Page 152 - Physics

Physics for Secondary Schools

Example 7.4 The principle of transmission of pressure
in fl uid can be demonstrated using the
Calculate the pressure exerted on a apparatus shown in Figure 7.17. The
diver at a depth of 20 m below the
surface of water in a sea. Atmospheric apparatus consists of a glass tube fi tted
pressure = 1.03 × 10 Pa, density of with a movable piston and a bulb pierced
5
water = 1 000 kg/m , g = 10 m/s or with uniform-sized holes. Water is fi lled
3
2
10 N/kg. in by dipping the bulb in water and slowly
pulling the plunger.
Solution
Pressure due to the water in the sea When pushing the piston in, the water
= hρg squirts out from all the holes with equal

3
=20 m × 1 000 kg/m × 10 N/kg strength. This shows that the piston exerts
force per unit area (pressure) on the
5
= 200 000 N/m = 2 ×10 Pa water which is then transmitted equally
2
Pressure exerted on the diver = throughout the water. Pascal’s principle is
Atmospheric pressure + pressure due widely used in many devices. Hydraulic
to the water. car jacks, car brakes, and most of the

5
= 1.03 × 10 Pa + 2.00 × 10 Pa mechanical diggers and bulldozers use
5
the principle of transmission of pressure
5
= 3.03 × 10 Pa in fl uid in their operations, and hence the
Transmission of pressure in fl uids name hydraulic machine.
Consider pressure applied to any part of
an enclosed vessel containing a fl uid. The Plunger
molecules of fl uid are free to move; thus,
transmit the pressure equally throughout
the enclosed container. This phenomenon Piston
was observed by a French scientist and
philosopher known as Blaise Pascal in
1650 and it is called the principle of
transmission of pressure in fl uid.

Pascal’s principle of pressure Water
transmission in fl uids states that “When
pressure is applied at any point on the
surface of a fl uid contained in a closed
container, it is transmitted undiminished
to all parts of the fl uid and the walls of Figure 7.17: Illustration of the Pascal’s
the container”. principle

146
Student’s Book Form One

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