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Solids, liquids and gases

Physics > Secion 5: Solids, liquids and gases

a) Units

5.1 use the following units: degrees Celsius (oC), Kelvin (K), joule (J), kilogram/metre3 (kg/m3), kilogram/metre3 (kg/m3), metre (m), metre2 (m2 ), metre3 (m3), metre/second (m/s), metre/second2 (m/s2 ), newton (N), Pascal (Pa).

Unit of temperature: degrees Celsius (oC)
Unit of temperature: Kelvin (K)
Unit of mass: kilogram/metre3 (kg/m3)
Unit of density: kilogram/metre3 (kg/m3)
Unit of distance: metre (m)
Unit of Area: metre2 (m2)
Unit of Volume: metre3 (m3)
Unit of Speed: metre/second (m/s)
Unit of Acceleration: metre/second2 (m/s2)
Unit of force: newton (N)
Unit of Pressure: Pascal (Pa)

b) Density and Pressure

5.2 know and use the relationship between density, mass and volume:


5.3 describe experiments to determine density using direct measurements of mass and volume

Suppose a rectangular block have a volume of 50 m3 and mass of 200kg. Its density will be 200/50 kg/m3, i.e. 4kg/m3.

5.4 know and use the relationship between pressure, force and area:

ρ=force/area p=F/A

5.5 understand that the pressure at a point in a gas or liquid which is at rest acts equally in all directions

Pressure in liquids and gases act equally in all directions, as long as the liquid or gas are not moving.

5.6 know and use the relationship for pressure difference:

pressure difference = height × density × g
p = h × ρ × g

c) Change of state

5.7 understand the changes that occur when a solid melts to form a liquid, and when a liquid evaporates or boils to form a gas

When a solid is heated, the molecules starts vibrating. At a time they lose their attraction force and move slowly. This time they reach the state liquid i.e. solid has melted. When more heat is given, the molecules totally lose their attraction force and move randomly. They have reached the state gas i.e. liquid has boiled.

5.8 describe the arrangement and motion of particles in solids, liquids and gases

Features Solid Liquid Gas
Arrangement Regular Irregular Random
Movement Cannot move, vibrate only Particles can move throughout the liquid slight past each other The particles have the most kinetic energy
Energy of Particles Particles have least kinetic energy Particles have more kinetic energy than solid The particles have the most kinetic energy
Distance between Particles Closely packed Not closely packed Far apart
Shape 3d Structure Takes the shape of the container No fixed shape

d) Ideal gas molecules

5.9 understand the significance of Brownian motion, as supporting evidence for particle theory

One piece of evidence for the continual motion of particles in a liquid or a gas is called Brownian motion. Particles of a liquid or gas are moving around continually and bump into each other and into tiny particles such as pollen grains. Sometimes there will be more collisions on one side of a pollen grain than on another, and this will make the pollen grain change its direction or speed of movement.

5.10 understand that molecules in a gas have a random motion and that they exert a force and hence a pressure on the walls of the container

Gases are made up of particles that are moving. The particles in gases are spread out and constantly moving in random. They hit the walls of the container and create pressure.

5.11 understand why there is an absolute zero of temperature which is –273oC

Temperature affect the pressure of particles of gases. The higher the temperature, the higher the energy in particles and more the pressure. If we decrease the temperature the result will be the exact opposite. As we cool the gas, the pressure keeps decreasing. The pressure of the gas cannot become less than zero. The temperature at which the pressure of the gas is decreased to 0, that temperature is called absolute zero. It is approximately –273oC.

5.12 describe the Kelvin scale of temperature and be able to convert between the Kelvin and Celsius scales

Temperature in K = temperature in oC + 273
Temperature in oC = temperature in K - 273

5.13 understand that an increase in temperature results in an increase in the average speed of gas molecules

If we heat gas molecules, they gain more kinetic energy. As they do so, they begin to move faster and the average speed of the molecules increases.

5.14 understand that the Kelvin temperature of the gas is proportional to the average kinetic energy of its molecules

Temperature in Kelvin is directly proportional to the average kinetic energy of molecules. If we increase the temperature, kinetic energy as well as pressure will increase as well.

5.15 describe the qualitative relationship between pressure and Kelvin temperature for a gas in a sealed container

The number of gas particles and the space, or volume, they occupy remain constant. When we heat the gas the particles continue to move randomly, bu with a higher average speed. This means that their collisions with the walls of the container are harder and happen more often. This results in the average pressure exerted by the particles increasing.

When we cool a gas the kinetic energy of its particles decreases. The lower the temperature of a gas the lass kinetic energy its particles have – they move more slowly. At absolute zero the particles have no thermal or movement energy, so they cannot exert pressure.

5.16 use the relationship between the pressure and Kelvin temperature of a fixed mass of gas at constant volume:

p1 / T1 = p2 / T2

5.17 use the relationship between the pressure and volume of a fixed mass of gas at constant temperature:

p1V1 = p2T2

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