It's a good idea to read
Examples of energy store conversions in systems first
and
Specific latent
heat is dealt with in a separate section
Whenever you get an increase in
temperature of a system, energy must be transferred from one energy store to
another.
However, for the same quantity of
heat energy transferred, the temperature rise will vary.
The temperature rise will depends on
the amount of material heated and its structure.
Don't confuse heat and temperature!
When some object is heated, the
thermal energy ('heat') transferred increases the thermal energy
store of the object.
The temperature increases, but
the temperature only indicates how hot or cold the object is.
When you heat a material, thermal energy is
absorbed and its
internal energy is increased
due to an increase in its
thermal energy and potential energy stores.
At a particle level
this is due to:
(i) An increase in the kinetic energy
store caused by increased vibration of solid particles or increased kinetic energy of the free movement
of liquid and gas particles from one place to another.
From kinetic particle theory, a
temperature value is a measure of the average kinetic energy of the
particles - much of the average internal energy of the material.
(ii) An increase in the potential
energy caused by the increase in kinetic energy opposing the
inter-particle forces of attraction - the particles on average a bit
further apart with increase in temperature.
The internal energy store is the sum of
the kinetic energy store plus the potential energy store - the latter can
often be ignored in the situations described here concerning heat capacity.
The energy transferred to a given material
acting as a thermal energy store to raise its temperature by a specific amount can vary quite widely.
e.g. you need over four
times more heat energy to raise a given mass of water to specified temperature
than that for the same mass of central heating oil or aluminium (they have
different specific heat capacities - but more on this later).
Application: Solar panels may contain
water that is heated by radiation from the Sun.
Water has a high heat capacity
and can store a lot of thermal energy.
This water may then be used to
heat buildings or provide domestic hot water.
Water is the usual conveyer of
thermal energy in central heating systems.
Water is a very good thermal
energy store in a hot bottle for cold winter nights in bed.
Different substances store different amounts
of energy per kilogram for each °C temperature rise.
To put it another way, different
materials require different amounts of heat energy to raise a given
amount of material by the same increase in temperature.
This is called the specific heat capacity and varies from material to material, whether it be a gas,
liquid or a solid - its all to do with the nature and arrangement of the
particles - atoms, ions or molecules.
Materials with a high heat capacity will
release lots of heat energy when cooling down from a higher to a lower
temperature.
The
specific heat capacity (SHC
or just c) of
a substance is the amount of energy required to change the temperature of
one kilogram of the substance by one degree Celsius.
This is a way of
quantifying an increase or decrease in a material's thermal energy store.
The formula for expressing the
amount of heat transferred
between energy stores is given by the equation.
change in thermal energy store (J) = mass
(kg) x specific heat capacity (J/kgoC) x change in temperature (oC)
∆E = m x c x ∆θ
∆E
= energy transferred in Joules (change in thermal energy)
m = mass of material in kilograms kg
c = SHC = specific heat
capacity J/kgoC,
∆θ =
∆T = temperature change in Celsius oC
The specific heat capacity of
water is 4180 J/kgoC (Joules per kilogram per degree),
this means it takes 4180 J of heat energy
to raise the temperature of 1 kg of water by 1oC.
See 2.2
for Worked
out practice questions
involving specific heat ....
... where you
have to use the formula and correct units described above and you MUST
be able to rearrange the equation.
The amount of energy stored in
(transferred to) or
released from a system as its temperature changes can be calculated using
the above equation.
Other specific heat capacity values (J/kgoC):
ice 2100, aluminium 902, concrete 800,
glass 670, steel 450, brass 380, copper 385, lead 130
Because each material has a different
heat capacity, although you can heat the same mass of substance from one
temperature to another, you cannot assume they store the same amount of
thermal heat energy per kilogram.
The materials with the highest heat
capacity will store the most thermal energy per kilogram for the same
increase in temperature - they are effectively a more concentrated
thermal energy store.
Conversely, when allowing materials
to cool, the materials with the highest specific heat capacity will
release more thermal energy per kilogram for the same decrease in
temperature.
Be able to evaluate different materials according to their
specific heat capacities.
The heat specific heat capacity
in simple terms is how much energy (J) is needed to heat a specific mass (1
kg) by one degree oC.
Examples may have studied
include the use of water, which has a very high
specific heat capacity,
oil-filled radiators and
electric storage heaters containing concrete or bricks.
