Topic 3
Thermal physics
Last lesson?
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
The hotter the temperature, the faster the
average speed of the particles
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
• Lord Kelvin
• 1824-1907
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.
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.
Moles!
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
How big is 6 x 1023?
Imagine the whole of the United states
covered in unpopped popcorn to a depth
of six miles!
Moles!
For example, Hydrogen (H2) has a relative
molecular mass of 2, so 2 grams of
hydrogen (one mole) contains the same
number of molecules as atoms in 12g of
carbon-12 (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
Relative formula mass
We can use the idea of moles and apply it to
molecules using relative formula mass.
C2H5OH
RFM = (2 x 12) + (6 x 1) + (1 x 16) = 46
46g of ethanol = I mole of ethanol molecules
Mole of gas
One mole of any gas occupies 24 dm3
(24000 cm3) at standard temperature and
pressure (20°C and 1 atmosphere)
You will be
given this in a
question
Moles of gas = volume (cm3)/24000 = volume
(dm3)/24
Learn
this!
Let’s try some questions:
Pages 161 and 162
Questions 4, 5 and 6
Don’t forget to read the relevant
pages in your textbook (Pages
158-162)
Today’s lesson
• Define specific heat capacity and thermal
capacity.
• Solve problems involving specific heat
capacities and thermal capacities.
Imagine if…………
Imagine if……..
Two beakers of water were heated by
identical heaters for an equal amount of
time.
The beaker with less water would end up
hotter. WHY?
Imagine if………
Imagine if……..
Two beakers, one containing water and one
containing kerosene (equal masses) were
heated by identical heaters for an equal amount
of time.
The beaker containing the kerosene would be
twice the temperature! WHY?
Since the amount of heat energy supplied
is the same to both substances, it seems
that different substances require different
amounts of heat energy to cause the same
temperature rise.
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.
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
Let’s try some
questions!
Page 171 Qs 2, 3
and 4
Investigation time!
Let’s do an experiment to measure
specific heat capacities