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3.6.1 Bernoulli’s Principle

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Bernoulli's Principle states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.


Venturi Effect

The Venturi effect is the fluid pressure that results when an incompressible fluid flows through a constricted section of a pipe.

Experiment 1

Figure above shows that when water flow from left to right, the water level decreases from left to right. This indicates that, the water pressure decreases from left to right.

Explanation:
Liquids flow from places with higher pressure to places with lower pressure.


However, if the experiment is repeated by using a Venturi tube where the diameter at B is made smaller than A and C as in the diagram above, the water level become lowest at B.

Explanation:
The pressure at B is the lowest because the liquid flow the fastest at B. According to Bernoulli's Principle, the faster the water flow, the lower the water pressure.


Experiment 2


Figure above shows some air is blow through a tube from left to right. The water level in the capillary tube increases from left to right.
This indicates that the pressure in the tube decreases from left to right.

Explanation:
Gases flow from places with higher pressure to places with lower pressure.


However, if the tube is replaced by a Venturi tube, the water level become highest at B. This indicates that, the pressure of the air is the lowest at B.

Explanation:
The pressure at B is the lowest because the gas flow the fastest at B. According to Bernoulli's Principle, the faster the gas flow, the lower the gas pressure.

Experiment 1

A paper will be lifted upwards when air is blown rapidly above it.

Explanation:
  1. Air move rapidly above the paper, causes the pressure above the paper to decrease.
  2. Pressure below the paper becomes relatively higher.
  3. Owing to the difference of the pressure, a net force is produced to push the paper upward.

Experiment 2

When air is blown rapidly between the 2 ping pong balls, the 2 balls will move towards each other.

Explanation:
  1. Air move rapidly between the 2 balls, causes the pressure between the 2 balls to decrease.
  2. Pressure at the other side of the balls becomes relatively higher, push the 2 ball close to each other.

Experiment 3

The ping pong ball will not fall when water is allowed to flow through the filter funnel.

Explanation:
  1. Water above the ping pong ball flow rapidly, causes the pressure above the ping pong ball to decrease.
  2. As a result, the pressure below the ping pong ball is relatively higher.
  3. Owing to the difference of the pressure, a net force is produced to push the ping pong ball upward.