Q.State and prove Bernoulli's theorem. OR Derive expression for height of the liquid rise
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Start your 14-day free trial to unlock the full solution →Applying the work-energy theorem to a fluid element flowing through a tube of varying cross-section and height gives .
Statement: For the steady, streamlined flow of an ideal (non-viscous, incompressible) fluid, the sum of pressure energy, kinetic energy, and potential energy per unit volume remains constant along any streamline:
Proof: Consider fluid flowing through a pipe of varying cross-section, entering at point 1 (area , speed , height , pressure ) and leaving at point 2 (area , speed , height , pressure ), in a small time interval .
By the equation of continuity (mass conservation for incompressible flow), the volume of fluid entering equals the volume leaving:
and the mass moved is at both ends.
Work done by pressure forces:
- At the inlet, the fluid behind pushes the element in, doing positive work .
- At the outlet, the element pushes fluid ahead of it, so work is done AGAINST it (i.e. the fluid loses this much energy pushing forward).
Net work done on the fluid element by pressure forces:
By the work-energy theorem, this net work equals the sum of the changes in kinetic energy and potential energy of the fluid element (mass ) as it moves from 1 to 2:
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