O Level Physics Chap 19 Density and Pressure

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Prepared By: Shakil Raiman

Density: Density of a substance is defined as its
mass per unit volume.
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Unit: kg/m3, g/cm3
For Regular Solid:
 Measure the mass of the object.
 Measure the dimensions. (Suppose
for a cuboid length, width, and
height)
 Calculate volume by using
dimensions. (Suppose for a cuboid
Volume = length×width×height)
 Calculate density by using formula
density = mass/volume
For Irregular Solid:
 Measure the mass of the object.
 Take some suitable liquid within a measuring cylinder.
The liquid should be enough so that the object sink
completely. Measure the volume of the liquid. (V1)
 Put the object into the liquid of the measuring cylinder.
Measure the new volume of the liquid level. (V2)
 Volume = V2 – V1
 Calculate density. (density = m/ (V2 – V1))
For Irregular Solid:
 Measure the mass of the object.
 Take some suitable liquid within a
displacement can. Wait for no liquid
fall for the cal.
 Put the object into the liquid and
use measuring cylinder to measure
the displaced liquid
 Calculate density by using formula,
density = mass/ volume.
For Liquid:
 Take a measuring cylinder. Measure the mass of it.
(m1)
 Take the liquid within measuring cylinder. Read the
volume of the liquid level. (V)
 Measure the mass of the measuring cylinder with
the liquid. (m2)
 Mass of liquid = m2 – m1
 Calculate density by using formula, density =
mass/volume
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Mass: Mass of a body is the measure of the
amount of matter that it contains.
Weight: Weight of a body is the attractive force
exerted on it due to gravity.
Inertia: Any object tends to remain in its own state
of rest or motion. Such tendency is called inertia.
Mass of an object
is always the same
Weight of an object
varies from place to place
is a measure of inertia
is measured using a beam
balance or an electronic
balance.
is a measure of force
is measured using a spring
balance or a compression
balance
is measured in kg
is measured in N
mass: 1 kg
Weight on Earth: 10 N
Weight on Moon: 1.6 N
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Pressure is force acting normally per unit area.
Pressure = Force / Area
Unit: Pascal (Pa), Nm-2, Ncm-2
If area is constant,
Pressure increases if force increases and pressure
decreases if force decreases.
If force is constant,
pressure increases if area decreases
pressure decreases if area increases
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We have air around us and therefore there is
atmospheric pressure.
Atmospheric pressure exists because of
molecular bombardment of energetic
molecules.
The normal atmospheric pressure is about
1.013×105 Pa. It is called 1 atmosphere ( 1 atm. )

The pressure due to a
liquid column depends
on the depth, height
and the density of
liquid.

Liquid Pressure =
height × density × g

P=hg

Mass of water = volume × density
= 𝐴ℎ

Force = Weight of Liquid Pressure
= 𝐴ℎ𝑔

So, 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =
𝐹𝑜𝑟𝑐𝑒 𝐴ℎ𝜌𝑔
=
𝐴𝑟𝑒𝑎
𝐴
=ℎ𝜌𝑔

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Open end:
 Pressure at X = P0+hpg
Close end:
 Pressure at Y = hpg
The pressure in a liquid depends
on
1. the density of the liquid
2. the depth of the liquid
 Note: the pressure in liquid does
not depend on the area at any
depth.
 In the following picture, there is
equal pressure at point A, B or C.

Simple mercury barometer is used to measure
atmospheric pressure.
 Construction 1. a thick-walled glass tube (
about 1m long) is filled with mercury
completely
2. the open end of the is covered with a
finger and inverted
3. place the inverted tube in a mercury
container
 Observation: The height of the mercury
column is found to be about
 Atmospheric pressure=760 mmHg or 76 cmHg

Manometer is used to
measure gas pressure.
 Construction: The manometer
consists of a U-tube containing
a column of liquid. The liquid
can be mercury, water or oil.
 Pressure of Gas = P0+hpg

Pressure can be transmitted through a liquid in hydraulic
presses and hydraulic brakes on vehicles.
 A small force on smaller piston can cause greater force on
greater piston.
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Wish you all very good luck and excellent
result.
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