# Applications on pascal’s principle , Manometer types and uses

**The aqueous manometer is preferred to the mercuric one to measure small pressure differences because the density of water is small compared to that of mercury , So , the difference in water levels in the two branches of the manometer becomes clear and easy to be measured which decreases the error in measurement .**

**Manometer**

**The manometer consists of U-shaped tube containing proper amount of liquid of known density , One of its ends is connected to the gas reservoir whose pressure to be measured and the other end exposed to air , The pressure at all points in the same horizontal plane in a homogeneous liquid is the same .
**

**Manometer is used in measuring the pressure of enclosed gas , It is used in measuring the difference between the pressure of enclosed gas and the atmospheric pressure .**

**Manometer types**

**Aqueous manometer : the liquid used is water .****Mercuric manometer : the liquid used is mercury .**

**It is preferable to use the mercuric manometer for measuring high pressure difference because the mercury has high density , so , mercury neither rash out the tube nor into the gas reservoir .**

**How it is used ?**

**If the liquid level in the free end branch is the same liquid level in the other branch that connected to the gas reservoir , Then , ****P _{a} = P_{gas }**

**Δ P = P _{gas} − P_{a} = zero**

**Δ P = zero , ****If the liquid used is mercury **

**If the liquid level in the free end branch is higher than the liquid level in the other branch that connected to the gas reservoir , Then , P _{gas } > P_{a} .**

** P _{gas} = P_{a} + ρ g h**

**Δ P = P _{gas} − P_{a}**

**Δ P = + ρ g h ( N / m² )**

**If the liquid used is mercury then : ****P _{gas} = P_{a} + h**

**Δ P = P _{gas} − P_{a} , Δ P = + h ( cm Hg ) .**

**If the liquid level in the free end branch is lower than the liquid level in the branch that connected to the gas reservoir , Then , P _{gas } < P_{a} .**

**P _{gas} = P_{a} − ρ g h**

**Δ P = P _{gas} − P_{a}**

**Δ P = − ρ g h ( N / m² )**

**If the liquid used is mercury then : ****P _{gas} = P_{a} − h**

**Δ P = P _{gas} − P_{a} , Δ P = − h ( cm Hg ) .**

**Pascal’s principle**

**When a liquid is put in a glass container equipped with a piston at the top , then the pressure at point A at depth h is : **

**P = P**_{1} + ρ g h

_{1}+ ρ g h

**Where : P _{1} is the pressure at the liquid surface and it is due to the atmospheric pressure and the weight of the piston , ρ g h is the pressure of the liquid column on point A , when putting extra load on the piston , the pressure increases by Δ P . **

#### **P = P**_{1} + ρ g h + **Δ P**

_{1}+ ρ g h

**The piston does not move down because the liquid is incompressible , If the pressure on the piston is increased to a certain limit , the glass container breaks down , This means that the pressure acting on the piston is transferred to all parts of the liquid and to the walls of the container , ****The French scientist Pascal set his principle based on this result .**

**Pascal’s principle : When pressure is applied on a liquid in a container , the pressure is transmitted in full to all parts of the liquid as well as the walls of the container , ****Pascal’s principle is not applied to gases because unlike liquids , they can be compressed due to the big intermolecular spaces between their molecules .
**

**Applications on pascal’s principle **

**The hydraulic press****The hydraulic brakes of the car****The hydraulic lift ( It uses a liquid to lift cars in the petrol stations )**

**Chair of the dentist****The hydraulic drill****Diving suit**

**The hydraulic press**

**Hydraulic press consists of a tube with 2 pistons at its ends , one small of area ( a ) and the other big of area ( A ) where the space between them is filled with an appropriate liquid such as oil , It is used in lifting high loads using a small force and it is based on Pascal’s principle .
**

**When a force ( f ) acts on the small piston , pressure ( P ) is produced , P = f / a , The pressure ( P ) transfers completely through the liquid to the lower surface of the big piston where a force ( F ) is produced , P = F / A , ****In this case the pistons are in equilibrium at one horizontal level where : **

**Pressure on the small piston = Pressure on the big piston **

**P = f / a = F / A**

**If the force ( f ) moved the small piston a distance ( y _{1} ) , then the big piston is affected by a force ( F ) that moves it a distance ( y_{2} ) .**

**Applying the law of conservation of energy ( case of ideal press ) , then :**

** Work done on the small piston = Work done on the big piston **

**f y _{1} = F y_{2}**

**F/ f = y**_{1} / y_{2}

_{1}/ y

_{2}

**Cases of hydraulic press**

**When the two pistons are at the same level
**

**P = f / a = F / A**

**When the two pistons are at different levels **

**P = ( f / a ) = ( F / A ) + ρ g h or P = ( f / a ) + ρ g h = F / A
**

**Where : ρ is the liquid density , h is difference in height between the two pistons .**

**The mechanic advantage of the hydraulic press **

**The mechanic advantage ( η ) is determined from the relation :**

**η =( F / A ) = ( f / a ) = ( A / a ) = ( R² / r² ) = ( D² / d² ) = ( y _{1} / y_{2 })= ( v_{1} / v_{2} )**

**Where : R = radius of the big piston , r = radius of the small piston **

**D = diameter of the big piston , d = diameter of the small piston **

** v _{1} = speed of the small piston , v_{2} = speed of the big piston **

**The mechanic advantage ( η ) is the ratio between the force produced at the big piston and the acting force at the small piston or it is the ratio between the cross-section area of the big piston to the cross-section area of the small piston . **

**When the mechanical advantage of a hydraulic press at equilibrium = 400 , It means that the ratio between the force produced at the big piston to that acting at the small piston = 400 .**

**Efficiency of the hydraulic press is the ratio between the work done at the big piston to the work done at the small piston , ****The efficiency of the hydraulic press is determined from the relation : F y _{2} ÷ f y_{1} **

**When the efficiency of the hydraulic press = 97% , It means that the ratio between the work done at the big piston to the work done at the small piston = 97 / 100 .**

**The efficiency of the hydraulic press does not reach 100 % because the friction between the piston and the walls of the tube , the gas bubbles in the liquid where work is consumed to reduce the volume of bubbles .**

**The hydraulic brake system in a car is two types**

**Drum brake ( rear brake ) : It uses Pascal’s rule as the braking system uses a brake fluid , Upon pushing on the brake pedal with a small force and a relatively long stroke ( distance ) , the pressure is transmitted in the master brake cylinder , hence , onto the liquid and the whole hydraulic line , then to the piston of the wheel cylinder outwardly and finally to the brake shoes and the brake drum , A force of friction results in , which eventually stops the car .**

**Front ( disk ) brake : In this case , the forces resulting from the braking action press on the brake pads which produce friction enough to stop the wheel , It should be noted that the distance travelled by the brake shoes in both cases is small because the force is large .**

** Diving suit **

**It protects the diver from the high pressure in the very deep seas where air is blown inside it to protect the head of the driver from the high pressure .
**

**Applications on the pressure at a point ( Connected vessels , U-shaped tube & Mercuric barometer )**

**Properties of fluids , Factors affecting density and pressure**

**Steady flow , Turbulent flow and Applications on the continuity equation**

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