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Question

A liquid flows through a horizontal tube. The velocities of the liquid in the two sections, which have have areas of cross section A1 and A2, are v1 and v2 respectively. The difference in the levels of the liquid in the two vertical tubes is h. Then :


A
The volume of the liquid flowing through the tube in unit time is A1v1.
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B
v2v1=2gh
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C
v22v21=2gh
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D
The energy per unit volume of the liquid is the same in both sections of the tube.
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Solution

The correct options are
A The volume of the liquid flowing through the tube in unit time is A1v1.
C v22v21=2gh
D The energy per unit volume of the liquid is the same in both sections of the tube.
Since there are no sources and sinks in between the two sections of the tube, by using the equation of continuity, we can say that A1v1=A2v2 = constant. Thus, option A is correct.
Using Bernoulli's equation for the two sections of the tube we have
P1ρg+v212g+h1=P2ρg+v222g+h2 .....(1)
As the tube is horizontal, we take h1=h2.
P1P2=ρ2(v22v21) .....(2)
where P1 and P2 are the pressures at section 1 and section 2.
But, P1P2=ρgh .....(3)
(from the difference in heights of the liquid columns)
Thus, from (2) and (3), we have
ρgh=ρ2(v22v21)
or, v22v21=2gh.
Thus, option (c) is correct.
Bernouli's theorem plays the role of law of conservation of energy.
From (1), we can deduce that there is no change in the energy per unit volume between the two sections. Thus, option (d) is also correct.
Hence, options (a), (c) and (d) are the correct answers.

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