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Forces and pressure: 6.1
Particle theory of states of matter with
respect to density, weight and pressure in fluids due to their weight and depth
Doc Brown's Physics exam study revision notes
What is a fluid?
Sub-index for physics notes on 6.
Forces and Pressure in fluids and calculations
6.1
Particle theory - revision of states of matter and density
Gas and liquid
fluid
particles are free to move around from place to place, but not in solids!
Fluids are materials that flow, usually
gases or liquids.
Therefore fluids have virtually no
tensile strength, a very viscous fluid ('very sticky') has a very small
tensile strength.
In solids, the particles can just vibrate
from a fixed position and cannot move to another position in the lattice of
atoms, but the bonding is strong, so solids exhibit a wide range of
tensile strength and are difficult to either compress, stretch or bend.
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very low density FLUID |
high density FLUID |
highest density, but NOT a FLUID |
Solids have the highest density, the
particles are the closest together.
In fluids the inter-particle attractive
forces are sufficiently weak to prevent a solid forming, allowing free
random movement of the molecules of a liquid or gas.
Because of the weaker inter-particle force,
the particles of gas will spread out to fill any space available giving gases by
far the lowest density of the three states of matter.
When a solid is
The particles in a liquid are held much
closer together because of greater inter-particle forces giving liquids a much
greater density than gases.
Apart from water, liquid densities of
materials are
usually a few % less than that of their solid form.
The density of liquids is so high, with
so little space between the particles, they are almost impossible to
compress to a smaller volume.
Gases have so much space between the
particles that they are readily compressed to a smaller volume.
The closer together the particles are,
the more compact the substance is, the greater density of it.
See also
Particle
theory models of gases, liquids and solids and the particle
diagram above.
Liquids have a uniform density (same
throughout its bulk) which only increases very slightly under extremely high
pressures because there is so little free space to squeeze the molecules into.
However, there is considerable space between
gaseous particles and its is relatively easy to compress the particles closer
together e.g a bicycle pump filled with air or water.
The air is easily
compressed and the water isn't. The water can ruin the pump but you can't
compress the water in it. High pressure water pistols rely on compressed air NOT
compressed water.
Liquid fluids have a much greater density
than gaseous fluids
So, for similar depths (or heights) of gas and liquid,
liquids will create a much greater pressure because of the greater weight (due
to gravity) of substance acting on the same surface area.
The greater the density of a material,
the greater the number of collisions can take place, creating a greater
pressure.
This concept is most applicable to gases, which are so easily
compressed under pressure, so considerably increasing their density - this
happens with the
Earth's atmosphere.
Therefore do NOT assume gases cannot create a
pressure due to their weight.
Also, the greater the column of fluid
e.g. water, the greater the pressure created - the greater weight acting on
a given area.
See later section on pressure in
liquids.
See also
Particle models of gases–liquids–solids, explaining properties, state changes
(GCSE chemistry)
The density of materials and the particle model of matter
(GCSE physics notes)
Gas law calculations - P, V and T
relationships
(GCSE chemistry notes)
Index physics Forces notes 6. Forces & pressure in
fluids, calculations
Keywords, phrases and learning objectives for forces involving
pressure situations
Be able to explain using particle model theory, the
relative
density and pressure in gases, liquids and solids due to their weight.
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Forces and Pressure
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