Thermal Physics

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STATES OF MATTER

Matter exists in 3 forms.

Solid- occupies a fixed volume-has a fixed shape

Liquid-occupies a fixed volume-takes the shape of the container.

Gas-expands to fill its container-takes the shape of the container.

Changes of state

Heat a solid and it melts to become a liquid. Heat the liquid and it boils to become a gas. These are examples of changes of state.

Melting- from a solid to a liquid

Boiling- from a liquid to a gas

Condensing- from gas to a liquid

Freezing- from liquid to a solid.

THE KINETIC MODEL OF MATTER

The particles of which matter is made of are very tiny. We refer to them as particles.

Solid- The particles are packed in closely together in contact with every particle surrounding it. Because they are so tightly packed they cannot move around. They vibrate in their fixed position. The hotter it gets the more it vibrates.

Liquid- The particles are packed slightly less closely together. Each particle is still in close contact but fewer than a solid. The arrangement is less ordered. Because the particles are slightly less packed they have the opportunity to move around.

Gas- The particles are widely separated from one and another. They are no longer in contact unless they bump into each other. The particles now use their energy to bounce around at about 400m/s.

Evidence for the Kinetic Model

Brownian Motion

Robert brown used a microscope to study pollen grains when he saw particles moving around. At first he thought these were alive but then saw they were moving with the other particles. This became known as Brownian motion.

Kinetic Model

Liquids take up the shape of their container because their particles are free to move around.

Gases fill their container because their particles can move freely about.

Solids retain their shape because the particles are packed closely together.

Gases diffuse because they are freely mobile.

dissolved substances diffuse through a liquid. Sugar crystals in a liquid dissolve and molecules spread through the liquid carried by the mobile particles.

Most solids expand when they melt as the particles are moving.

Liquids expand a lot when they boil.

EVAPORATION

The boiling point of water is 100 degrees Celsius but it does not have to be heated to 100 degrees before it evaporates. After a downpour of rain the puddles eventually dry up even though the temperature is much lower than 100 degrees. This has become water vapour in the air.

A liquid evaporates more quickly as its temperature approaches its boiling point.

Evaporation

Evaporation is when a liquid becomes a gas due to boiling. A liquid evaporates more quickly as its temperature reaches boiling point.

DESIGNING A THERMOMETER

Mercury-in-glass thermometers are used in many different situations.

Mercury expands at a steady rate as it is heated

COMPRESSING A GAS

A gas exerts pressure on the walls of its container because its particles are constantly colliding with the walls. They bounce of the walls exerting a force as they do so.

Gas trapped in a box

If the box is made smaller the volume of gas decreases. At the same time its pressure increases. The particles of gas have been squashed into a smaller volume, so they will collide with the walls of the container a lot more.

TEMPERATURE AND INTERNAL ENERGY

A thermometer tells us about the average energy of the particles in the object whose temperature is being measured. It does this by sharing the energy of the particles. If they are moving rapidly the thermometer will detect a higher temperature. Placing a thermometer into an object to measure its temperature is the most common way.

Internal energy is the total energy of all the particles

Temperature is a measure of the average kinetic energy of the individual particles.

Kinetic Model of Matter

Temperature and temperature scales