CHAPTER 4

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

PRESSURE

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

PRESSURE Pressure is the force for every surface-unit that is pressured

vertically.

This definition firstly stated by Blaise Pascal. As the tribute to him, the unit in International unit system is N/m2, or pascal.

A. PRESSURE OF SOLID SUBSTANCE

Pressure at solid substance is a real illustration to understand the definition of pressure.

The unit of pressure is N/m² = pascal (pa). It is formulated as follows :

P = F / A In International Unit, the unit of pressure is Pascal (Pa) that means

the unit of force divided by the unit of area, or Newton/meter2

With reference from the above formula, we know that :a. The bigger the force, the bigger the pressure.b. The narrower the area, the bigger the pressure.

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

THE PRESSURE OF SOLID SUBSTANCE

B. The pressure of liquid substance Liquid can give pressure to any

object

The object under liquid would receive pressure from any side.

Some items related to solid pressure are hydrostatic pressure, Pascal Laws, interconnected-container laws, and Archimedes Laws.

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

THE PRESSURE OF LIQUID SUBSTANCE

1) Hydrostatic pressure Hydrostatic pressure is a pressure

under water that is caused by the weight of the liquid itself.

Theory of Hydrostatic pressure : “ hydrostatic pressure in any point of flat-object under the same liquid that is in balance is equal”

The instrument used for observing hydrostatic pressure is Hartl.

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

PRESSURE OF LIQUID SUBSTANCE

2) Pascal Laws Pascal Laws states that the

pressure given to liquid in closed-room would be forwarded to any direction on a par”

Pascal’s statement then recognized as Pascal Laws, mathematically, it is stated as follows:

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

PRESSURE OF LIQUID

3) Interconnected-Container Interconnected-container Laws : “ if the

interconnected-containers are filled by the same liquid and it is on balance, the surface of all pipes will be on horizontal line”

Hydrostatic pressure at A will be equal at B, so that the equation is :

 

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

LIQUID PRESSURE The application of Interconnected-container

Laws

some of the following things are used very often in our daily life :

The interconnected-container in laboratory is like in the following figure :

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

LIQUID PRESSURE

3) Archimedes Pressure Archimedes Laws : “ If one object is plunged into

liquid, the object will get float force that is equal to the weight of water removed ”

There are 3 possibilities of plunging an object into liquid : sinking, floating, and flying under liquid.

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

GAS PRESSURE Gas pressure is usually called atmosphere pressure

In principle, gas pressure is the same with liquid pressure

air pressure greatly influences the weather

1. The influence of height to air pressure

Air pressure above the sea level = 76 cmHg or 1 atm. If the height goes up 100 m, the air pressure will go down 1 cmHg. You can feel it if you go to a high-place.

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

GAS PRESSURE2. Gas pressure in closed-room

The tool to measure the air pressure in closed-room is manometer. There 2 kinds of manometer, manometer mercury and metallic manometer.

There 2 kinds of mercury manometer namely open air manometer, and manometer with compressed air.

PHYSICS FOR JUNIOR HIGH SCHOOL VIII

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