Note Hydraulic Form 4 Download 2013

8/12/2019 Note Hydraulic Form 4 Download 2013

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Understanding Pressure1. Pressure on an area, A is the normal force, F, which is being applied perpendicularly to the

area.

2. Pressure on an area, A is expressed as the normal force, F per unit area, A.

3. P = (F/A)

4. This SI unit for pressure is the pascal, Pa, where 1 Pa = 1 N/m2( square meter ).

5. Pressure is increased:

if the force, F applied to a given area, A is increased.

if a given force, F is applied to a smaller area, A.

6. If a balloon is pressed against by a finger, the balloon will only change its shape a bit. If the

balloon is pushed against by a needle with the same force, the balloon will burst. This is because

a finger has a larger surface area (A) than a needle. Hence, the needle exerts more pressure

than the finger and perforates through the surface of the balloon and making a hole and freeing

the air inside the balloon.

Applications of Pascal's Principle in Everyday Life

A hydraulic system is a device in which a small applied force can give rise to a larger force.

The principle in the hydraulic system is widely used in jacks, vehicle brake systems, hydraulic

presses and heavy machinery.

Hyraulic Jacks

Hydraulic jacks are used to lift a heavy load such as when changing a car tyre. When the handle

is pressed down, a valve closes and the small piston forces hydraulic fluid through another valve

to the larger cylinder. The pressure transmitted results in a large force on the load.

When the handle is raised, valve B closes and hydraulic fluid flows from the buffer tank through

valve A into the small cylinder. The handle is moved up and down repeatedly until the load is

sufficiently lifted up.
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The large piston can be lowered at the end by opening the release valve to allow all the

hydraulic fluid to flow back into the buffer tank.


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Hydraulic Brakes

Hydraulic brakes are used in cars, lorries and motorcycles.

In a hydraulic brake system, a liquid, known as brake fluid,

is used to transmit pressure from the brake pedal to all the wheels of the vehicle.


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

brake fluid and this pressure is transmitted, via a system of pipes, to each cylinder at the wheels.

The cylinder at the wheels cause a pair of pistons to push a pair of friction pads to press against

the surface of the brake discs or brake drums. The frictional forces between these brake
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components cause the vehicle to slow down and stop.

When the brake pedal is released, a spring restores the brake discs to their original positions.Hydraulic Pumps

Hydraulic pumps are used to raise cars in a motor workshop.

The machine is equipped with a small cylinder connected to a large cylinder. Both cylinders are

filled with oil.

Compressed air is introduced into the small cylinder in which the compressed air exerts a

pressure on the surface of the oil.

This pressure is transmitted by the oil to the large cylinder where the pressure acts on a large

piston to produce a force which is large enough to lift a car.

Basic Hydraulic System

A hydraulic system operates based on Pascal's principle.

In this hydraulic system, a small force, F1 is applied to the small piston resulting in a large force ,

F2 at the piston K. The pressure, due to the force, F1, is transmitted by the liquid to the large

piston.


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Pressure, P = F1/A1

This pressure is transmitted through the liquid and acts on the base of the large piston.

Force on the large piston, F2 = P X A2.= (F1/A1) X A2.

The large force causes the load to rise.

Also F2/F1 = A2/A1

Output force / input force = output piston area / input piston area

Because of the much larger surface area, A2 of the piston K compared to the surface area, A1 of

the piston, the resultant force, F1.


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This shows that a large force can be produced by a small force, using Pascal's principle.

Hydraulic systems act as a force multiplier where A2/A1 is the multiplying factor.

For example, if A2=5A1, then F2 = 5F1

since F2 = F1 X (A2/A1)

A hydraulic system must not containany air bubbles in any portion of its hydraulic fluid system.
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The presence of air bubbles in the hydraulic fluid system will reduce the efficiency of the system

as part of the applied force will be used to compress the air bubbles.

Instruments for measuring Gas Pressure

1. Manometer

A manometer consists of a U-tube filled with a liquid (mercury, water or oil) with a certain

density.
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The manometer is used to measure the difference in pressure between the two sides of the U-

tube.

When the manometer is not connected to the gas supply, i.e. when both arms are open to the

atmosphere, the liquid levels in both arms are equal.

To measure the pressure of a gas, the other arm is connected to the gas pipe and the gas

pressure acts on the surface of the liquid in the respective arm.

if the gas pressure is greater than the atmospheric pressure, the liquid in the respective arm (say

arm B) will be pushed downwards. Under equilibrium conditions, (same pressure from both

arms), the level of the liquid will be at the same level.

2. Bourdon Gauge

A Bordon gauge consists of a coil of flattened copper tube with an oval cross section connected

to a lever system.


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When the gas supply is connected, the pressure in the gas acts to straighten the copper coil.

The movement of the copper coil is transferred to the lever system which actuates a pointer to

move across a scale which has been calibrated to give readings of pressure.

The unit of measurement used in the Bourdon gauge is Pascal. Bourdon gauges are normally

connected to gas cylinders to give an indication of the quantity of gas in the cylinders.

Bourdon gauges are more robust than manometers and more suitable for measuring higher

pressures. But they have to be calibrated before they can be used.

Instruments for measuring atmospheric pressure:

1. Mercury Barometer

A mercury barometer consists of a thick-walled glass tube, which is closed at one end.


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The tube is completely filled with mercury and inverted several times to remove air bubbles. The

tube is then completely filled again with mercury.

After all air has been removed, the open end of the glass tube is inverted into a container of

mercury.

The mercury column drops until it reaches a height about 76 cm above the lower surface. The

space between the top of the mercury and the end of the tube should contain no air; it is a

complete vacuum.


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Atmospheric pressure and altitude

1. Atmospheric pressure decreases with altitude, or the height above of sea level. At higheraltitudes, the density and temperature of the air are lower. As a result, the frequency of

collisions of the molecules is decreased (lower). Hence, atmospheric pressure is lower.


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Total pressure below the surface of a liquid.

1. The formula for liquid pressure, P = hpg, is used to determine the additional pressure due to
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the weight of the liquid at any point below the liquid's surface.

2. As a result, the total pressure acting at a depth, h below the liquid's surface is the sum ofthe pressure due to the weight of the liquid (P) and the atmospheric pressure acting on the

liquid's surface.

Total pressure acting on an object below a water with a depth of h

= atmospheric pressure + hpg.\

Applications of atmospheric pressure:

Drinking Straw

1. When drinking with a straw, one has to suck the straw. This causes the pressure in the straw


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to decrease.

2. The external atmospheric pressure, which is greater, will then act on the surface of the water

in the glass, causing it to rise through the straw.

Rubber Sucker

1. When the rubber sucker is press onto a smooth surface, usually a glass or tiled surface, the air

in the rubber sucker is forced out. This causes the space between the surface and the sucker to

have low pressure.

2. The contact between the rubber sucker and the smooth surface is airtight.

3. The external atmospheric pressure, which is much higher, acts on the rubber sucker, pressing

it securely against the wall.
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Siphon

1. A rubber tube can be used to siphon liquid from a container at a higher level to another at a

lower level. For example, we can remove petrol from the petrol tank of a vehicle or dirty waterfrom aquarium.

2. The tube is first filled with the liquid and one end is placed in the liquid in the container A. The

other end is placed at a level which must be lower than the surface of the liquid in container A.

3. The pressure in the rubber at the lower end is equal to atmospheric pressure plus the

pressure due to h cm column of liquid. As the pressure at the lower end is greater than the

atmospheric pressure, the liquid flows out.
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Vacuum Cleaner

1. vacuum cleaner applies the principle of atmospheric pressure to remove dust particles. When

it is switched on, the fan sucks out the air from space inside the vacuum (space A). Space A then

becomes a partial vacuum.
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2. The atmospheric pressure outside, which is greater, then forces air and dust particles into the

filter bag. This traps the dust particles but allows the air to flow through an exit at the back.


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Lift Pump

1. A lift pump is used to pump water out of a well or to a higher level. The greatest height to

which the water can be pumped is 10 m. This is equivalent to the atmospheric pressure.

2. When the plunger is lifted, the upper valve closes and the lower valve opens. The atmospheric

pressure, acting on the surface of the water, causes water to flow past valve B into the cylinder.

3. When the plunger is pushed down, the lower valve closes and the upper valve opens. Water

flows above the plunger.

4. When the plunger is next lifted, the upper valve closes again and the lower valve opens once

more. the atmospheric pressure, acting on the surface of the water, forces water past the lower


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valve into the cylinder. Simultaneously, the water above the plunger is lifted and flows out

through the spout.

5. This process is repeated until sufficient water is obtained.


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. The collisions of the gas molecules with the walls of the container are elastic collisions and the

molecules rebound with the same speed which results in a change in momentum for each

molecule.

.The total change of momentum when the gas molecules collide with the walls of the container

in one second produces a force which acts on the walls of the container.

.By the definition of Pressure = Force / Area


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3. Factors Affecting Air or Gas Pressure

a. Pressure increases when the density of gas increases.b. Pressure increases when temperature increases due to kinetic energy of molecules increases.

Atmospheric Pressure

1. Existence of Atmospheric pressure

. According to the kinetic theory of gases, gases consist of molecules which are far apart and inrandom motion at high speeds.

.The gas molecules possess mass and experience the gravitational pull. The result is that gases

have weight.

.The atmosphere is a thick layer of air that surrounds the Earth.

.The atmosphere exerts a pressure called atmospheric pressure which is caused by the weight of

the thick layer of air above the Earth's surface.


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.Atmospheric pressure acts on every object on the surface of the earth.

Activity to show the existence of Atmospheric Pressure

. Fill a glass with water to the brim. Cover it with a thick cardboard. Invert it downwards. Thewater does not fall down. Why? because atmospheric pressure supports the cardboard (and

water) from falling. The resultant force on the cardboard is greater than the weight of water.

Even in the existence of gravity!

.Boil an empty tin half-filled with water. Cap the tin. Let it cool under running tap water.

Wallaa....the tin will get crumpled as the water cools down. As the steam condenses, the

pressure inside the metal tin decreases, The external atmospheric pressure which is higher,

crushes the tin.

Mercury Barometer1. A mercury barometer consists of a thick-walled glass tube, which is closed at one end.

2. The tube is completely filled with mercury and inverted several times to remove air bubbles.

The tube is then completely filled again with mercury.

3. After all air has been removed, the open end of the glass tube is inverted into a container of

mercury.

4. The mercury column drops until it reaches a height of about 76cm above the lower surface.


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The space between the top of the mercury and the end of the tube should contain no air, it is a

complete vacuum.

5. The column of mercury in the tube is supported by the atmospheric pressure and its heightdepends on the magnitude of the atmospheric pressure.

6. Since the atmospheric pressure at sea level can support a vertical column of mercury 76 cm

or 760 mm high, we can, for convenience, express mm Hg as a unit of pressure. 1 Standard

atmospheric pressure (1 Patm) = 76 cm Hg or 760 mm Hg (also known as one atmosphere).

Patm = 76 cm Hg = 10 000 Pa.

1 P atm = 76 cm Hg = 10 000 Pa = 1 bar. In unit m water: Patm = 10 m water.

Understanding Pressure in Liquids

1. For liquid at rest, the pressure at a certain point in the liquid is the same in all directions.

2. The pressure in a liquid is due to


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a) Density of the liquid,p.

b) Depth of the liquid,hbelow the surface liquid.c) Acceleration of the gravity,g.

3. The pressure on a liquid is proportional to the density of the liquid,p and the depth, h, at

which the liquid is measured.

4. The pressure in a liquid at rest (static liquid) is independent of the shape (area and slope)o

of the container.
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5. The applications of pressure in liquids are:

i) Damsii) Domestic Water supplies

i) Dams

- Dam is very much thicker at the bottom than at the top, since the pressure at the bottom is the

greatest.

- Large dams are built for the hydroelectric generation of electricity.

- The high pressure on the deep-water side of the dam causes water to flow through these holes

at great speed turning the turbines in the holes and generate the electricity.

ii) Domestic Water Supplies

- The main water comes from a reservoir but in order to maintain a constant high pressure tothe consumer, it is pumped to the top of a water tower located on high ground.

- The main pressure is determined by the height,h.

Note Hydraulic Form 4 Download 2013

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