# PHYB5 - Breaking Matter Down

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

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• Created by: tom
• Created on: 26-06-11 18:58

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Breaking Matter Down
Accelerating Charged Particles
An electric field is a region in which a charged object experiences a force. The electric field
strength is defined as the force pet unit positive charge. Electric forces may be either
attractive or repulsive due to positive and negative charges.
Qq
F = 4 2
0r
1
o 40 = constant
o 0 = permittivity of free space
o Q and q = charges in coulombs
Coulombs' Law can be used to find the force of attraction/repulsion between two charged
objects. Conventionally, a positive answer denotes a repulsive force, and a negative one
denotes an attractive force. Any charges are said to be point charges.
It follows from this equation that the electric field strength around a point charge (or a
charged sphere ­ which behaves as if the charge were concentrated at its centre) will be
given by:
Q
E=F
q = 40r2
Electric fields can be represented by drawing lines of force. Radial field lines of force will
radiate from the centre of the charge in all directions. They will not be uniform as charged
objects will experience different forces and accelerations at different positions in the field. In
a uniform field on the other hand, charged object will experience a constant force and
acceleration at any position within the field. To produce a uniform electric field, a potential
difference needs to be connected across a pair of parallel metal plates. Field lines will be
parallel and perpendicular to the plates
E=V d
E = Electric field strength (NC-1) or (Vm-1)
V = Potential difference across the
plates (V)
d = Separation (m)
Generating Electrons (The Thermionic Effect):
A heated metal filament or metal cathode in a vacuum will eject electrons from the surface
if it can overcome the electrostatic forces holding the electrons to their parent atoms.

## Other pages in this set

### Page 2

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There are two heating methods; direct heating (current is passed through the filament) or
indirect heating (cathode is heated by a separate filament close to it).
The emitted electrons produce an electric field. The electrons repel other electrons and
disperse whilst forming a cloud around the filament or cathode ­ which will have become
positive due to the loss of electrons.…read more

### Page 3

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Deflection by a Uniform Electric Field:
Electric fields can be used to deflect the path of a beam of charged particles when the field
is applied at right angles to the path of the beam. This is the similar to projectile motion in a
gravitational field
A particle of charge Q and mass m initially moving with a speed u, moving into an electric
field of strength E = V

### Page 4

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Q, travelling at velocity V in a magnetic field of strength B is
given by
F = BQv
This force, when at a right angle to the magnetic field, provides a centripetal force, keeping
the particle in circular motion. The diagram below shows a particle of charge Q and mass m
moving in a uniform magnetic field of strength B.…read more

### Page 5

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Mass spectrometer separate ions according to
the ratio of their mass to their charge. As most of the ions have a charge of +1 it can be
considered that the mass spectrometer separates ions with different masses.
Velocity selector: From the ion chamber the ions are accelerated through a hole in the
cathode into the velocity selector. The purpose of this chamber is to select only particles
with a certain velocity and charge.…read more

### Page 6

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The beam passed through several hollow metallic cylindrical electrodes
called drift tubes. They are connected to a radio frequency alternating supply. The ions are
accelerated by the electric field that exists between each pair of drift tubes. In order to use a
constant frequency supply each drift tube needs to be longer than the previous one as they
are moving faster charged particles which will travel a greater distances in the time for which
the alternating supply has the wrong polarity.…read more

### Page 7

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For particles travelling close to speed of light if the half-life was measured from earth it
would be found to be much longer than that of similar stationary particles. The time is
dilated or prolonged as a result of travelling at such high speeds.…read more

### Page 8

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As they travel faster, they trace a larger arc meaning they always take the
same time to travel each semi-circle. This allows the voltage applied to the dees to be of a
constant frequency
m
2r 2( QB ) 2m
T= V = V V = QB
The time period is equal to the inverse of the frequency, meaning the orbital frequency is
QB
f cyc = 2m
This does not depend on the velocity of the particle.…read more

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This
accelerates the charged particles to high energies. The particles are initially injected into the
ring with relatively high energy before having their energy boosted as they are accelerated by
the electrodes.
A property of any accelerated charged particles is that it emits electromagnetic radiation.
The charged particles in a synchrotron are accelerating when they are deflected in a circular

### Page 10

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K, along with 1232 dipole magnets to bend the protons and
392 quadrupole magnets to focus the beams
Way Forward
The production of synchrotron radiation is a limiting factor when accelerating electrons in a
circular accelerator. The low rest mass and availability of the electron makes it an ideal
particle to accelerate in a linac.…read more