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					    EDEXCEL IGCSE PHYSICS 7-4

            Particles
Edexcel IGCSE Physics pages 226 to 231
           December 4th 2010
       Edexcel IGCSE Specification
Section 7: Radioactivity and particles
c) Particles
describe the results of Geiger and Marsden’s experiments with gold foil and
alpha particles
describe Rutherford’s nuclear model of the atom and how it accounts for the
results of Geiger and Marsden’s experiment and understand the factors
(charge and speed) which affect the deflection of alpha particles by a nucleus
understand that a nucleus of U-235 can be split (the process of fission) by
collision with a neutron, and that this process releases energy in the form of
kinetic energy of the fission products
recall that the fission of U-235 produces two daughter nuclei and a small
number of neutrons
understand that a chain reaction can be set up if the neutrons produced by
one fission strike other U-235 nuclei
understand the role played by the control rods and moderator when the fission
process is used as an energy source to generate electricity.
 The Plum Pudding Atomic Model
Before about 1910
many scientists
believed that an atom
consisted of:
Positively charged
matter spread out like a
pudding embedded by
negatively charged         The ‘Plum Pudding’ Model
electrons (like plums in
a pudding).
     Rutherford’s Atomic Model
In 1909 Ernest Rutherford suggested that an atom
consists of a a tiny positively charged nucleus
surrounded by negatively charged electrons.




     Lord Rutherford
       1871 - 1937
   Geiger & Marsden’s alpha particle
        scattering experiment
In 1909 Hans Geiger
and Ernest Marsden
performed an
experiment using alpha
particles to determine
which of the two
models was the better
in describing the        Geiger and Marsden
structure of an atom.
    The apparatus
2
                    5




                        4




                            3

1
               What was observed
   alpha               thin metal foil
  source




1. Virtually all of the alpha particles went straight through the metal foil.
2. A few alpha particles were deflected through a small angle.
3. About 1 in 10 000 were deflected backwards.
           How the results can be explained
                                        atom
1. Deflections occur because there is
   a force between the charged
   nucleus and the positively charged
   alpha particles.
2. Most of the alpha particles do not
   go near enough to the nucleus to
   be deflected.
3. Backwards deflections occur when
   the alpha particles make near
   head on collisions with the
   positively charged nucleus.



                                               nucleus (highly enlarged)
How their results supported
Rutherford’s atomic model
            1. The relatively small number of
               deflections indicates that most of
               the atom is empty space with only
               a very small nucleus.
            2. The backward deflections can
               only occur if the nucleus is
               positively charged and contains
               most of the atom’s mass.
            3. The ‘plum pudding’ model would
               not produce backward
               deflections.
Choose appropriate words to fill in the gaps below:
According to __________ an atom consists of a tiny,
              Rutherford
___________ charged __________ surrounded by a cloud of
  positively             nucleus
________ electrons. The nucleus also contains most of the
 negative
______ of an atom.
 mass

This model was supported by the ______ particle scattering
                                    alpha
experiment in 1909. In this experiment most alpha particles
passed ________ through a thin metal foil with only about 1
        straight
in 10000 being deflected _________.
                          backwards

                    WORD SELECTION:
            Rutherford mass backwards negative
              straight positively alpha nucleus
                 Nuclear fission
Nuclear fission is the splitting
of an atomic nucleus.

Nuclear fission can be used as
an energy source in a nuclear
reactor.

There are two fissionable
substances in common use in
nuclear reactors, uranium 235
and plutonium 239.
                  Chain reaction
The fission of a nucleus of
Uranium 235 can be initiated
by a neutron.

When this nucleus splits       neutron
further neutrons are
produced.

These neutrons in turn can
cause more nuclei to split.

An avalanche effect, called a
‘chain reaction’ can then                A chain reaction
occur.
    Nuclear fission reactor

                      5
    2



6




1
             3&4



7
                Nuclear reactor parts
1. Fuel rods                           4. Coolant
These contain U235 or Pu239. They      This transfers the heat energy of the
become very hot due to nuclear         fuel rods to the heat exchanger.
fission.                               Coolant be water, carbon dioxide gas
                                       or liquid sodium.
2. Control rods
Made of boron, when placed in-         5. Heat exchanger
between the fuel rods these absorb     Here water is converted into high
neutrons and so reduce the rate of     pressure steam using the heat energy
fission. Their depth is adjusted to    of the coolant.
maintain a constant rate of fission.
                                       6. Reactor core
3. Moderator                           This is a thick steal vessel designed to
This surrounds the fuel rods and       withstand the very high pressure and
slows neutrons down to make further    temperature in the core.
fission more likely. The moderator
can be water or graphite.              7. Concrete shield
                                       This absorbs the radiation coming
                                       from the nuclear reactor.
Choose appropriate words to fill in the gaps below:
Nuclear fission is the _________ up of the nucleus of an atom
                         splitting
into two smaller nuclei. ________ and neutrons are also
                           energy
usually emitted.
Nuclear ________ use Uranium _____ or Plutonium _____to
           reactors                235               239
produce energy by nuclear ________. A controlled chain
                              fission
reaction is maintained by the use of _______ rods which
                                       control
absorb some of the _________ produced.
                     neutrons

An _______ bomb is the consequence of an uncontrolled
    atomic
chain reaction.
                      WORD SELECTION:
             reactors energy 239 atomic splitting
                neutrons 235 fission control
                  Particles
       Notes questions from pages 226 to 231

1. Answer the questions on page 231.
2. Verify that you can do all of the items
   listed in the end of chapter checklist on
   page 231.
Online Simulations

				
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