FORM -1-PHYSICS-PRESSURE

PRESSURE

Concept of Pressure

Pressure due to Solids

Pressure in Liquids

A liquid will exert pressure on an immersed object as well as on the walls of the container holding it. Note that the pressure exerted by liquids is due to the weight of the liquid. Also increase in water level in liquid cause increase in pressure.

The Characteristics of Pressure in Liquids

Describe the characteristics of pressure in liquids

Note: Pressure in liquids =Force/Area

But force= hApg/A

=Hsg

Hence pressure in liquids is given as hsg.

H= Height of the liquid column

A= Area of the base

The pressure at any point in a liquid at rest then depends

Depth
Density of liquid

Note: It does not defend on the area

Pressure in liquids is characterized using the following parameters

Pressure in a liquid increase with depth
Pressure in a liquid acts equally in all directors
Pressure in a liquid increases with increase in density of the liquid.

Mercury exerts more pressure than an equal volume of water. This because mercury is denser than water.

The Variation of Pressure with Depth in Liquids

Examine the variation of pressure with depth in liquids

Demonstration of water spurting from holes at different heights.

As pressure at point A is low due to small height of water above it but at point B the height increase and the distance of water increase while at point C the height is greatest therefore the level of water distance will be large distance.

Demonstration of pressure in a communicating vessel

The shape of vessel found in point A, B, C and D is different but the pressure is the same due to the equal height (L) of the liquid above

Problems Involving Pressure in Liquids

Solve problems involving Pressure in Liquids

Example 4

A cube of side 2cm is completely submerged in water so that the bottom of the cube is at a depth of 10cm. use g=10m/s² and s=1000kg/m³

What is different between the pressure on the bottom of the cable and the pressure on its tap?
Determine the difference in the force on the top and bottom.
What is the weight of the water displaced by the cube?

Solution;

Data.

Depth = 10cm = 0.1m

s = 1000kg/m³

g = 10m/s

h= 8cm= 0.08

formular

p= sgh

p=1000kg/m³x10m/s²x0.1m=1000N/m²

At the h= 0.08m p= 1000x10x0.08= 800N/m²

Change in pressure = 1000N/m =200N/m²

So that different in pressure between the bottom the top is 200N/m²

Pressure = Force/Area = P= F/A

Area =L²= (10.02)²= 0.0004m²

Distance in water 200N/m²x0.0004m²

This is the upthrust acting on the cube

Water displaced by the cube in weight= volume of the cube

Volume = l3 (2cm)³ = 8cm³

For water, 1cm³= 1g 50.8cm³ = 8g

Weight of displaced water

mass of displaced water/100=8g = 0.08N

Example 5

Calculate the pressure at the bottom of tank of water 15m deep due to the water above it is (s=1000kg/m³).

Solution:

Data:

Height =15m

S=1000kg/m³

g= 10m/s²

Formula

Pressure in liquid sgh

= 1000x10x15=150000N/M²

The Principle of a Hydraulic Pressure

Explain the principle of a hydraulic pressure

It state that “Any external pressure applied to the surface of an enclosed liquid will be transmitted equally throughout liquid.”

Illustration of Pascal’s Principle.

Note: in all side of vessel will experience equal pressure.

The hydraulic press works on Pascal’s principle, when using this principle a small force converted into a large force and vice versa.

From Pascal’s Principle

Note: The distance moved (d) is inversely proportional to the cross sectional area.

Example 6

The pistons of a hydraulic press have their areas given as 3.^0x10-4m2 and 2x10²m² respectively. If the smaller piston is pushed down with a force of 12N, what is the force required to push the larger piston?

Soln:

Data:

F₁ = 120N, A₂= 2X10²m²

A₂ = 3X10⁴m²

Application of Hydraulic Press

Measure pressure of a liquid

Hydraulic press is used in industries to express bulk items.

Hydraulic brake system

When pressure is applied to the brake pedal, it pushes the piston in the master cylinder forward creating a pressure in the brake fluid. This pressure is transferred to the slave cylinders where it is multiplied and pushes the brake shoes against the brake dram that is attached to the wheel of the vehicle.