Gauss' theorem
ICSE · Class 12 · Physics
Most important questions from Gauss' theorem for ICSE Class 12 Physics board exam 2026. MCQs, short answer, and long answer questions with marks.
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Sample Questions
A large plane sheet has surface charge density 1.77 × 10^-11 C·m^-2. Find the electric field near the sheet and the field between two such sheets carrying equal and opposite charge densities of this magnitude.
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E near one sheet = 1.0 N·C^-1, field between opposite sheets = 2.0 N·C^-1
For one infinite plane sheet, E = σ/(2ε₀). With σ = 1.77 × 10^-11 C·m^-2, E = 1.77 × 10^-11 /(2 × 8.85 × 10^-12) = 1.0 N·C^-1. For two equal and opposite sheets, the fields add between the sheets, so E = σ/ε₀ = 2.0 N·C^-1.
A conducting sphere of radius 0.30 m carries charge 2.7 μC. Find the electric field just outside the surface and the electric field inside the sphere.
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E just outside = 2.7 × 10^5 N·C^-1, E inside = 0
Just outside a charged conductor, E = σ/ε₀ = q/(4πR²ε₀). With q = 2.7 × 10^-6 C and R = 0.30 m, E = 9.0 × 10^9 × 2.7 × 10^-6 / 0.09 = 2.7 × 10^5 N·C^-1. Inside a charged conducting sphere, the electric field is zero.
A uniformly charged non-conducting sphere has radius 0.50 m and total charge 6.0 μC. Find the electric field at r = 0.20 m from the centre.
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4.32 × 10^4 N·C^-1
Inside a uniformly charged non-conducting sphere, E = (1/4πε₀)(qr/R³). Substituting q = 6.0 × 10^-6 C, r = 0.20 m, R = 0.50 m gives E = 9.0 × 10^9 × 6.0 × 10^-6 × 0.20 / 0.125 = 4.32 × 10^4 N·C^-1.
A cube encloses a charge of 1.5 μC at any position inside it. Find the total electric flux through the cube, and then the flux if a second charge of -0.5 μC is also enclosed.
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Flux = 1.69 × 10^5 N·m²·C^-1, then 1.13 × 10^5 N·m²·C^-1
Net flux through a closed surface is Φ_E = q/ε₀. For 1.5 μC, Φ_E = 1.5 × 10^-6 / 8.85 × 10^-12 ≈ 1.69 × 10^5 N·m²·C^-1. With an added -0.5 μC, net charge becomes 1.0 μC, so flux becomes 1.0 × 10^-6 / 8.85 × 10^-12 ≈ 1.13 × 10^5 N·m²·C^-1.
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