Transcript www.PhysicsGCSE.co.uk P1a (ii) Heating Houses You will learn about: Factors that affect the amount of energy needed to change the temperature of a substance.

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P1a (ii) Heating Houses
You will learn about:
Factors that affect the amount of energy needed to change
the temperature of a substance (Specific Heat Capacity)
Factors that affect the amount of energy needed to change
the state of a substance (Specific Latent Heat)
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Heating a substance
For a substance to be
heated to a particular
temperature three
considerations are needed
to determine the amount of
energy required:
1.
More liquid
requires more
energy
How much liquid there
is in the beaker
2.
What the liquid is
3.
The rise of
temperature required
Dependent on
Specific Heat
Capacity
Higher
temperature rise
means more
energy needed
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Changes of State
Gas
Temperature (°C)
Liquid
Solid
Change of
State: Liquid to
a Gas
Time (seconds)
Change of
State: Solid to a
Liquid
When a solid is heated, its
temperature rises until it
changes into a liquid. If the
liquid is heated eventually
the liquid will turn into a gas.
The inclines
represent
the state of matter (Solid,
Liquid or Gas). When heat
energy is added the
temperature rises over a
period of time.
The plateaus
represent where the
substance is changing state.
There is no temperature rise
even though heat energy is
being applied!
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Change of State explained
Specific Heat Capacity:
The amount of energy needed to
change the temperature of one
kilogram of a substance by one
degree Celsius. The unit is (J/kg °C)
Specific Latent Heat:
The amount of energy needed to
change the state of one kilogram
of a substance without a change in
temperature. The unit is (J/kg)
Represented by
inclines
This means that a substance
that is heated will continue
to rise in temperature.
The thermal energy
transfers into kinetic energy
and the molecules of the
substance vibrate more and
more.
This means that sometimes when
a substance is heated it will not
rise in temperature.
Instead of rising in temperature
the energy is used to break the
bonds holding the molecules
together.
Represented by
plateaus
Plates are flat. The word plateau
has plate in it. So remember that a
plateau is like a flat plate!
Energy cannot do two things at the
same time! It cannot raise the
temperature AND break the bonds of a
substance. If it raises the temperature
then this is SHC. If the state changes
then this is SLH.
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Specific Heat Capacity Calculations
Different substances have different SHC values.
The higher the SHC value then the more energy
needed to raise its temperature.
A higher SHC also means a longer time to heat up.
Important!
SHC:
Substance
SHC (J/kg °C)
Energy transferred = mass x SHC x temperature change
Copper
390
Aluminium
897
Rubber
1600
Seawater
3900
Water
4200
Calculate the energy transferred when 60 kg of
seawater is heated from
15°C to 70°C.
Energy transferred = mass x SHC x temperature change
Energy transferred = 60 kg x 3900 J/kg°C x (70°C-15°C)
Energy transferred = 60 x 3900
x
55
Energy transferred = 12870000 J
= 12.87 MJ
So water requires more
energy to raise its
temperature as it has the
largest SHC value in this table
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Specific Latent Heat Calculations
Substances require different SLH
values to change state.
A higher value means that more
heat energy is needed to be
absorbed (if heated up) or
released (if cooling down) to
change that state.
SLH:
Energy transferred = mass x specific latent heat
Important!
Change of State
SHC (J/kg)
Ice to water
340,000
Water to Steam (Gas)
2,260,000
Calculate the energy transferred when
1.25kg of ice melts at 0°
Energy transferred = mass x specific latent heat
Energy transferred = 1.25kg x 340,000 J/kg
Energy transferred = 425,000 J
= 425 kJ
So water requires more
energy to be absorbed to
change from liquid to gas than
solid to liquid
Remember there is no temperature change in the calculation. The 0°C information is telling you
that you are dealing with a solid. Recall that water is a solid (ice) at 0°C
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Quick Questions
1. Eve says that when you heat an object it gets hot.
Adam says it does not. Explain who is right.
2. The SHC of quartz glass is 700 J/kg °C. How much
energy is needed to raise the temperature of 1kg of
quartz glass by just 1 °C?
3. A graph shows a substance melting and then boiling.
Where on the graph would you locate where the
substance is changing state?
4. An aluminium saucepan has a mass of 2.4kg. How
much energy is needed to raise its temperature from
22 °C to 100 °C so it can boil some water?
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Quick Questions
1.
2.
3.
4.
Eve says that when you heat an object it gets hot. Adam says it does
not. Explain who is right. Both are correct: Eve is correct when
discussing an object that is not changing state. Adam is correct when
discussing an object that is changing state.
The SHC of quartz glass is 700 J/kg °C. How much energy is needed to
raise the temperature of 1kg of quartz glass by just 1 °C? 700J
A graph shows a substance melting and then boiling. Where on the
graph would you locate where the substance is changing state? This is
where there is a plateau (remember plateau is the straight plate part)
An aluminium saucepan has a mass of 2.4kg. How much energy is
needed to raise its temperature from 22 °C to 100 °C so it can boil
some water?
Energy transferred = mass x SHC x temperature change
Energy transferred = 2.4kg x 897 J/kg °C x 78 °C
Energy transferred = 167918.4 J
= ~ 168kJ