physics f4 pressure

7/23/2019 Physics f4 Pressure

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In SPM, you need to know

1. The definition of pressure

2. The formula of pressure (you need this formula to do lot of calculation.)

. The SI unit of pressure

!. "actors affectin# the ma#nitude of pressure

Pressure

Pressure is defined as the force actin# normally per unit area. ($ere, the word %normally% means

perpendicularly.)

Example

&s shown in the dia#ram a'oe, a 2* force acts on a !cm+ surface. The force is shared eually 'y the

surface, hence each 1 cm+ of the surface withstand a force of -* (2*!).

The force (-*) acts on 1 unit area (1cm+) is said to 'e the pressure actin# on the surface. Therefore, the

pressure actin# on the surface is -*cm+.

Mathematically,

Pressure=Force/AreaP=F/A=20N/4cm2=5N/cm

Unit of PressureThe SI unit of pressure is Pascal (Pa). 1 Pa is eual to1 newton per metre2 (*m+).

1 N/m = 1 Pa


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Factors Affecting the Magnitude of Pressure

a. Magnitude of the force.

The lar#er the force, the hi#her the pressure.

'. ontact area.

The lar#er the contact area, the lower the pressure.

"indin# the force acted on a surface when the pressure and surface area are #ien.

/ifficulty 0eel 1

Example

& force " is actin# on a surface of area 2cm+, produces a pressure 2-Pa on the surface. "ind the

ma#nitude of the force. (&*S -*)

Ans!er

Example "

& 'lock of wood m lon#, - m wide and 1 m thick is placed on a ta'le. If the density of the wood is

k#m3, find

a. the lowest pressure

b. the hi#hest pressure on the ta'le due to the 'lock.

Ans!er


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Example #Two cu'es made of the same material4 one has sides twice as the other, lyin# on a ta'le. Standin# on oneface, the small cu'e e5erts a pressure M on the ta'le. 6hat is the pressure (in term of M) e5erted 'y thelar#er cu'e standin# on one of its faces, on the ta'le7

Ans!er

"or the small cu'e,

Surface area 8 5+

9olume 8 5:

Mass 8 m

6ei#ht 8 m#

Pressure,

P=FAP=mgx2=M

"or the 'i# cu'e,

Surface area 8 !5+

9olume 8 ;5:

Mass 8 ;m (


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Pressure in 0iuids

1. Pressure in liuid is o!ing to the !eightof the liuid actin# on the surface of any o'=ects in the

liuid.2. Pressure of a liuid is directl' proportionalto

a. the gra(itational field strength

'. the depth

c. densit' of the liuid.

. Pressure in liuids is N)* AFFE*E+'y thesi,e or shapeof the o'=ect.

!. The pressure caused 'y a liuid and the pressure in a liuid can 'e determined 'y usin# the

euation 'elow

Pressure Caused by Liquid

Pressure in Liquid


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Example 1

The dia#ram shows 2 fishes in water. The density of the water is 12- k#m:. The surface area of fish & is cm+ and the surface area of fish < is 2cm+. "inda. the pressure e5erted 'y the water on fish &.'. the pressure e5erted 'y the water on fish


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d. %urface area of fish - A"= "cm = ." m

Force exerted ' the li0uid pressure-

P=FAF=PAF=(20500)(0.2)F=4100N

A5ample 2

Figure ao(e sho!s the cross section of a sea near a seaside. Find the difference of the pressureet!een point A and point . 2+ensit' of sea!ater = 134g/m56

Ans!er$/ensity, ? 8 1-k#m:@raitational "ield Stren#th, # 8 1 *k#

&t point &/epth, h 8 .; m

Pa=hag+PatmPa=(0.8)(1050)(10)+PatmPa=8400+Patm

&t point


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P=hg+PatmP=(20)(1000)(10)+(100000)P=300000Pa

Pressure 7n 8i0uid 9 *he U9*ue

1. Dsually, a D3tu'e is used to compare and measure density of liuids.

2. The density of the 2 liuids is related 'y the euation

h11=h22

Example 1$

"i#ure a'oe shows a D3tu'e filled with water and liuid P. 0iuid P is insolu'le in water. @ien that the

density of water is 1k#m:, find the density of liuid P.

Ans!erh18 1cmh28 12 cm?18 1k#m:?28 7

h11=h22(10)(1000)=(12)22=1000012=833kgm3


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Example "$

The dia#ram shows a D3tu'e filled with two types of liuid, E and F which are not mi5a'le. If the density of

liuid E and liuid F are 12 k#m: and ; k#m: respectiely, find the alue of h.

Ans!erh18 1cmh28 h?18 ;k#m:?28 12k#m:

h11=h22(10)(1200)=h(800)h=12000800=15cm

haracteristic of li0uid pressure$

1. Pressure in liuid +)E% N)* depends on

%hape of container

%i,e of container

Area of it surfaces


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Pressure increases !ith depth

1. The deeper the liuid, the faster the liuid spurts out.

2. In conclusion, the pressure in a liuid increases with depth.

.

Pressure depends on the (ertical depth- ut not the length of li0uid column.

Pressure at A = Pressure at

All points at the same le(el in a li0uid are at the same pressure

The leel of the surface of a liuid in a container is always the same 'ecause for a #ien liuid, the

pressure is always eual at the same leel.


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Pressure does not depend on the surface area of the o:ect.

Pressure e5erted on the small fish

8 Pressure e5erted on the 'i# fish

Pressure acts in all direction

The pressure at any point of a liuid acts eually in all direction.


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Application of li0uid pressure

+am

The wall of the dam is 'uilt thicker at the 'ottom to

withstand a hi#her pressure.

1. The #enerator is placed at the lower part so that

the pressure of the water is hi#h enou#h to drie the

tur'ine.

Su'marine

In deep sea, the pressure of the water is

tremendously hi#h. $ence the 'ody of the

su'marine is thick and 'uilt 'y stron#

material

Measuring lood Pressure

6hen measurin# 'lood pressure, the inflata'le cuff of the

sphy#momanometer should 'e at the same leel with the

heart so that the pressure measured is eual to the 'lood

pressure of the heart.

Intraenous Transfusion

"or intraenous transfusion (I9), the 'ottle

is hun# at an eleated position to ensure

that the liuid in the I9 'ottle #ains sufficient

pressure to flow into the ein of the patient.

6ater Tower

The water tower is 'uilt at hi#h place so that the water has


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sufficient pressure to flow to consumerGs house.

;as Pressure

@as pressure is the force per unit area e5erted 'y the #as molecules as they collide with the surface of an

o'=ect.

1. In SPM, especially in paper 2 essay uestion, you need to know how the #as pressure is

produced. (See uestion 'elow)


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Unit Used to Measure Atmospheric Pressure

1. The followin# are the unit used to measure atmospheric pressure

a. Pascal (Pa)

1 Pa 8 1 *m+

'. Standard &tmospheric Pressure (atm)

1 atm 8 &tmospheric Pressure at sea leel ( 8 11,2- Pa)

c. mm$# (also known as torr)

1 mm$# 8 1J atm (rou#hly eual to the liuid pressure e5erted 'y a millimetre of mercury).

d. mili'ar (*ot used in SPM)

". 7n %PM- usuall' !e use the unit cm>g- instead of mm>g.

Proof of Existence of Atmospheric Pressure

The e5istence of the atmospheric pressure can 'e proed 'y the followin# e5periments.

1. Brushin# can e5periment

2. 6ater coer with card'oard does not flow out

. Ma#de'ur# $emisphere

rushing can experiment

1. 6hen a can fil led with hot water is closed and is

cooled down rapidly 'y pourin# cold water on it, it will

crush instantly.

2. This e5periment proes that there is a hu#e

atmospheric pressure e5erts on eerythin# on the

surface of the earth.

?ater co(er !ith cardoard does

not flo! out

1. The card'oard does not fall and

the water remains in the #lass een

thou#h itKs not supported 'y

anythin#.

2. This is 'ecause the force

caused 'y the atmospheric pressure

acts on the surface of the card'oard

is #reater than the wei#ht of the


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water in the #lass. This e5periment

proes that atmospheric pressure is

present on the surface of the earth.

Syrin#e

1. 6hen the piston is pulled up, the atmospheric

pressure inside the cylinder will decrease.

2. The atmospheric pressure outside pushes the

liuid up into the syrin#e.

Magdeurg >emisphere

1. 6hen the air inside the

hemisphere is pumped out so that it

'ecomes a acuum, the hemisphere

cannot 'e separated een 'y a ery

#reat force.

2. This is 'ecause when the air is

pumped out, the pressure inside the

hemisphere 'ecomes ery low.

. The atmospheric pressure

e5erts a stron# force on the outer

surface of the hemisphere, holdin#

the hemisphere ti#htly to#ether.

0ift Pump Siphon6orkin# Mechanism of Siphon


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Sucker $ook

6hen the sucker is pressed into place, the air inside is

forced out. &s a result, the pressure inside the sucker

'ecome ery low. The sucker is then held in position 'y the

hi#h atmospheric

pressure on the outside surface.

Straw

6hen a person sucks throu#h the straw,

the pressure in the straw 'ecome low.

The atmospheric pressure outside

which is hi#her will force the water into

the straw and conseuently into the

mouth.

Lu''er Sucker 9acuum Bleaner


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9acuum Bleaner

Instruments Dsed to Measure &tmospheric Pressure

1. &tmospheric pressure can 'e measured 'y either

a. a simple 'arometer,

'. a "ortin 'arometer

c. an &neroid 'arometer.

2. In SPM, most of the uestions asked are related to the simple 'arometer.

. "or "ortin 'arometer and &neroid 'arometer, you only need to know their workin# principle.


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Simple Mercury


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The Aneroid Barometer

Simple


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Characteristics of the Mercury Barometer

< and A


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"ind the pressure at point &,


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"i#ure a'oe shows a simple mercury 'arometer. 6hat is the alue of the atmospheric pressure shown 'y

the 'arometer7 /ensity of mercury 8 1J k#m:N

Ans!er

&tmospheric Pressure,

P 8 J cm$#

or

P 8 h?# 8 (.J)(1J)(1) 8 1J Pa

Example #

In a'oe, the hei#ht of a mercury 'arometer is h when the atmospheric pressure is 11 Pa.6hat is the pressure at E7

Ans!er

&tmospheric Pressure,

Patm8 h cm$# 8 11 Pa

Pressure at E,

PE8 (h 3 Oh) 8 h cm$#

PE8 5 11 8 - - Pa


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Example @

"i#ure a'oe shows a mercury 'arometer where'y the atmospheric pressure is J mm $# on aparticular day. /etermine the pressure at pointa. &,'.


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&tmospheric pressure 8 Patm

Pair C -- cm$# 8 Patm

Pair

8 Patm 3 -- cm$#

8 11 3 (.--)(1 J)(1)

8 11 3 ! ;

8 2J 2 Pa

A5ample J

If the atmospheric pressure in a housin# area is 1 Pa, what is the ma#nitude of the force e5erted 'y

the atmospheric #as on a flat hori>ontal roof of dimensions -m Q !m7

&nswer

&rea of the roof 8 - 5 ! 8 2 m+

"orce acted on the roof

" 8 P&

" 8 (1 )(2)

" 8 2,, *

"ortin


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. 9ernier scale is used for takin# readin# for e5tra accuracy.

< A


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1. &


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1. & manometer is a D3shape tu'e filled with some liuid (usually mercury).

2. Manometer is a deice used to measure #as pressure in a container.

. The pressure of the #as is eual to the sum of the atmospheric pressure and pressure due to the

column of liuid.

P#as8 PatmC Pliuid

Note:

There are a few points we need to know when usin# a manometer

a. /ifference in #as pressure at difference leel can 'e i#nored.

'. Pressure on the surface of liuid is eual to the #as pressure in contact.

c. Pressure that cause 'y liuid 8 h?#.

d. "or a #ien liuid, the pressures at any point of the same leel are the same.

e. "or different liuid with different density, pressure at two different leel will 'e different.

f. The pressure of the #as trapped in a capillary tu'e depends on the position of the tu'e.


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#. "i#ure 'elow shows the pressure of the #as when the capillary tu'e is hori>ontal, ertical and

ertically upside down.

physics f4 pressure

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