Venturi meter is an application of Bernoulli's equation. Its basic principle also depends on the Bernoulli equation, which is
- Aboth velocity of moving fluid and pressure within fluid increases
- Bvelocity of a moving fluid increases and the pressure within the fluid decreasesCorrect
- Cboth velocity of moving fluid and pressure within fluid constant
- Dpressure within the fluid increases and velocity of moving fluid increase
Solution
Concept
From Bernoulli's equation, when velocity increases at a constriction, pressure decreases.
Formula
p1/ρg + V12/2g + z1 = p2/ρg + V22/2g + z2
Step-by-step solution
1. Total energy along a streamline stays constant (Bernoulli). 2. At the venturi throat the area is smaller, so velocity increases (continuity A1V1 = A2V2). 3. Kinetic energy rises, so pressure energy must fall. 4. The pressure difference between inlet and throat is measured to find the discharge.
Explanation
In a venturi meter the flow passes through a narrow throat. The velocity increases there, so the pressure drops. This pressure drop is used to calculate the flow rate.
Why the other options are wrong
A and D: velocity and pressure cannot both increase, because total energy is constant. C: if both stayed constant there would be no pressure difference to measure.
Shortcut
Velocity up means pressure down (Bernoulli).
Common mistake
Thinking that higher velocity also means higher pressure.
Correct answer: B) velocity of a moving fluid increases and the pressure within the fluid decreases