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3.6 Bernoulli Principle

Bernoulli’s principle.

Bernoulli’s principle states that The pressure of a moving liquid decreases as the speed of the fluid increases and vice versa.

The conditions which enable the Bernoulli’s Principle to use

(1)     The fluid is incompressible and flows easily

(2)     The fluid is streamline or in layers,i.e. there are no eddies or turbulence

Factors affecting the speed of the  moving liquid.

(1)     The speed of moving object increases  as the area of moving  fluid  decreases

(2)     The speed of moving object increases  as the distance of moving  fluid 

Bernoulli’s principle and the law of conservation of energy

Energy is always conserved and this also applies to fluids, whether static or in motion. A fluid in motion has kinetic energy. This kinetic energy imparted to the fluid comes at the expense of the pressure of the fluid. Thus, when the speed (kinetic energy) of the fluid increases, its pressure decreases.

Activities to show Bernoulli’s Principle

Activity 1

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When the air is blown up in the surface of a piece of paper,its observed that the paper moves up.

This happened because the air moved at a very high velocity.

According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases .

The higher atmospheric pressure which acts at the bottom of the paper pushes up the paper.

Activity 2

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When the air is blown harder through the straw ,

The two ping-pong balls will move closely to each other.

The air moved  at a very high velocity between the balls.

 According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases .

The higher atmospheric pressure caused the ping-pong balls closer to each other.

Activity  3

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When the air blows harder ,the ball is not falling down.

It is because the air moved  at a very high velocity between the balls and the wall of  the filter tunnel.

According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases .

The bottom of the ball has the higher atmospheric pressure  which can hold the ball from falling down.

 Activity 4- Using Venturing tubes

Mark the water level in the vertical tubes P,Q and R in the following figures.

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Use of Bernoulli’s Principle in Everyday Life  

(1)       A person who stands near a railway feels like falling  into it  when suddenly a train moves with a high speed passes him. It is because the velocity of the air in front of him increases. According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases. The higher atmospheric pressure behind  pushes  him forward.

(2)     When two speed bots move faster and closely to each other, an accident may be occurred. It is because the  water moved  at a very high velocity between the boats. According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases .

The higher water pressure on either sides of the boats  caused its  closer to each other .

(3)     A table-tennis player often gives the ball ‘top spin’ enabling a hard drive to be hit over the net and land on the table. A ball hit with spin swerves considerably in its path.

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The figure above shows the rotating surface of the ball, due to spin, and v the air velocity. The spin of the ball carry some air with it, and the resultant velocity of air on the upper surface will therefore be less than v, and the lower surface greater than v. There is therefore a pressure difference between the upper and lower surfaces which deflects the ball, originally hit from right to left in downwards direction

Use of Bernoulli’s Principle in

(1)   Bunsen burner

 

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When  the jet of gas flows out from the nozzle with high velocity , the pressure in the  Bunsen burner becomes low(Bernoulli’s principle)

A higher external atmospheric pressure will be sucked into the air hole and be mixed with the gas .

The mixture of gas and air allows more complete combustion of the gas.

(2)  Insect Piston Spray

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When the piston is pushed, air is forced out through the jet of gas at a high speed.

According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases .

The higher atmospheric pressure in the insect poison container will push the insect poison  liquid up through the narrow metallic tube.

(3)    Filter pump

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Water from a tap flows at high speed out of the jet of water(nozzle).

According to Bernoulli’s Principle , the pressure of the moving air decreases as the speed of the air increases

The higher air pressure in the vessel ‘pumps’ air from the side tube to which the vessel is connected.

(4)   Carburettor

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When the engine is switched on , air is sucked into the the venture tube.

Fast moving air in the narrow section of the tube  causes  a low pressure at this region.

The higher atmospheric pressure in the petrol compartment  will push the petrol into the region.

The petrol will mix with air before it flows into the cylinder engine for combustion.

(5)           Aerofoil

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The above figure shows the streamlines round a thin wing at an angle of attack to the air stream.

It will be seen that the path of air is longer over the upper surface than the lower, and therefore the speed is greater on the upper surface. The lift is produced by the difference in pressure between the two surfaces , which helps the plane to take-off.

In addition to its use in airplanes, aerofoil is also used in racing cars. In this case, the downward force helps to stabilize the car at high speeds.

2 Responses

  1. thanx u teacher……huhuhuhu

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