PHYB5 - Energy from the Nucleus

Revision notes for AQA Physics B: Physics in Context. Covers a section of the unit 5 exam.

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  • Created on: 26-06-11 19:01
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Energy from the Nucleus
The law of conservation of energy states that "energy cannot be created nor destroyed, but
may be changed from one form to another"
E = mc2
E = Energy (J)
m = Mass (kg)
c = Speed of light (m/s)
When using the above equation, there are various units that can be used, and some of which
can be complicated due to mass and energy being equivalent.
Atomic Mass Unit ­ the atomic mass unit (u) is defined so that the mass of Carbon12 is
The mass of a Carbon-12 atom divided by 12 gives the value of u = 1.993×10-27kg . When
using Einstein's equation, the masses can be given in terms of this atomic mass unit.
Electron Volts ­ the electron volt is a unit of energy. It is equal to the kinetic energy gained
by an electron being accelerated by a p.d. of 1 volt. 1eV is equal to 1.6×10-19J
The relationship between these two units is that 1u = 934MeV . This fact can be used rather
than the above equation if you have a mass in terms of u and need to work out the energy
Mass Defect
The mass of a nucleus is always less than the total mass of its constituent nucleons. The
difference is called the mass defect.
mass defect = mass of nucleons - mass of nucleus
The reason for this is that combining the nucleons causes some of their mass to be released
as energy. If you wanted to separate these nucleons, you would have to give them the same
amount of energy. This is called the binding energy.
Nuclear Stability and Instability
Whether a nucleus is stable or not depends on the balance between the protons and
neutrons in the nucleus. As protons are positively charged they repel each other, meaning a
nucleus made of protons only would fly apart. Hydrogen-1 is the only nucleus that does not
contain any neutrons. For other light, stable nuclei the number of protons (Z) and neutrons

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N) are the same. After Z=20 the stable nuclei contain more neutrons than protons. The
difference between them becomes greater as Z increases.
This graph shows how N and Z vary. Isotopes
that decay by beta emission do so because they
contain too many neutrons, as a new neutron
turns into a proton. Isotopes that decay by
positron emission do so because they contain
too few neutrons, as a protons turns into a
neutron. Alpha emitters all have a proton
number greater than 84.…read more

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This binding energy is the energy that would be necessary to split the nucleus into its
constituent protons and neutrons.
Binding Energy per Nucleon (BEPN)
The binding energy per nucleon of an element is the energy required to overcome the
electrostatic forces needed to form that element to allow the strong force to take over,
and thus it is also the energy required to separate the nucleons in an element.…read more

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The change in mass or binding
energy can be used to work out the energy released from nuclear fission.…read more

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Carbon was used in early reactors, however water is now used in
Pressurised Water Reactors.
Absorption Cross Section
The chance of a neutron being absorbed depends on the absorption cross section. Two solid
objects, such as billiard balls, will collide provided that the centre of each ball falls within a
circle of radius equal to twice the radius of one of the balls. For billiard balls this area
depends only on the physical size of the balls.…read more

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Energy from Nuclear Fusion
The energy that we receive on Earth from the Sun and stars comes from nuclear fusion
Fusion in the sun
The principle process by which energy is generated in the Sun is a cycle called the Hydrogen
Cycle. It involves the fusion of hydrogen nuclei. At temperatures at the core of the sun,
hydrogen atoms are stripped of their electrons meaning the sun is a plasma of positively
charged nuclei of atoms and free electrons
1.…read more

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Why is it difficult to achieve?
One promising fusion reaction appears to be fusing of deuterons (nuclei of hydrogen-2).
There is an abundance of this fuel on Earth. The problem is that the nuclei both carry a
positive charge. In order to fuse they have to get close enough for the nuclear strong force
to come into play, meaning the separation has to be equal to the sum of their radii. This is
1x10-15m for the deuterons.…read more

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Energy could be
extracted by pumping a coolant through the lithium blanket and then producing electricity as
What's the Risk?
Risk is the probability of harm being caused by an activity. An activity could be harmful to the
health of the person undertaking the activity or of others not directly involved in the activity,
the equipment being used, or the natural environment.…read more

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Low level waste can be stored in metal drums and buried in large trenches…read more



Hi tom

im currently doin A2 physics! Im hoping to maintain my B grade as i dont think ill be able to improve. any tips

also how well did u do on your a-levels and wat are u doin now if its not too personal

Paul Davis

Fucking amazing notes mate and viren lad, good luck in your exams

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