Topic 3
Thermal physics
Temperature
TEMPERATURE determines
the direction of flow of thermal
energy between two bodies in
thermal contact
HOT
This is another way of saying that
if an object is hotter than another,
heat energy will flow from the
hotter object to the colder!
COLD
Obvious, but important to
remember!
Temperature
Temperature is also a measure of the
average random kinetic energy of the
particles in a substance.
Note that they are
not all travelling at
the same speed.
Thermal equilibrium
Two bodies in thermal contact
will eventually reach the same
temperature. The two bodies
are now said to be in thermal
equilibrium.
WARM
WARM
Kelvin Temperature
Kelvin temperature is proportional to the
average random kinetic energy of the
particles in a substance.
Note that they are
not all travelling at
the same speed.
Absolute/Kelvin temperature and
Celsius
T (in Kelvin) = T (in degrees Celcius) + 273
Just to mention for now that zero Kelvin is
the lowest possible temperature.
Measuring temperature
The thermometer has to be placed in
thermal contact with whatever is being
measured until the thermometer and object
are in thermal equilibrium.
We’ll measure some
temperatures in a
moment
So what is heat?
Heat is the amount of thermal energy. For
example, the sparks from a sparkler are at
around 800°C but do not burn your skin.
However, a hot cup of tea at around 100°C
will burn your hand badly. This is because the
tea contains more heat energy, even though
it is cooler.
Internal Energy
This is the sum of the kinetic energies and
potential energies of the particles in a
substance
If you imagine the forces between particles as a spring, you
can see if the particles are pulled apart or squashed together
that energy is stored in the spring. Similarly there is potential
energy between the particles in a substance.
Heat transfer
Conduction, convection and radiation.
I’m not going to say anything about
these, you should know it already.
Can you construct a heat transfer
mind-map?
Colours
Few words
Connections
Drawings
Pages 108 to 113 IGCSE Physics
Moles!
You need to learn this
definition.
• One mole of a substance contains the
same number of molecules/atoms as in 12
grams of carbon-12.
• This number (of atoms or molecules) is
known as the Avogadro constant (NA)
which is equal to 6.02 x 1023
Moles!
It follows therefore that 7g of lithium
(atomic mass 7), 20g neon (atomic mass
20) or 39 g potassium (atomic mass 39) all
contain the same number of atoms (1
mole or 6.02 x 1023 atoms)
Moles!
• The number of moles of a substance can
thus be found by dividing the mass of
substance by its relative atomic or
molecular mass
n = mass/RAM
Example
How many moles of sulphur atoms are
there in 80g of sulphur? How many grams
of carbon would have the same number of
atoms?
Example
How many moles of sulphur atoms are
there in 80g of sulphur? How many grams
of carbon would have the same number of
atoms?
N = mass/RAM = 80/32 = 2.5 moles
Example
How many moles of sulphur atoms are
there in 80g of sulphur? How many grams
of carbon would have the same number of
atoms?
N = mass/RAM = 80/32 = 2.5 moles
Mass of carbon = RAM x n = 12 x 2.5 = 30 g
Heat Capacity
The relationship between the amount of
heat energy a substance requires to raise
its temperature by a given amount is
called its thermal capacity. It is measured
in J.°C-1 or J.K-1.
Definition to learn
• Thermal capacity is the amount of energy
needed to raise the temperature of a
substance by 1K.
Why are thermal capacities different?
• When a substance is heated, its internal
energy increases (potential and kinetic).
The stronger the force between the
particles in the substance, the more heat
energy goes into potential energy (and less
into kinetic), so the temperature rise is less
than in substances with little force between
particles. Obviously the more partciles there
are too, the more heat energy can be
absorbed.
Calculations using Thermal
capacity
Energy absorbed = Thermal capacity x Temp rise
J
J.°C-1
E = QΔT
°C
Specific heat capacity
Specific heat capacity is the amount of
energy needed to raise the temperature of
unit mass of a substance by 1K
Specific heat capacity of water = 4186 J.kg-1.°C-1
Specific heat capacity of kerosene = 2010 J.kg-1.°C-1
Specific heat capacity of mercury = 140 J.kg-1.°C-1
Calculations using S.H.C.
Energy absorbed = Mass x Specific Heat capacity x Temp rise
J
kg
J.kg-1.°C-1
Q = mcΔT
°C
For example
500 g of olive oil is heated until its temperature rises by
120°C. If the specific heat capacity of olive oil is 1970
J.kg-1.°C-1, how much heat energy was used?
Energy absorbed = Mass x Specific Heat capacity x Temp rise
Energy absorbed = 0.5 x 1970 x 120
Energy absorbed = 118200 J
Solids, liquids and gases
Melting?
• Changes in kinetic energy and potential
energy?
Evaporation?
• Changes in kinetic energy and potential
energy?
Condensing?
• Changes in kinetic energy and potential
energy?
Freezing?
• Changes in kinetic energy and potential
energy?
Evaporation
Consider a beaker of water at room
temperature
Evaporation
The molecules of water are moving around
at different speeds, some fast, some slow.
# of molecules at
a particular speed
Average
speed
speed of
molecule (m/s)
Evaporation
If a molecule is at the surface, and moving
fast enough, it may escape the liquid. This
is called evaporation.
Freedom!
Evaporation
Since the average speed of the remaining molecules
must now be lower, the temperature of the liquid drops
(since temperature is a measure of the kinetic energy of
the molecules).
Freedom!
Evaporation
Evaporation can thus take place at any
temperature.
Increasing the rate of evaporation
Increasing the
temperature means that
more molecules are
moving fast enough to
escape.
Increasing the rate of evaporation
Increasing the surface
area means that more
molecules are at the
surface.
Increasing the rate of evaporation
Increasing the air flow
over the surface so that
molecules are carried
away before they can fall
back into the liquid
Increasing the rate of evaporation
Decreasing the humidity
of the surrounding
atmosphere to stop
water molecules from the
atmosphere entering the
liquid.
Boiling
The bubble
contains only
water vapour,
not air!
Boiling occurs when
vapour is produced in
the body of the liquid.
This only happens at the
boiling point of the liquid.
To summarize:
Evaporation takes place only at the
surface of the liquid and can take place at
any temperature.
To summarize:
Boiling
means
bubbles!
Boiling occurs
when vapour is
produced in the
body of the
liquid. This only
happens at the
boiling point of
the liquid.
Latent heat
Latent heat
In last year’s experiment, you will have noticed that the
temperature of the salol stopped changing as the salol
changed from a liquid to a solid.
Temp
(°C)
Melting point
Time (mins)
Latent heat
When the molecules of a substance settle
into the regular pattern of a solid, energy is
released as bonds are formed. This
energy released is called latent heat. This
stops the temperature from falling.
(“latent” = “hidden”)
Latent heat
The opposite happens when a solid makes.
Heat is needed to break the bonds between the
solid particles (increasing their potential energy
instead of raising the temperature (kinetic
energy))
liquid
Temp
(°C)
Melting point
solid
Time (mins)
Specific Latent heat
The specific latent heat of a substance
tells us how much energy is needed to
change the state of 1 kg of substance at
constant temperature.
Solid to liquid/liquid to solid
or
liquid to gas/gas to liquid
Specific Latent Heat
The specific latent heat of fusion (melting) of ice at 0 ºC,
for example, is 334000 J.kg-1. This means that to convert
1 kg of ice at 0 ºC to 1 kg of water at 0 ºC, 334000 J of
heat must be absorbed by the ice.
All at 0°C
1 kg
1 kg
334000 J absorbed
Specific Latent Heat
Conversely, when 1 kg of water at 0 ºC freezes
to give 1 kg of ice at 0 ºC, 334000 J of heat will
be released to the surroundings.
1 kg
1 kg
334000 J released
All at 0°C
Specific Latent Heat of
Vaporisation
For water at its normal boiling point of 100 ºC, the latent
specific latent heat of vaporization is 2260000 J.kg-1.
This means that to convert 1 kg of water at 100 ºC to 1
kg of vapour at 100 ºC, 2260000 J of heat must be
absorbed by the water.
All at 100°C
1 kg
2260000 J input
1 kg
Latent heat
Conversely, when 1 kg of steam at 100 ºC
condenses to give 1 kg of water at 100 ºC,
2260 kJ of heat will be released to the
surroundings.
All at 100°C
1 kg
1 kg
2260000 J released
Another formula!
Energy = mass x specific latent
heat
Q = mL
An example calculation
Calculate the amount of heat required to completely
convert 50 g of ice at 0 ºC to steam at 100 ºC. The
specific heat capacity of water is 4.18 kJ.kg-1.°C-1. The
specific latent heat of fusion of ice is 334 kJ.kg-1, and the
specific heat of vaporization of water is 2260 kJ.kg-1.
50g
50g
0°C
100°C
An example calculation
Heat is taken up in three stages:
1. The melting of the ice.
0°C
0°C
2. The heating of the water.
100°C
0°C
3. The vaporization of the water.
100°C
100°C
Stage 1
1. Heat taken up for converting ice
at 0ºC to water at 0ºC
0°C
0°C
mass of water x latent heat of fusion
= 0.050 (kg) x 334 (kJ.kg-1)
= 16.7 kJ
Stage 2
2. Heat taken up heating the water
from 0 ºC to the boiling point, 100 ºC
0°C
100°C
mass of water x specific heat capacity x
temperature change
= 0.05 (kg) x 4.18 (kJ.kg-1.°C-1)x 100 (ºC)
= 20.9 kJ
Stage 3
3. Heat taken up vaporising the
water
100°C
100°C
mass of water x latent heat of vaporization
0.05 (kg) x 2260 kJ.kg-1
= 113 kJ
The answer
The sum of these is
16.7 + 20.9 + 113
= 150.6 kJ (151 kJ)
Formulae you will probably
need
• Q = mcΔT (warming/cooling – change in temp)
• Q = mL (change of state at constant temp)
• Power = Energy/time
• Intensity(W.m-2) = Power(W)/area(m2) (waves topic)
• Many questions will involve sun shining on ice,
probably linked with questions about global
warming.
Kinetic theory/ideal gas
We can understand the behaviour of
gases using a very simple model, that of
an “ideal” gas.
The model makes a few simple
assumptions;
Ideal gas assumptions
• The particles of gas (atoms or molecules)
obey Newton’s laws of motion.
You should know these
by now!
Ideal gas assumptions
• The particles in a gas move with a range
of speeds
Ideal gas assumptions
• The volume of the individual gas particles
is very small compared to the volume of
the gas
Ideal gas assumptions
• The collisions between the particles and
the walls of the container and between the
particles themselves are elastic (no kinetic
energy lost)
Ideal gas assumptions
• There are no forces between the particles
(except when colliding). This means that
the particles only have kinetic energy (no
potential)
Do you remember what internal
energy is?
Ideal gas assumptions
• The duration of a collision is small
compared to the time between collisions.
Pressure – A reminder
Pressure is defined as the normal
(perpendiculr) force per unit area
P = F/A
It is measured in Pascals, Pa (N.m-2)
Pressure – A reminder
Collisions of the gas particles with the side
of a container give rise to a force, which
averaged of billions of collisions per
second macroscopically is measured as
the pressure of the gas
Change of
momentum
Explaining the behaviour of gases
When we heat a gas at constant volume, the
pressure increases. Why?
Increased average kinetic energy of the
particles means there are more collisions
with the container walls in a period of time
and the collisions involve a greater change
in momentum.
Explaing the behaviour of gases
When we heat a gas a constant pressure,
the volume increases. Why?
Increasing the volume reduces the chance
of particles colliding with the container
walls, opposing the effect of the particles
increased kinetic energy.
Explaing the behaviour of gases
When we compress (reduce the volume) a
gas at constant temperature, the pressure
increases. Why?
A smaller volume increases the likelihood
of a particle colliding with the container
walls.
Explaing the behaviour of gases
In this way we are explaining the
macroscopic behaviour of a gas (the
quantities that can be measured like
temperature, pressure and volume) by
looking at its microscopic behaviour (how
the individual particles move)