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Fission and Fusion – Revision Pack (P4)
Nuclear Fission:
Natural uranium contains two isotopes, uranium-235 and uranium-238.
The (enriched) uranium used in nuclear power stations contains a higher amount of
uranium-235 than occurs naturally.
Fission occurs when a large unstable nucleus is
split up and energy is released as heat.
The heat is used to boil water to produce
steam.
The pressure of the steam acting on the blades
makes it turn
The rotating turbine turns the generator,
making electricity.
When uranium fissions, a chain reaction starts. A nuclear bomb is an example of a
chain reaction that is not controlled.
In a nuclear power station, atoms of uranium-235 are bombarded with neutrons. This
causes the nucleus to split, releasing energy.
The extra neutrons emitted cause further
uranium nuclei to split up. This is described as a
chain reaction and released huge amounts of
energy.
Controlling Nuclear Fission:
The output of a nuclear reactor can be controlled:
-
Boron control rods are used to control the rate of fission. The rods can be
raised or lowered. Boron absorbs neutrons, so few are available to split more
uranium nuclei.
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Graphite Moderators between the
uranium fuel rods slows down the
fast moving neutrons emitted
during fission. Slow-moving neutrons
are more likely to be captured by
other uranium nuclei.
Fusion:
Nuclear fusion happens when two light
nuclei fuse (or join) together and in doing
so release large amounts of energy.
Fusion needs very high temperature and
pressure that is near impossible to replicate
on earth.
Research in this area is very expensive, so it
is carried out as an international joint venue
where costs, expertise and benefits are all
shared.
Fusion happens in stars at very high
temperatures and pressures.
Fusion bombs are started with a fission reaction which creates exceptionally high
temperatures.
There have been a few unsuccessful attempts to replicate the conditions safely on
earth BUT scientists are working hard to solve the safety issues presented. Cold fusion
is not accepted as a realistic representation as results are impossible to verify thus
far.
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Resistance:
Resistance slows the flow of electrical current or
flow of electrons.
A variable resistor, or rheostat, changes the
resistance. Longer lengths of wire will have more
resistance and shorter lengths will have less
resistance.
Thinner wires have higher resistance and thicker
wire have lower resistance (see left).
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Voltage (potential difference) is measured in volts on a voltmeter which must be
connected in parallel to the resistor.
For a fixed resistor, as the voltage increases so does the current.
For a fixed power supply, as the resistance increases, the current decreases.
The formula for resistance is:
RESISTANCE = Voltage / Current
Resistance is measured in Ohms (Ω).
Voltage is measured in Volts (V).
Current is measured in Amps (A).
Live, neutral and earth wires:
The live wire carries a high voltage into and
around the house. This wire is brown.
The neutral wire completes the circuit; it provides
a return path for the current which enters through
the live wire and exits via the neutral wire. This
wire is blue.
The earth wire is there as a safety measure. It is
connected to the case of the appliance to
prevent it from going ‘live’. This is yellow and
green.
A fuse contains a wire which breaks the circuit if
the current gets too high. It is a safety feature.
The earth wire and fuses work together to prevent
people from experiencing an electric shock. If the
live wire was to touch the casing of the
appliance, like an electric cooker, then you
would get a shock when you touched the
The fuse is in the live wire. If there was a
appliance. However, the earth wire is connected
fault, like the live wire touches the case,
to the metal casing so takes the current away
then the earth wire allows a huge current
it, stopping
it from
‘live’.
image tofrom
the left
highlights
whatbecoming
was discussed
to surge through the live wire which The
melts
before;
the
earth
wire
will
take
the
current
away
the fuse and breaks the circuit.
from the casing if it comes into contact with the live
wire.
NOTE – A re-settable fuse (circuit-breaker) doesn’t
need to be replaced to restore power, it can be reset.
Electrical Power:
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The rate at which an appliance transfers energy is its power rating:
(Electrical) Power = Voltage x Current
NOTE – the mains voltage of a home in the UK is always 230V
The formula for electrical power can be used to calculate the correct fuse to use in
an electrical device; for example:
A microwave oven has a power rating of 900W. Which of these fuses is the most
appropriate for this microwave which will be used at home?
7A fuse
2A fuse
4A fuse
13A fuse
ANSWER
Power = voltage x current
Current (in Amps) = Power / Voltage
Voltage at home = 230V
Power = 900W
900 / 230 = 3.913 A -SO, the 4A fuse is the most appropriate fuse to use.
Double insulation:
An appliance with a outer plastic case doesn’t need to be
earthed, this is because their outer case is an electrical
insulator NOT (like metal) a conductor – so it CANNOT
become live.
The symbol for a double insulated appliance is shown the
left.
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Electrons:
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An atom consists of a small positively charged nucleus surrounded by an equal
number of negative electrons. For an atom to be neutral there needs to be the
same amount of negative charges as positive charges.
NOTE – all electrostatic effects are due to movement of electrons – it is ONLY the
electrons that can be transferred.
REMEMBER - like charges repel and opposite charges attract.
When an object gives away electrons, it has been good
so we give it a + (positive) charge.
When an object has taken away electrons, it has been
bad so we give it a – (negative) charge.
If we rub a polythene rod against a duster, electrons
move to the polythene from the duster – this means that
the polythene is negative because it has stolen electrons
and been bad.
Atoms that have become charged are called ions.
REMEBMBER – electrons travel between two insulators.
Electrostatic Shocks:
In lorries that carry inflammable gasses or vapours or a high concentration of
oxygen, a spark can ignite an explosion. This is why they have to be earthed before
they are unloaded. Friction can cause a charge to build.
If a person touches something at a high voltage – a surge of electric charge may
flow through their bodies through to the earth. Even a small amount of electric
charge can be fatal to a person.
Static electricity is a nuisance but not fatal; for example:
-
Dusts and dirt are often attracted to insulators, like a TV screen
Clothes made from synthetic materials (like nylon) often ‘cling’ to each other
and the body
To avoid dangerous electric shocks:
-
-
Any objects that are likely to become charged should be connected to
earth; any flow of charge would immediately flow down the earth wire
In factories where a machine is likely to become charged, the worker will
stand on a rubber mat or wear shoes with insulating soles so that charge
cannot flow through their bodies to earth
Fuel tankers are connected to an aircraft by a conducting cable during
refuelling
PastAnti-static
Papers: sprays, liquids and cloths made from conducting materials (like brass or
steel) carry away electric charge and leave a conducting layer. This ensures that
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a charge doesn’t build up.
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Using Radiation:
Radiation emitted from the nucleus of an unstable atom can be alpha (α), beta (β)
or gamma (µ).
Alpha radiation is absorbed by the skin, so is NOT useful for diagnosis or treatment.
Beta radiation passes through the skin but not bone. Its medical uses are limited
because of this; they are for example used to treat the eyes.
Gamma radiation is very penetrating and is widely used in medicine. Cobalt-60 is a
gamma-emitting radioactive material that is widely used to treat cancers.
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When nuclear radiation passes through a material it causes ionisation. Ionising
radiation damages living cells, increasing the risk of cancer. Alpha radiation is the
most ionising and gamma radiation is the least ionising.
Cancer cells can be killed using radiation. The affected area is exposed to large
amounts of radiation in a process called radiotherapy.
NOTE – materials can be made radioactive in a nuclear reactor whereby their nuclei
absorb extra neutrons.
X-rays and Gamma rays:
When X-rays pass through the body, the tissues absorb some of the ionising radiation.
The amount absorbed is dependent on the density of the absorbing material.
Gamma and X-rays have similar (short) wavelengths, but are made in different ways.
X-rays are made in the following way:
STEP 1 – high-speed electrons are fired (from
the cathode) at a metal target (located at the
anode)
STEP 2 – most of the kinetic energy is lost as
heat, but some is transferred to x-rays
STEP 3 – the x-rays then exit via a glass window
Through using an x-ray machine, you can
control the production and energy of the xrays. With gamma rays you CANNOT control or
change the gamma radiation emitted from a
particular source.
When the nucleus of an atom of a radioactive substance decays, it emits an alpha
or beta particle and loses any surplus energy by emitting gamma rays.
T racers:
Some radioactive tracers are used to investigate inside a patient’s body without the
use of surgery.
For example technetium-99m is used as a medical tracer. It only emits gamma
radiation.
Iodine-123 is also used as a medical tracer.
It emits gamma rays and is used to
investigate the thyroid gland (see left).
The radioactive tracer is mixed with food
or drink and consumed or injected into the
body.
Its progress through the body can be
monitored by using a detector such as a
gamma camera connected to a
computer.
This avoids using surgery to investigate.
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Treating Cancer:
Source of
radiation
Source of
radiation
A radioisotope is used to destroy a tumour in the
body.
STEP 1 – three sources of radiation, each
providing a 3rd of the required dose, are
arranged around the patient with the tumour at
the centre.
Source of
radiation
STEP 2 – the healthy tissue only receives a third of
the dose so the damage to healthy tissue is
limited.
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- each
radiation source slowly rotates around the patient. The tumour will receive
constant radiation BUT healthy tissue receives only small inconsistent doses of radiation,
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limiting damage to healthy tissue.
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Longitudinal Waves:
Ultrasound is a sound above the frequency at which humans can hear, which is
about 20,000Hz.
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This type of wave travels as a pressure wave; it contains:
-
Compressions – areas of higher pressure where particles are squashed
together
Rarefactions – areas of lower pressure where particles are more spread out
Longitudinal waves:
-
-
-
Cannot travel
through a
vacuum; the
denser the
medium, the
faster a sound
wave travels
The higher the
frequency or
pitch, the
smaller the
wavelength
Wave Direction
The louder the sound, or the more powerful the ultrasound, the more energy is
carried by the wave and the larger the amplitude
The direction of wave travel is parallel to the vibrations of particles in longitudinal
waves. The direction of wave travel is at right angles to the vibrations of particles in
transverse waves.
Uses of Ultrasound:
Ultrasound is used to break down kidney stones in these steps:
STEP 1 – a high powered ultrasound beam is directed at the kidney stones
STEP 2 – the energy (vibrations) from the ultrasound breaks the stones down into
smaller pieces
STEP 3 – the smaller pieces are then excreted in the normal way
Ultrasound is also used in body scans where a pulse of ultrasound is sent into the
body. At each boundary between different body tissues some ultrasound is reflected
and the rest is transmitted. The returning echoes are recorded and used to build a
picture of the internal structure.
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Ultrasound can be used in body scans because:
-
-
-
When ultrasound is reflected from a
different part of the body, the depth of
each structure can be calculated using
the distance = speed / time equation –
we know the speed at which ultrasound
travels and the time it takes for the echo
to return
If the tissues are very different (e.g. blood
and bone) then most of the ultrasound
will be reflected and little will be left to
penetrate more into the body
The proportion of ultrasound reflected at each part depends on the densities
of each of the adjoining tissues
The information gathered can be used to produce an image of what was
scanned
Ultrasound (image a) is generally preferred to X-ray (image b) because:
-
It can produce images of soft tissue
It doesn’t damage any living cells
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Dust Precipitators:
As waste gases from factories and
power stations travel up the
chimney,
STEP 1 - they meet the negatively
charged metal grid. The dust or
smoke particles gain this charge so
are also negative (1 on the image)
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STEP 3 – the collecting plates are knocked or at intervals receive vibrations (or
earthed) and this causes the dust or smoke particles fall down into a collector (3 on
the image)
This all means that harmful particles that pollute the air are removed before they exit
the chimney.
REMEMBER – opposite charges attract so in STEP 2 smoke and dust is attracted to the
plate.
UNDERSTAND that for the dust particles to become charged they have to gain or
lose electrons.
Paint Spraying:
Static electricity is used in paint spraying.
STEP 1 – the paint is given a negative charge
and all of the paint particles become
charges with the same charge
STEP 2 – when the paint is sprayed, the
particles spread out because like charges
repel
STEP 3 – the object to be painted is given
the opposite charge to the paint (positive).
Opposite charges attract so the paint is
attracted to the object and sticks to it
This means that there is limited paint
NOTE
– if and
the object
to be
paintedan
is not
charged, the paint moves onto it, but:
waste
the object
receives
even
coating of paint.
(1) The object becomes charged from the paint, gaining the same charge
(2) Further paint droplets are repelled away from the object
This is why the paint is given the opposite charge to the object. If the object is
positive, having lost electrons, the paint should be negatively charged having
gained electrons.
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Defibrillators:
Defibrillation is a procedure to restore a regular
heart rhythm by delivering an electric shock
through the wall of the chest to the heart.
STEP 1 – two paddles are charged from a high
voltage supply.
STEP 2 – They are then placed firmly on the
patient’s chest to ensure good electrical contact
STEP 3 – Electric charge is passed through the
patient to get their heart to contract
The handles of the paddle are made of an insulating material to ensure the operator
doesn’t experience an electric shock as well.
Power can be calculated using:
POWER
=
Energy
Time
For example:
A defibrillator is turned on for 0.008 seconds. The energy used is 600J – what was the
power?
600 / 0.004 = 75,000 W
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Background Radiation:
The large majority of
background radiation
comes from:
-
-
Radioactive
substances from
the ground which
are present in rocks
and soil
Cosmic rays which
come from space
Man-made or
artificial sources,
like radioactive
waste from industry
and hospitals.
Most background radiation comes from natural sources, but some is artificial.
Tracers:
Gamma sources are used as tracers in
underground pipes because its radiation can
penetrate to the surface. There are three steps
when trying to identify a leak using a tracer in an
underground pipe:
STEP 1 – a very small amount of gamma emitter is
put into the pipe
STEP 2 – a detector is passed along the ground
above the path of the pipe (see image)
STEP 3 – an increase in activity is detected where
the leak is, and little or no activity is detected after
this point
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Smoke Detectors:
Smoke detectors contain a radioactive source which emits alpha particles.
Without smoke, the alpha particles ionise the air. This creates a tiny current which
can be detected by the circuit of the alarm.
When smoke fills the detector, the alpha particles are partially blocked. There is now
less ionisation of the air. The change in current is detected and the alarm goes off.
Dating rocks:
Time
% of Uranium
% of Lead
0
100
0
Uranium:
Lead ratio
1:0
After 1 halflife
After 2 halflife
After 3 halflife
50
50
1:1
25
75
1:3
12.5
87.5
1:7
A lot of rocks contain traces
of uranium, a radioactive
material.
Unstable uranium isotopes
present in these rocks go
through a series of decays,
to eventually form a stable
isotope of lead.
By knowing the
uranium to lead ratio, we can estimate how old a rock is. The half-life of uranium is
about 4500 million years. So if the uranium to lead ratio is high, the rock is fairly
young. However, if the uranium to lead ratio is low (that is, there is more lead than
uranium) then the rock is older.
If the percentage of uranium in a rock was at 12.5% then we can estimate that the
rock is 13500 million years old (4500 million x 3).
Radiocarbon Dating:
Time
% of carbon14
% of
nitrogen-14
Carbon-14:
nitrogen-14
ratio
All living organisms have
the carbon isotope
carbon-14 present
because it is part of the
food chain.
SO, by measuring the
amount of carbon-14 in
an archaeological find,
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0
100
0
1:0
After 1 halflife
After 2 halflife
After 3 halflife
50
50
1:1
25
75
1:3
12.5
87.5
1:7
Carbon-14 decays into nitrogen-14
over time, so if we know the ratio of
carbon-14 to nitrogen-14 we can
estimate how old an object, for
example a tree trunk, is. When living
things die, they stop taking in
carbon-14 so the over time the
carbon-14 decays and the activity
decreases. The half life of carbon-14
is about 5700 years so if the
percentage of carbon-14 is 12.5%
then we can estimate that the tree
trunk is 17,100 years old (5700 years x
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halfcan
life).also compare the current activity of carbon-14
in living
matter to the sample activity which will also
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Radiation:
There are three different types of radiation:
Alpha (α)
Alpha particles can
also be described as a
helium nucleus:
Beta (β)
A beta particle is a
high speed electron:
Gamma (µ)
This is a type of
electromagnetic
radiation.
It has NO charge.
Least penetrating; can
be absorbed by air or
a sheet of paper.
This can be absorbed
by a few sheets of
aluminium or a thin
piece of lead.
It is the most
penetrating and can
be absorbed by a
thick piece of lead.
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Radioactive Decay:
Radioactive substances decay naturally, giving out alpha, beta or gamma
radiation. Radioactive substances have unstable nuclei – the nuclear particles are
not held together very strongly. When these substances break down into different
types of atom, it is called radioactive decay.
Nuclear radiation causes ionisation - this is where the electrons from an atom are
removed or when the atom gains electrons.
It is impossible to predict radioactive decay because it is a completely random
process.
Radioisotopes are where an atom has a different number of neutrons than what the
actual element has – e.g. Carbon (12) has six neutrons but in its radioisotope
(carbon 14) it has 8 neutrons. The further away the isotope is from the original atom
from the element, the more unstable it is. It is unstable nuclei that decay and emit
radioactivity.
The idea of the decay is that it makes the nucleus more stable. The atomic number
will change and a new element will be formed.
The nucleus:
A nucleon is a particle found in the nucleus. A
nucleon consists of protons and neutrons.
A = atomic mass (or nucleon number)
Z = atomic number (or proton number)
X = chemical symbol for the element
Alpha and Beta Particles:
The number of neutrons = A – Z
Nucleons can never be lost. Charge is always
When a nucleus emits an alpha or beta particle, the remaining nucleus is a new
conserved.
element.
Alpha particles are very good ionisers. They are also the largest of the three different
particles that can be emitted. This means that they are more likely to strike atoms of
the material they are moving through, ionising them as it goes through.
During alpha decay, the
atomic mass of the parent
nucleus decreases by 4 and
the atomic number
decreases by 2.
The parent nucleus has two
fewer protons and two fewer
neutrons.
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During beta decay, the
atomic mass is unchanged.
The atomic number
increases by 1 – this means
that the nucleus has one
more proton and (as such)
one less neutron.
C-14 has 6 protons
Half Life:
N-14 has 7 protons
The half life of a radioisotope is the
average amount of time it takes
the nuclei present to decay. The
half life will stay the same; it does
NOT change.
Look at the graph on the left – you
will get a similar one in the exam.
Look for one number on the y-axis –
for example 80cpm. This is at zero
days.
Look for half of this amount
(40cpm) – this is at two days.
So the half life is two days.
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