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Question #42966

Difficulty: 8/10 | 🟢 Free

The Question

A rectangular container of length LL is placed along the xx-axis from x=0x = 0 to x=Lx = L. It is filled with an incompressible diamagnetic fluid of density ρ\rho and magnetic susceptibility χ\chi (where χ<0\chi < 0). The system is permeated by a non-uniform magnetic field B⃗=B0xk^\vec{B} = B_0 x \hat{k}, where B0B_0 is a positive constant and k^\hat{k} is the unit vector in the vertically upward zz-direction. The container is given a constant horizontal acceleration a⃗=−a0i^\vec{a} = -a_0 \hat{i} (a0>0a_0 > 0). In the steady state, the free surface of the fluid is observed to be perfectly horizontal at the edge x=Lx = L. Assuming the fluid does not spill, what is the difference in height of the fluid surface between the edges x=Lx = L and x=0x = 0, given by Δh=h(L)−h(0)\Delta h = h(L) - h(0)? (Here gg is the acceleration due to gravity and μ0\mu_0 is the vacuum permeability).

Options

  • A.

    −χB02L22μ0ρg-\frac{\chi B_0^2 L^2}{2 \mu_0 \rho g}

  • B.

    −χB02L2μ0ρg-\frac{\chi B_0^2 L^2}{\mu_0 \rho g}

  • C.

    χB02L22μ0ρg\frac{\chi B_0^2 L^2}{2 \mu_0 \rho g}

  • D.

    −χB02L2μ0ρg-\frac{\chi B_0^2 L}{2 \mu_0 \rho g}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Multi-Concept_Bridge

Master problems that require linking physics and math concepts to solve complex competitive questions.

Concepts Behind This Question

Diamagnetic Property of Materials|Accelerated Frame Hydrostatics|Vector Cross-Products

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to synthesize fluid statics in an accelerated frame with the magnetic body force on a diamagnetic fluid.

A common trap is miscalculating the magnetic force density by either forgetting the factor of 1/2 in the formula f⃗m=χ2μ0∇(B2)\vec{f}_m = \frac{\chi}{2\mu_0} \nabla(B^2) or incorrectly using ∇B\nabla B instead of ∇(B2)\nabla(B^2), which leads to Option B.

Furthermore, students often fail to recognize that the 'horizontal' condition at x=Lx=L is a boundary condition on the pressure gradient that fixes the unknown pseudo-force.

Ignoring the pseudo-force entirely leads to a sign-flipped, incorrect magnitude (Option C).

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterMagnetism and Matter - Class 12
ConceptDiamagnetic Property of Materials
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree