Q.State Bernoulli's theorem. Prove that total energy possessed by a flowing ideal fluid is always conserved? Can Bernoulli's equation be used to describe the flow of Water through a rapid in a River?
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Start your 14-day free trial to unlock the full solution →Bernoulli's theorem (P + (1/2)ρv² + ρgh = constant) follows from the work-energy theorem applied to an ideal fluid in streamline flow; it cannot describe rapids because rapids are turbulent, not streamline, flow.
Statement of Bernoulli's Theorem: For the streamline (laminar), non-viscous, incompressible flow of an ideal fluid, the sum of the pressure energy, kinetic energy, and potential energy per unit volume at every point along a streamline is constant:
P + (1/2)ρv² + ρgh = constant
where P = pressure, ρ = fluid density, v = flow speed, h = height above a reference level.
Derivation (energy conservation): Consider an ideal fluid flowing through a pipe of varying cross-section and height. Let a fluid element enter at point 1 (area A1, speed v1, height h1, pressure P1) and leave at point 2 (area A2, speed v2, height h2, pressure P2) in a small time Δt.
By the equation of continuity (mass conservation for incompressible flow), A1 v1 = A2 v2, so the volume of fluid entering equals that leaving: ΔV = A1 v1 Δt = A2 v2 Δt.
Work done by pressure forces pushing the fluid:
- Work done ON the fluid at point 1: W1 = P1 A1 (v1 Δt) = P1 ΔV
- Work done BY the fluid against pressure at point 2: W2 = P2 A2 (v2 Δt) = P2 ΔV
Net work done on the fluid element by pressure forces:
Wnet = (P1 − P2) ΔV
This work changes the kinetic and potential energy of the mass Δm = ρΔV of fluid that effectively moved from point 1 to point 2:
ΔKE = (1/2)Δm v2² − (1/2)Δm v1²
ΔPE = Δm g h2 − Δm g h1
By the work-energy theorem:
Wnet = ΔKE + ΔPE
(P1 − P2)ΔV = (1/2)ρΔV(v2² − v1²) + ρΔV g(h2 − h1)
Dividing throughout by ΔV:
P1 − P2 = (1/2)ρ(v2² − v1²) + ρg(h2 − h1)
Rearranging:
P1 + (1/2)ρv1² + ρgh1 = P2 + (1/2)ρv2² + ρgh2
Since points 1 and 2 were arbitrary points along the streamline:
P + (1/2)ρv² + ρgh = constant
This confirms the total energy per unit volume of an ideal fluid in streamline flow is conserved.
Can Bernoulli's equation describe flow through a rapid in a river? No. Bernoulli's theorem is derived strictly for steady, streamline (laminar), non-viscous, incompressible flow. The flow of water through a rapid is turbulent — the water forms eddies, swirls and vortices, and a significant amount of mechanical energy is dissipated as heat/sound due to internal friction (viscosity) and turbulence. Since these conditions violate the basic assumptions of the theorem (no energy loss, streamline flow), Bernoulli's equation cannot be used to accurately describe the flow of water through a rapid.
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