Without symmetry, this simplicity is absent.
Correct answer: A. Three times
Explanation: Step 1: Net flux depends on net enclosed charge. Step 2: Surface area alone does not decide total flux. Step 3: Since enclosed charge is tripled, net flux becomes three times.
Correct answer: B. Negative
Explanation: Step 1: Add the charges inside the closed surface. Step 2: Positive two and negative five give a negative net result. Step 3: Therefore by Gauss's law, total flux is negative.
Correct answer: A. Net charge inside is zero
Explanation: Step 1: Outgoing lines give positive flux and incoming lines give negative flux. Step 2: If both are equal, net flux is zero. Step 3: By Gauss's law, net enclosed charge is zero.
Correct answer: A. It remains same
Explanation: Step 1: Net flux is decided by net enclosed charge. Step 2: Outside charges can change field distribution on the surface. Step 3: Still net flux remains same if enclosed net charge is unchanged.
