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Q.State and prove Bernoulli's theorem. OR What do you mean by viscosity? Obtain an expression for the terminal velocity of a ball falling in a viscous liquid.

Haryana BsehBoard of School Education Haryana (Senior Secondary Part-I / Class 11) 2023Subjective· 5mImportance★★★★★
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Applying the work-energy theorem to a fluid element moving along a streamline in an ideal fluid gives Bernoulli's equation, P+12ρv2+ρgh=constantP + \frac{1}{2}\rho v^2 + \rho gh = \text{constant}.

Statement: For the streamline flow of an ideal fluid (incompressible, non-viscous, and flowing steadily/non-turbulently), the sum of pressure energy, kinetic energy, and potential energy per unit volume remains constant at every point along a streamline:

P+12ρv2+ρgh=constantP + \frac{1}{2}\rho v^2 + \rho gh = \text{constant}

Proof (using the work-energy theorem): Consider an ideal fluid flowing steadily through a tube of varying cross-section, from a point 1 (area A1A_1, height h1h_1, velocity v1v_1, pressure P1P_1) to point 2 (area A2A_2, height h2h_2, velocity v2v_2, pressure P2P_2). By the equation of continuity, A1v1=A2v2=constant mass flowA_1v_1 = A_2v_2 = \text{constant mass flow}.

Considering a small volume ΔV\Delta V (mass Δm=ρΔV\Delta m = \rho \Delta V) of fluid pushed from position 1 to position 2:

  • Work done by the pressure force pushing the fluid in at point 1: W1=P1A1Δx1=P1ΔVW_1 = P_1 A_1 \Delta x_1 = P_1 \Delta V
  • Work done against the pressure force at point 2 (fluid pushing out against the fluid ahead): W2=−P2A2Δx2=−P2ΔVW_2 = -P_2 A_2 \Delta x_2 = -P_2\Delta V
  • Total work done by pressure forces: W=(P1−P2)ΔVW = (P_1 - P_2)\Delta V

By the work-energy theorem, this net work equals the change in total mechanical energy (kinetic + potential) of the fluid element:

(P1−P2)ΔV=[12Δm v22+Δm gh2]−[12Δm v12+Δm gh1](P_1 - P_2)\Delta V = \left[\frac{1}{2}\Delta m\, v_2^2 + \Delta m\, g h_2\right] - \left[\frac{1}{2}\Delta m\, v_1^2 + \Delta m\, g h_1\right] …

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