2. The Particle Model
● Suppose that all objects are made of particles.
● Solid objects have lower temperatures than
their liquid phase.
● Therefore the particles have lower average kinetic
energy in the solid phase than in the liquid phase.
● We assume that in a solid the particles can
vibrate around fixed points, in liquids they can
move slowly and in gases they can move
quickly.
3. The Particle Model
● An object in its solid phase has a fixed shape.
● When it melts it moves to fill the shape of its
container.
● In the gaseous phase it will completely fill its
container.
● We assume that in solids, the particles are
tightly bound in fixed shapes (crystalline), that
in liquids they are bound close together but in
no fixed shape, and in gases they are virtually
unbound.
5. Changing Phases
● When thermal energy flows
into an object 2 things can
happen.
● Either the kinetic energy of
the particles can increase
(Temperature goes up)
● Or the potential energy of
the particles can increase
(bonds break and
temperature remains
constant)
● This can be shown on a
phase change graph.
Temperature/K
Time /s
6. Evaporation versus Boiling
● Evaporation is the process by which particles in
the liquid phase change into the gaseous phase
at temperatures below the boiling point of the
liquid.
● This is because the individual particles have a
range of kinetic energies even though the
average kinetic energy gives rise to a
temperature less than the boiling point.
● If a particular particle close to the surface has
enough energy to escape the bulk of the liquid
then it will evaporate and form a vapour.
7. Evaporation versus Boiling
● Now that the highly energetic particle has
escaped, the average kinetic energy of the
remaining liquid is reduced
● The temperature has decreased.
● This is the basis of evaporative cooling.
● A volatile liquid evaporates rapidly.
● A liquid's volatility is controlled by a factor called
its equilibrium vapour pressure.
8. Boiling
● A liquid boils when its vapour pressure falls
below that of atmospheric pressure.
● This means that the temperature at which water
boils is dependent on air pressure.
● It boils at lower temperatures at the top of a
mountain where the pressure is less than at sea
level.
9. Thermal Capacity
● An object that is heated is able to store thermal
energy as Internal energy.
● The amount of energy required to raise an
object's temperature by 1K is known as its
thermal capacity.
● An object with a large thermal capacity will take
longer to cool down than an object with a low
thermal capacity.
● This is because the rate of cooling depends on the
temperature difference but the total thermal energy
available depends on the thermal capacity.
10. Thermal Capacity
● The thermal capacity of an object is given by:
Where
● ΔQ is the thermal energy transferred into the object
in J
● ΔT if the temperature rise of the object in K
Thermal Capacity=
Q
T
11. Specific Heat Capacity
● In order to compare the thermal capacity of
different objects made of different materials the
thermal capacity per unit mass is calculated.
● This is called the specific heat capacity
● The specific heat capacity of a material is the
energy required to raise 1kg of sample by 1K.
● The symbol for specific heat capacity is c.
● Often this is quoted in data books as per g not kg!!!
c=
Q
m T
12. Questions
● A 750g sample of unknown metal is heated until
its temperature is raised to 70o
C from 15o
C. In
doing this 20kJ of energy is transferred into the
sample. What is the specific heat capacity of
the metal?
● How much energy is required to bring a 90g
sample of water from 10o
C to boiling point?
● A 600kg block of Pyrex loses 8.7x106
J of
energy as it cools from 95o
C. What is its
temperature after cooling? c=8.4x102
Jkg-1
K-1
13. Mixing materials
● Often two materials of different temperatures
are added together.
● The hotter material loses energy and the colder
one gains this. They then both end up at the
same equilibrium temperature.
● This can be used to find the specific heat
capacity of different materials and is known as
the method of mixtures.
14. Questions
● A 2kg block of copper at 150o
C is placed into a
3L bucket of water at 15o
C. What is the final
temperature of the water?
● A 4kg block of aluminium at -100o
C is placed
into a 2L vat of mercury at 250o
C. What is the
final temperature.
15. Latent Heat
● The energy required to break the bonds in a 1kg sample of
substance is known as the specific latent heat.
● The specific latent heat of transformation is the energy
required to completely change a the sate of a 1kg sample
of material.
● The latent heat of vaporisation LV
changes liquids to gases
● The latent heat of fusion LF
changes solids to liquids
● The latent heat of sublimation LS
changes solids to gases
L=
Q
m
16. Questions
● How much energy is required to completely
melt a 35kg block of water ice?
● How much energy is required to raise a 2kg
block of water ice from -20o
C to 45O
C.
● A 500g block of ice at -10o
C is added to a 1.5L
beaker of warm water at 70o
C. What is the final
temperature of the mixture?
17. Questions
● How much energy is required to completely boil
5L of water?
● A pressure kettle is used to heat 0.5L of water
from 15o
C to 110o
C in 5 minutes. What is the
minimum input power needed to do this?
● Hot rocks at 450O
C are placed into a kettle.
Each rock has a mass of 0.5kg. If the kettle
has 5.5L of water at 20o
C in it then how many
rocks are needed to boil the water.
● crock
= 980 Jkg-1
K-1
