Thermal Energy

 think of matter as existing in three states, solid, liquid, and gas.
What are the characteristic of each one of these three states?
We need to think about shape and size.


To distinguish between solid, liquid, and gas, it may help you to think about ice, water, and steam as examples of three states   of matter.

Changes of state

Heat a solid and it melts to become a liquid.
Heat a liquid and it boils to become a gas.
Cool a gas and it becomes first a  liquid and then a solid. These are the changes of state.

- from solid to liquid.
Boiling - from liquid to gas.
Condensing - from gas to liquid.
Freezing - from liquid to solid.

Another term for a liquid changing to gas is Evaporation .

   In order to understand the three states of matter and its changes we will use the kinetic model of matter.
The word kinetic means (related to movement). In this model the things that are moving is particles of which matter is made. the model thus has an alternative name: the particle model of matter

The particles of which the matter is made are very tiny. They may be atom, molecules, or ions, but we will simplify things by disregarding these differences  and referring only to spherical particles.

Why do liquids take up the shape of its container?
 Because their particles are free to move about within the bulk  of the liquid.
Why do gases fill their container?
 Because their particles can move about freely.

Why do solids retain their shape?
  Because their particles are packed tightly together.

Why do gases diffuse (spread out) from place to place?
Why do we can smell perfume across the room?
The perfume particles spread about because they are freely mobile.

 state  Arrangement of particles Movements of particles The attraction force between the particles 

  • The particles are packed tightly together.
  • Each particle is in close contact with all of its neighbors.
  • The particles can not move around.  However they do move a bit, they are able to  vibrate about fixed position.
  •  Each particle of a solid is strongly bonded to its neighbours. This is because the force between particles are strongest when the particles are closely together. 

  • The particles are packed slightly less closely together compared with a solid.
  • Each particle is still in close contact with most of its neighbors, but fewer than the case of solid.
  • The general arrangement is slightly more jumped and disorderly.  
  • The particles have the opportunity to move around within the bulk of the liquid. 
  •  The force between the particles are slightly weaker.

  • The particles are widely spread  from one another.
  • They are no longer in contact,unless they collide with each other 
  •  The particles are moving freely about
  •  The particles are far a part so they do not attract each other and can move freely about.

Thermal expansion:
Substances expand or get bigger when they are heated up. They contract or get smaller when 
they are cooled down. 

When substances expand or contract, their particles stay the same size. It is the space between the particles that changes:

The particles in a solid vibrate more when it is heated, and take up more room.

The particles in a liquid move around each other more when it is heated, and take up more room.

The particles in a gas move more quickly in all directions when it is heated, and take up more 

This is an interactive link to demonstrate the motion of particles in solid, liquid, and gas in the normal temperature and when we apply heat to the matter.

What's the difference in How do the particles in solids, liquids, and gases change?
(interactive video) .Press on start.

the motion of the particles

thermal expansion

This is an experiment to demonstrate that a metal bar is expand when heated.
where a cast iron peg is broken by the large forces set up in a steel bar due to thermal expansion or compression.

Uses of thermal expansion:

1- Thermometers use the idea of liquid expansion with the increase of temperature.

2- A metal lid or cap may stick on a glass jar and be hard to unscrew. Heating the lid causes (for 
example running hot water on it) causes it to expand (as the glass expand much less) so that the lid loosens
and can be removed.

3- A steel tire may be fitted on to the wheel of the railway locomotive while  the
tyre is very hot. It then cools and contracts, so that it fits tightly on to the wheel.

4- Ring and ball.

A steel ball will pass easily through a steel ring when its cold. when its hot, it will not pass through the ring.

Materials expanding and contracting can also cause problems.

For example, bridges expand in the summer heat and need special joints to stop them bending out of shape.

Heat is thermal energy. It can be transferred from one place to another by:
 Conduction, convection and evaporation involve particles, but radiation involves electromagnetic waves