Download AS90184 - NBCPhyyear11

6/07/2017
AS90184
Demonstrate understanding of
heat transfer and nuclear physics
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• What is heat?
• What is temperature?
• How does heat flow?
Heat
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• Think of two glass beakers filled with water
at the same temperature (80oC). One
beaker contains 100 ml of water the other
is larger and contains 500ml of water.
• Which beaker would take longer to cool?
• Therefore which beaker contains more
heat (energy)
Temperature
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Temperature is a measure of the heat
energy contained in an object. The
greater the heat energy the greater the
temperature.
The molecules within some material are all
moving around and have kinetic energy.
Temperature is a measure of the average
kinetic energy of the material.
Scales
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We are familiar with the Celcius scale, also
known as the Centigrade scale.
There are two fixed points on this scale
0oC is melting ice and 100oC is boiling water.
The range of temperatures in between are
divided into 100 equal divisions.
The Kelvin scale has the same size unit as
Celcius, but a different zero point. It starts at
absolute zero, when all molecules within
some material will stop moving (this is
-273oC).
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We only use the unit Kelvin (K), unlike
Celcius where we use degrees Celcius
(oC)
We will often use K to describe a
difference in temperature values.
e.g. to raise the temperature of some
water by 20 K is the same as raising
the temperature by 20oC.
Thermometer
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A thermometer is usually constructed of glass
with a bulb at one end containing a reservoir of
coloured liquid. There is a thin capillary running
the length of the thermometer.
Heat applied to the bulb transfers easily through
the thin outside shell of the bulb and heats the
liquid within. As heat energy is added to the
liquid it expands and flows up the capillary. A
scale marked on the side of the capillary allows
us to read the value of the temperature.
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Scale
Bulb with thin wall
containing liquid
Capillary
Transfer of heat
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There are generally three methods of heat
transfer:
Conduction: Through some material which allows
heat to travel through it, a thermal conductor.
Convection: Within a fluid when hot material is less
dense and it rises, cooler material get heated by
the source and rises up
Radiation: By being emitted from the source
directly as heat energy, travels through space in
all directions, by infra red light energy.
Black or White
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Which of these two colour options will warm up
in the sun quickest?
If we filled two flasks with hot water at the same
temperature, one was black and the other white,
which would cool quickest?
We find that black surfaces will absorb heat
more easily than white surfaces, but they will
also radiate heat more easily.
This effect is more pronounced when we
compare a matt black surface with a shiny silver
surface.
Conductors/Insulators
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What materials make good conductors of heat?
What materials make good heat insulators?
What is the key point about most good heat
insulators?
Specific Heat capacity
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Consider a beaker of water containing 100ml
and another containing 200ml.
If we heated both beakers up from 20oC to
30oC, which would require more energy?
The amount of energy required depends
upon the amount of material to be heated.
We state that the amount of energy required
to heat 1 kg of material by 1oC is called the
Specific Heat Capacity of that material.
Specific Heat Capacity of water
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The specific heat capacity of pure water is 4200
J per kg per 1oC
( 4200 Jkg-1oC-1 )
We find a formula to calculate the amount of
energy to heat an amount of material is given as:
Q=mcΔT
Where Q= heat energy
m = mass of material
ΔT= change in temperature
c = Specific heat capacity of the
material.
Example
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How much heat energy is required to raise by
15oC 2.25kg of water.
Q =
mcΔT
Q =
2.25 x 4200 x 15
Q =
141750 J
How much does the temp rise for 3 l of water
when 252000J of energy is used to heat
Q =
mcΔT
252000 =
3 x 4200 x ΔT
ΔT
=
252000/12600 = 20oC
Specific heat capacities
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Different materials will have different heat
capacities.
Latent heat
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When we boil a kettle of water not all the water
instantly turns into steam! There is a delay as
parts of the water turn into steam. This takes
some time and requires a lot of energy. The
larger the amount of water the more energy is
required.
During this process there is no change in
temperature as the H2O goes from water at
100oC to vapour at 100oC.
The energy required to do this is called the
latent heat.
Is there another heat point where this situation
occurs?
Latent heat
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We call the energy to change 1 kg of a
material from solid to liquid the latent heat of
fusion (melting), occurs at 0oC for water.
We call the energy to change 1 kg of a
material from liquid to gas the latent heat of
vaporisation, occurs at 100oC for water.
For water
latent heat of fusion is 334000 Jkg-1
latent heat of vaporisation is 2260000 Jkg-1
Temp/Energy Graph
The graph opposite
shows what happens to
temperature as we heat
something up through
it’s melting point and
boiling point.
If we turn the graph
around as materials
cool then as they
condense or fuse then
the temperature will
remain fixed.
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