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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. As if the whole charge were at the centre
Explanation: Step 1: A spherical shell has spherical symmetry. Step 2: For an outside point, the Gaussian surface encloses the whole charge. Step 3: The result is like a point charge placed at the centre.
Correct answer: B. It becomes half
Explanation: Step 1: Electric field of an infinite line charge is inversely proportional to distance. Step 2: When distance is doubled, the field becomes half. Step 3: For line charge, remember inverse relation with distance.
Correct answer: C. It remains same
Explanation: Step 1: Electric field of an infinite plane sheet does not depend on distance. Step 2: Even when distance is doubled, field remains the same. Step 3: Remember the difference between distance rules for plane sheet and point charge.
Correct answer: B. Perpendicular to the surface
Explanation: Step 1: An infinite plane sheet has symmetry along all directions in the plane. Step 2: Therefore field cannot prefer any direction along the sheet and must be normal to it. Step 3: Remember normal direction for plane sheet.
Correct answer: C. Zero
Explanation: Step 1: Net flux through a closed surface depends on net enclosed charge. Step 2: Equal positive and negative charges add to zero. Step 3: Therefore net flux is zero, even though electric field may exist inside.
Correct answer: B. Only the positive charge inside
Explanation: Step 1: In Gauss's law, only charge enclosed by the closed surface is counted. Step 2: The outside negative charge may change local field, but not net flux. Step 3: If positive charge is inside, net flux is determined by it.
Correct answer: A. Because electric field is not uniform or simply directed on the surface
Explanation: Step 1: Net flux is a total over the whole closed surface. Step 2: To find electric field, the field must be separated in a simple way. Step 3: In asymmetry this is not possible, so direct field calculation is difficult.
Correct answer: A. Because it is an imaginary closed surface for calculation
Explanation: Step 1: The purpose of a Gaussian surface is to simplify calculation. Step 2: It may be an imaginary closed surface and need not be a real object. Step 3: Choose it according to symmetry, not material.
Correct answer: C. Positive
Explanation: Step 1: Outgoing field lines give positive flux. Step 2: If more lines leave than enter, net flux is positive. Step 3: Positive net flux means positive net charge inside.
Correct answer: B. Negative
Explanation: Step 1: For a closed surface, outward direction is taken positive. Step 2: Entering lines give negative flux. Step 3: If more lines enter, net flux is negative and enclosed net charge is negative.
Correct answer: C. It becomes double
Explanation: Step 1: Radius of the spherical surface is unchanged, so distance is unchanged. Step 2: Field of a point charge is proportional to source charge. Step 3: If charge doubles, field on the surface doubles.
Correct answer: C. It remains same
Explanation: Step 1: Total flux depends on net charge enclosed by the closed surface. Step 2: Changing radius does not change enclosed charge. Step 3: Therefore total flux remains same, though field value may change.
Correct answer: A. One fourth
Explanation: Step 1: Electric field of a point charge varies inversely as square of distance. Step 2: When radius doubles, square of distance becomes four times. Step 3: Hence field on the surface becomes one fourth.
Correct answer: A. Because enclosed charge is same but area increases
Explanation: Step 1: Total flux is decided only by enclosed charge, so it stays same. Step 2: On a larger sphere, the same flux spreads over larger area. Step 3: Therefore electric field per unit area decreases.
Correct answer: B. Radially outward
Explanation: Step 1: Field from a positive line charge points outward. Step 2: On the curved surface of the cylinder, the field is radial and normal to the surface. Step 3: For a positive wire, remember radially outward direction.
Correct answer: A. Because field is parallel to the flat ends
Explanation: Step 1: Field of a long wire is radial. Step 2: For the flat ends of the cylinder, the field lies parallel to those surfaces. Step 3: A field parallel to a surface gives zero flux through it.
Correct answer: A. Because field is parallel to the curved part
Explanation: Step 1: Field of an infinite plane sheet is perpendicular to the sheet. Step 2: For a pillbox Gaussian surface, this field is parallel to the curved side. Step 3: Therefore flux through the curved side is zero.
Correct answer: A. Where surface charge density is higher
Explanation: Step 1: Field just outside a conductor is related to surface charge density. Step 2: Higher surface charge density gives stronger electric field. Step 3: Remember stronger field near sharper regions.
Correct answer: A. Because it would move free charges along the surface
Explanation: Step 1: Free charges in a conductor can move easily. Step 2: If a field component existed along the surface, charges would keep moving. Step 3: Therefore, in electrostatic equilibrium, field is normal to the surface.
Correct answer: A. When surface charge density is uniform
Explanation: Step 1: An infinite uniformly charged sheet has strong symmetry. Step 2: This symmetry gives equal fields on both sides independent of distance. Step 3: For finite sheets, edge effects can disturb this simple result.
Correct answer: C. It increases
Explanation: Step 1: Inside a uniformly charged solid non-conducting sphere, enclosed charge increases with radius. Step 2: As a result, field generally increases as we move outward from the centre. Step 3: Remember the difference between conductor and non-conducting solid sphere.
Correct answer: B. Zero
Explanation: Step 1: At the centre of the sphere, symmetry exists in all directions. Step 2: Effects from opposite directions balance each other. Step 3: Therefore, electric field at the centre is zero.
Correct answer: A. Total charge placed at the centre
Explanation: Step 1: Outside the sphere, a Gaussian surface encloses the whole charge. Step 2: Due to spherical symmetry, outside field is like total charge at the centre. Step 3: From outside, a spherical charge distribution behaves like a point charge.
Correct answer: A. Product of electric field and total area
Explanation: Step 1: For uniform and normal field, each small part contributes simply. Step 2: Total flux is electric field multiplied by total area. Step 3: This is the simplicity of symmetry-based Gauss calculation.
Correct answer: B. Zero
Explanation: Step 1: Flux is related to field crossing a surface. Step 2: A field parallel to the surface does not pass through that part. Step 3: Therefore flux through that part is zero.
