A2 Advancing Physics OCR B - Chapter 17

A2 Advancing Physics OCR B - Chapter 17

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Chapter 17. Probing deep into matter:
Rutherford Scattering
Previously physicists thought that the atom was a positively charged object, with negative
electrons scattered around it.
Rutherford fired a stream of alpha particles at a thin foil of gold nuclei. They then observed
the angle at which the alpha particles were scattered and noticed that occasionally, the
particles were scattered by more than 90º. This meant that it must have been hitting
something larger than itself.
Rutherford also noticed that most of the alpha particles went straight through, without
scattering. From this he concluded that.
· Since most atoms where unaffected by the gold film, large parts of the atom is
empty space.
· A very small number were scattered significantly, so there must be a point with high
mass, but only a small volume.
· Alpha particles were repelled, so this mass, must be positively charged.
· Since atoms are neutral overall, the negative charge must be outside of the
nucleus, in the empty space between atoms.
Closest approach
An alpha particle, can get closest to the nucleus when it is fired directly at it. When this
happens, it will be deflected by 180º. Since you know the kinetic energy, which you give
the alpha particles, you can work out how close to the nucleus they can get using.
1 2 Qq
mv =k .
2 r
You know Q, the charge on the nucleus, which is the number of protons, multiplied by the
charge on a proton. The charge on the alpha particle, can be calculated in the same way,
since there are 2 protons.
Fundamental Particles
At the moment there are 16 known fundamental particles, with 2 more theorized, (the
higgs boson and the graviton). These 16 are split into 2 groups, Fermions and Bosons.
Bosons
Bosons are responsible for the forces between particles. There are 4 known types. The
photon, is the most well known, as is it the electromagnetic force carrier, so includes
forces such as magnetism.
Another boson is the gluon, creatively named it is the carrier for the nuclear strong force,
which "glues" the quarks together in the nucleus.
Finally the W and Z bosons together are the carriers of the nuclear weak force.
There is a theorized 5th boson, called the graviton, which carries the very weak force of
gravity.
Anthony Stewart

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Fermions
Fermions are any particles which aren't bosons, and are themselves split into 2 groups,
Leptons and Quarks.
Quarks
Quarks, are the particles which form the protons and neutrons as well as many other, but
much rarer particles. There are 6 flavours of quark, 3 positively chared. The Up, Charm
and Top (or Truth) quarks. These Quarks, have a charge of + 2/3. The other 3 quarks are
the Down, Strange and Bottom (or Beauty). These have a negative charge of ­ 1/3.…read more

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Quark Confinement
In theory if you provide enough energy to a Baryon, you could separate it to get an
individual quark. The problem with this is though that because quarks are held together by
the nuclear strong force, a very large amount of energy is required for this, and before you
manage to separate the quarks, the energy you put in would form a quark-antiquark pair.
Meaning that it is, in-fact, impossible to get a quark on its own.…read more

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