Thermal Physics

THERMAL PHYSICS


Solid Liquid Gas
Volume Fixed Fixed Volume of
container
Shape Fixed Shape of container Shape of container
Arrangem Closely packed Packed less closely Widely separated
ent together Few contact with No longer in
Close contact with neighbours contact unless they
neighbours More jumbled and collide
Strong bond disorderly No bond between
between molecules Weaker bond between molecules
molecules
Movemen Cannot move Attached but not as Move freely
t of around because of rigorous Completely
particles strong bond Move around more unattached
Vibrate at fixed within the bulk of the Bouncing off one
position liquid another and the
Hotter -> more Both vibrating and walls
vibration moving
Separatio <- Freezing <- Condensing
n Melting -> Evaporation and boiling
->
 When a gas is hot, its particles move quickly and collide often because heat
energy is given to the particles, giving them kinetic energy to move
 When gas particles move, they hit side of container, creating pressure on the
container. The pressure of gas depends on how often and how hard the
molecules are colliding with the side
 Each molecule moves along with constant velocity in a straight line, until it hits
something. The molecules exchange momentum, but total momentum of the
two molecules is conserved. When a molecule hits a wall, it bounces back and
its momentum changes
 The greater the number of molecules hitting a wall, the greater is the force ->
greater pressure

Brownian motion: motion of small particles suspended in a fluid, caused by
molecular bombardment

 Particles move around in random directions and collide with each other and
surroundings
 When they collide, they change direction -> spread out
 Massive particles may be moved by light, fast moving molecules

Evaporation: when a liquid changed to a gas at a temperature below its boiling
points

, THERMAL PHYSICS


 When energetic molecules escape which have high temperature escape, the
remaining temperature of liquid drops. Heat foes to colder region so heat energy
flow away from the body

Observation Explanation
Higher More particles are moving fast enough to escape from the
temperature surface
Great surface More particles are close to the surface so they can escape
area easily
Air flow When particles escape from the water, they are blown away
so they cannot fall back into water
 The greater the temperature, the faster the particles move -> more collision ->
pressure increases
 The lower the volume of a gas, the less space the particles have to move around
-> bump into container more frequently -> pressure increases
 Boyle’s Law: p1V1 = p2V2

Thermal expansion (at constant pressure)

 Molecules gain kinetic energy and vibrate more vigorously. Solid expands in all
directions
 Liquid molecules have more kinetic energy to take up more space so volume
increases
 Gas molecules have more kinetic energy to take up more space so volume
increases

Application and consequences

 A hot rivet is passed through holes in 2 metal plates and then hammered until
ends are rounded so when it cools, it contracts and pulls 2 plates tightly together
 Heating metal lid of a jar or bottle causes it to expand so the lid loosens and can
be removed easily
 Overhead cables have to be slack so on cold days when they contract, they don’t
snap or detach
 Railroad tracks become wider when the train wheels are heated
 Bridges have an expansion joint because it could expand or contract due to
weather conditions
 Bimetallic strip:
o 2 metal joined together and one expands more rapidly
o When heated, the strip bends
o Used in fire alarms and thermostats

Thermal equilibrium: describes the state of two objects (or an object and its
surroundings) that are at the same temperature so that there is no heat flow
between them

Temperature: a measure of the average kinetic energy of the individual particles