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Q.Write Bernoulli's Theorem. Prove that the sum of energies of a liquid in a streamline flow remains constant. OR Prove that due to surface tension, the excess pressure inside a drop is 2T/R and excess pressure inside a soap bubble is 4T/R, where T is the surface tension of the liquid and R is the radius of the drop or bubble.

Madhya Pradesh MpbseMP Board Higher Secondary (Class 11) 2025Subjective· 4mImportance★★★★★
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Bernoulli's theorem: P + (1/2) rho v^2 + rho g h = constant along a streamline, for an ideal fluid in steady flow.

Bernoulli's Theorem: For an ideal fluid (incompressible and non-viscous) undergoing steady, streamline flow, the sum of the pressure energy, kinetic energy, and potential energy per unit volume remains constant at every point along a streamline:

P + (1/2) rho v^2 + rho g h = constant.

Proof: Consider an ideal fluid flowing through a tube of varying cross-section, from point 1 (area A1, height h1, speed v1, pressure P1) to point 2 (area A2, height h2, speed v2, pressure P2). Consider a small mass of fluid, m, that moves from point 1 to point 2 in time dt.

Work done by pressure at the inlet (pushing fluid in) = P1 A1 (v1 dt) = P1 dV (where dV is the volume of fluid that flows, same at both ends by the equation of continuity for an incompressible fluid).

Work done against pressure at the outlet (fluid pushing out against P2) = P2 A2 (v2 dt) = P2 dV.

Net work done by pressure forces on this fluid element = (P1 - P2) dV.

By the work-energy theorem, this net work equals the change in the fluid element's total mechanical energy (kinetic + potential) as it moves from point 1 to point 2:

(P1 - P2) dV = [(1/2) dm v2^2 + dm g h2] - [(1/2) dm v1^2 + dm g h1]

where dm = rho dV is the mass of the fluid element. Substituting and dividing throughout by dV: …

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