chapter 3 : force & pressure (form 4)

34
Pressure Pressure is defined as force acting perpendicular on a surface area. Formula of Pressure Factors affecting the magnitude of Pressure: 1. Magnitude of the force: The larger the force, the higher the pressure. 2. Contact area: The larger the contact area, the lower the pressure. 3. S.I. Unit of Pressure: The S.I. unit of pressure is : Pascal Notes: 1Pa = 1 Nm -2 Other units: cmHg, atm Application Involving High Pressure CHAPTER 3 : FORCE AND PRESSURE

Upload: ruslawati

Post on 14-Apr-2015

2.690 views

Category:

Documents


9 download

DESCRIPTION

Teacher's Copy

TRANSCRIPT

Page 1: Chapter 3 : Force & Pressure (Form 4)

Pressure

Pressure is defined as  force acting perpendicular on a surface area.

Formula of Pressure

Factors affecting the magnitude of Pressure:

1. Magnitude of the force:The larger the force, the higher the pressure.

2. Contact area:The larger the contact area, the lower the pressure.

3. S.I. Unit of Pressure: The S.I. unit of pressure is : Pascal

Notes: 1Pa = 1 Nm-2

Other units: cmHg, atm

Application Involving High Pressure

Application Involving Low Pressure

Foundation of Building

Snow Shoes Tyre of Tractor Feet of Elephant

CHAPTER 3 : FORCE AND PRESSURE

Page 2: Chapter 3 : Force & Pressure (Form 4)

Pressure in Liquid

Pressure in liquid is due to the weight of the liquid acting on the surface of any objects in the liquid.

 Formula: Pressure Caused by Liquid

P = Pressureh = depth

ρ = density of liquid

Example 1

Figure above shows a fish in a lake. Find the pressure of water exerted on the fish. (Density of water = 1000kg/m3)

Answer:

Pressure caused by water,P = hρgP = (3-1.8)(1000)(10)=12000Pa

Page 3: Chapter 3 : Force & Pressure (Form 4)

Formula: Pressure in Liquid

P = Pressure

Patm = Atmospheric Pressure

h = Depth

ρ = Density of liquid

g = Gravitational Field Strength

Characteristic 1:

The pressure in a liquid increases with depth.

Characteristic 2:

The pressure at any point of a liquid acts equally in all direction.

Page 4: Chapter 3 : Force & Pressure (Form 4)

Characteristic 3:

For a given liquid, the pressure at a point within it varies only with the vertical depth of the point below the surface of the liquid.

Pressure in liquid does not depends on- the shape of the container.- the size of the container.- the area of its surface

Characteristic 4:

Pressure in liquid depends only on its vertical distance from the surface of the liquid.

Pressure at A = Pressure at B

Page 5: Chapter 3 : Force & Pressure (Form 4)

Application of Pressure in Liquid:

1. Construction of Dam

The wall of the dam is built thicker at the bottom to withstand a higher pressure.

The generator is placed at the lower part so that the pressure of the water is high enough to drive the turbine.

2. Public Water Supply System

The water tower is built at high place so that the water has sufficient pressure to

flow to consumer’s house.

Page 6: Chapter 3 : Force & Pressure (Form 4)

What is Atmospheric Pressure?

Atmospheric pressure is the pressure exerted on the surface of any objects by the atmospheric gas.

It is caused by the weight of the atmospheric gas above the earth surface.

Unit of atmospheric pressure

The units of atmospheric pressure used in the SPM syllabus include:

- Pascal (Pa)- centimetre/milimetre mercury (cmHg/mmHg)- atmosphere (atm)- metre water

Notes:

- Atmospheric Pressure at Sea Level = 1atm- 1 atm = 76cmHg

Atmospheric Pressure and Altitude

Atmospheric pressure decreases as the altitude increases.

Conversion of Pa and cmHg

h = height of mercury column

ρ = density of mercury

g = gravitational field strength

Page 7: Chapter 3 : Force & Pressure (Form 4)

Example 1:

Given that the atmospheric pressure at Genting Highland is 72cmHg. Find the equivalent

value of this pressure in the unit of Pascal (Pa). [density of mercury = 13600 kgm-3]

Answer:

h = 72cm = 0.72m

ρ = 13600 kgm-3

g = 10 ms-2

Pressure in the unit of Pascal,

P = hρg

P = (0.72)(13600)(10) = 97920 Pa

Existence of Atmospheric Pressure

The cardboard does not fall and the water remains in the glass even though it’s not

supported by anything.

Explanation:

The force caused by the atmospheric pressure acts on the surface of the cardboard is

greater than the weight of the water in the glass.

Page 8: Chapter 3 : Force & Pressure (Form 4)

Crushing Can

When a can filled with hot water is closed and is cooled down rapidly by pouring cold

water on it, it will crash instantly.

Explanation:

When the air inside the can is cooled, its pressure decreases. The high atmospheric

pressure outside exerts a great force on the can and causes it crashes.

Magdeburg Hemisphere

Figure above shows a Magdeburg Hemisphere. When the air inside the hemisphere is

pumped out so that it becomes a vacuum, the hemisphere cannot be separated even by

a very great force.

Page 9: Chapter 3 : Force & Pressure (Form 4)

Explanation:

The atmospheric pressure exerts a strong force on the outer surface of the hemisphere,

holding the hemisphere tightly together.

Instrument Used to Measure Atmospheric Pressure

-Simple Barometer

-Fortin Barometer

-Anaroid Barometer

Page 10: Chapter 3 : Force & Pressure (Form 4)

Simple Barometer

Pa = 0cmHg

Pb = 26cmHg

Pe = 76cmHg

Pf = 84 cmHg

For a given liquid, the pressure will be the same at same level. Therefore,

Pd = Pe = 76cmHg

The difference of gas pressure at different level can be ignored if the difference of the

height is not too large. Therefore,

Pc = Pd = 76cmHg

Page 11: Chapter 3 : Force & Pressure (Form 4)

Example 2

Figure above shows a simple barometer. Find the atmospheric pressure in the unit of

a) cmHg

b) Pa

[Density of mercury = 13,600 kgm-3]

Answer:

a) The atmospheric pressure,

P = 76cmHg

b) The atmospheric pressure,

P = hρg

P = (0.76)(13,600)(10) = 103,360 Pa

Page 12: Chapter 3 : Force & Pressure (Form 4)

Example 3

Figure above shows a column of mercury in a glass tube. Some air is trapped in the

glass tube. If the atmospheric pressure is 76cmHg, what is the pressure of the gas

inside the tube?

Answer:

Patm = Pair + Pmercury

(76) = Pair + 50

Pair= 76 - 50 = 26cmHg

Page 13: Chapter 3 : Force & Pressure (Form 4)

More About Simple Barometer

The height, h will remains unchanged when

i) the glass tube is lifted up from the dish 

ii) the glass tube is lowered  further into the dish

iii) the diameter of the glass tube increases 

iv) the glass tube is tilted

v) the quantity of mercury in the dish is increased

Rubber Sucker

-When the sucker is pressed into place, the air inside is forced out.

-The sucker is held in position by the high atmospheric pressure on the outside surface.

Page 14: Chapter 3 : Force & Pressure (Form 4)

Syringe

When the piston is pulled up, the atmospheric pressure inside the cylinder will decrease.

The atmospheric pressure outside pushes the liquid up into the syringe.

Straw

-When a person suck through the straw, the pressure in the straw become low.

-The atmospheric pressure outside which is higher will force the water into the straw and

consequently into the mouth.

Page 15: Chapter 3 : Force & Pressure (Form 4)

Vacuum Cleaner

When a vacuum cleaner is switched on, it sucks out the air inside the cleaner, causes

the pressure inside the cleaner become low.

The atmospheric pressure which is higher in magnitude, forces the air and duct particles

into the cleaner.

Page 16: Chapter 3 : Force & Pressure (Form 4)

How gas pressure is produced?

Gas molecules are at constant and random movements.

When the molecules collide with the wall of the container and bounce back, they

experience a change in momentum.

The momentum change exerts a force on the wall.

Force per unit area is the pressure exerted on the wall of the container.

instruments used to measure gas pressure

-Manometer

- Bourdon Gauge

Using Manometer

Difference of gas pressure at different level can be ignored.

Pessure on the surface of a liquid is equal to the pressure of the gas in contact.

For a given liquid (same liquid), the pressure at any point of the same level is the

same.

For different liquid with different density, pressure will be different at the same

level.

Page 17: Chapter 3 : Force & Pressure (Form 4)

Pgas = Patm + Pliquid

Pgas = Patm + hρg

Pgas = Gas Pressure

Patm = Atmospheric Pressure

g = Gravitational Field Strength

Page 18: Chapter 3 : Force & Pressure (Form 4)

Figure above shows the water levels in a water manometer used to measure

the pressure of a gas supply. Find the pressure of the gas in unit of

i) cm water

ii) Pa

[Atmospheric Pressure = 1000cm Water; Density of water = 1000kg/m3]

Answer: 

i) 

Gas Pressure, 

P = Patm + Pwater 

P = 1000 + (30-5) = 1025 cm Water 

ii) 

h = 1025cm = 10.25m 

ρ = 1000kgm3 

g = 10ms-2 

Gas Pressure, 

P = hρg 

P = (10.25)(1000)(10) 

P = 102500 Pa

Page 19: Chapter 3 : Force & Pressure (Form 4)

U-Tube

A U-tube can be used to determine density

Page 20: Chapter 3 : Force & Pressure (Form 4)

Figure above shows a U-tube filled with 2 liquids X and Y that do not mix. Given that the density of liquid Y is 1.6g/cm3. Find the density of liquid X.

Answer:

Page 21: Chapter 3 : Force & Pressure (Form 4)

Gas Pressure In A Capillary Tube

Pgas = gas pressure in the capillary tube

Patm = atmospheric pressure

h = length of the captured mercury column

ρ = density of mercury

g = gravitational field strength

Page 22: Chapter 3 : Force & Pressure (Form 4)

Example 3

Figure above shows a capillary containing a column of mercury. If the atmospheric pressure is 100,000Pa, find the pressure of the gas trapped in the capillary tube. [Density of mercury = 13,600kg/m3]

Answer:Gas Pressure,P = Patm + hρgP = 100,000 + (0.005)(13600)(10)P = 100680Pa

Bourdon Pressure Gauge

If pressure is applied, the copper tube will be straightened slightly.

The copper tube pushes the lever system and hence rotates the pointer.

Gas pressure can be read directly from the scale.

Page 23: Chapter 3 : Force & Pressure (Form 4)

Pascal's Principle

Pascal's principle states that any change in pressure applied to an enclosed liquid will

be transmitted to every point of the fluid without any change in magnitude.

Pascal's Priciple - Prove

When the plunger is pushed in, the water squirts equally from all the holes. This

shows that the pressure applied to the plunger has beentransmitted

uniformly throughout the water.

Hydraulic System

Page 24: Chapter 3 : Force & Pressure (Form 4)

F1 = Force exerted on the small piston

A1 = area of the small piston

F2 = Force exerted on the big piston

A2 = area of the big piston

The hydraulic system acts as a force multiplier.

In a hydraulic system the large piston has cross-sectional area A2 = 200 cm2 and the

small piston has cross-sectional area A1 = 5 cm2. If a force of 250 N is applied to the

small piston, what is the force F, on the large piston?

Answer

Page 25: Chapter 3 : Force & Pressure (Form 4)

Hydraulic Jack

-When the handle is pressed down, valve A is closed whereas valve B is opened. The

hydraulic fluid is forced into the large cylinder and hence pushes the piston moving

upward.

-When the handle is raised, valve B will be closed while vale A will be opened. Hydraulic

fluid from the buffer tank will be suck into the small cylinder.

-This process is repeated until the load is sufficiently lifted up.

-The large piston can be lowered down by releasing the hydraulic fluid back to the buffer

tank through the release vale.

Page 26: Chapter 3 : Force & Pressure (Form 4)

Hydraulic Brake

When the brake pedal is pressed, the piston of the master cylinder applies a pressure

on the brake fluid.

This pressure is transmitted uniformly to each cylinders at the wheel, cause the

pistons at the wheels to push the brake shoes to press against the surface of the

brake.

The friction between the brakes and brake shoes causes the vehicle to slow down and

stop.

Page 27: Chapter 3 : Force & Pressure (Form 4)

Bernoulli's Principle

Bernoulli's principle, physical principle formulated by Daniel Bernoulli that states that

"as the speed of a moving fluid (liquid or gas)increases, the pressure within the

fluiddecreases."

Experiment 1

Water flows from high pressure region to low region.

Pressure at A > Pressure at B > Pressure at C.

Experiment 2

Pressure at B becomes lowest because the speed of the water is the highest.

Page 28: Chapter 3 : Force & Pressure (Form 4)

Experiment 3

Pressure at B becomes lowest because the speed of the water is the highest.

Bunsen Burner

When the burner is connected to a gas supply, the gas flows at high velocity

through a narrow passage in the burner, creating a region of low pressure.

The outside air, which is at atmospheric

pressure, is drawn in and mixes with the gas.

The mixture of gas and air enables the gas to burn completely to produce a

clean, hot, and smokeless flame

Page 29: Chapter 3 : Force & Pressure (Form 4)

Aeroplane

When a wing in the form of an aerofoil moves in air, the flow of air over the top

travels faster and creates a region of low pressure. The flow of air below the

wing is slower resulting in a region of higher pressure.

The difference between the pressures at the top and underside of the wing

causes a net upward force, called lift, which helps the plane to take-off.

Insect Spray

When the plunger is pushed in, the air flows at a high velocity through a nozzle.

The flow of air at high velocity creates a region of low pressure above the metal

tube. The higher pressure of the atmospheric air acts on the surface of the

liquid insecticide causing it to rise up the metal tube.

The insecticide leaves the top of the metal tube through the nozzle as a fine spray.