Comprehensive Physics notes covering everything students need for both AS and A-Level Physics exams. This includes core topics- such as mechanics or quantum physics- as well as key formulae and calculations & practicals that students are tested on in the exam.
Particles, Quantum Phenomena and Electricity Mechanics, Materials and Waves
1 Constituents of the Atom 1 Scalars and Vectors
2 Particles and Antiparticles 2 Resolving Vectors
3 Quarks 3 Moments
4 Hadrons 4 Velocity and Acceleration
5 Leptons 5 Motion Graphs
6 Forces and Exchange Particles 6 Equations of Motion
7 The Strong Interaction 7 Terminal Velocity and Projectiles
8 The Weak Interaction 8 Newton’s Laws
9 Feynman Diagrams 9 Work, Energy and Power
10 The Photoelectric Effect 10 Conservation of Energy
11 Excitation, Ionisation and Energy Levels 11 Hooke’s Law
12 Wave Particle Duality 12 Stress and Strain
13 QVIRt 13 Bulk Properties of Solids
14 Ohm’s Law and I-V Graphs 14 Young’s Modulus
15 Resistivity and Superconductivity 15 Progressive Waves
16 Series and Parallel Circuits 16 Longitudinal and Transverse Waves
17 Energy and Power 17 Superposition and Standing Waves
18 EMF and Internal Resistance 18 Refraction
19 Kirchhoff and Potential Dividers 19 Total Internal Reflection
20 Alternating Current 20 Interference
21 The Oscilloscope 21 Diffraction
,The Nuclear Model (Also seen in GCSE Physics 1 and 2) Constituent Charge (C) Mass (kg)
We know from Rutherford’s experiment that the structure of Proton 1.6 x 10-19 1.673 x 10-27
an atom consists of positively charged protons and neutral Neutron 0 1.675 x 10-27
neutrons in one place called the nucleus. The nucleus sits in Electron - 1.6 x 10-19
9.1 x 10-31
the middle of the atom and has negatively charged electrons
orbiting it. At GCSE we used charges and masses for the constituents relative to each other, the table above
shows the actual charges and masses.
Almost all of the mass of the atom is in the tiny nucleus which takes up practically no space when compared to
the size of the atom. If we shrunk the Solar System so that the Sun was the size of a gold nucleus the furthest
electron would be twice the distance to Pluto.
If the nucleus was a full stop it would be 25 m to the first electron shell, 100 to the second and 225 to the third.
Notation (Also seen in GCSE Physics 2)
A
We can represent an atom of element X in the following way: Z X
Z is the proton number. This is the number of protons in the nucleus. In an uncharged atom the number of
electrons orbiting the nucleus is equal to the number of protons.
In Chemistry it is called the atomic number
A is the nucleon number. This is the total number of nucleons in the nucleus (protons + neutrons) which can be
written as A = Z + N.
In Chemistry it is called the atomic mass number
N is the neutron number. This is the number of neutrons in the nucleus.
Isotopes (Also seen in GCSE Physics 1 and 2)
Isotopes are different forms of an element. They always have the same number of protons but have a different
number of neutrons. Since they have the same number of protons (and electrons) they behave in the same way
chemically.
Chlorine If we look at Chlorine in the periodic table we see that it is represented by 3517.5Cl . How can it have 18.5
35 37
neutrons? It can’t! There are two stable isotopes of Chlorine, 17 Cl which accounts for ~75% and 17 Cl which
35.5
accounts for ~25%. So the average of a large amount of Chlorine atoms is 17 Cl .
Specific Charge
Specific charge is another title for the charge-mass ratio. This is a measure of the charge per unit mass and is
simply worked out by worked out by dividing the charge of a particle by its mass.
You can think of it as a how much charge (in Coulombs) you get per kilogram of the ‘stuff’.
Constituent Charge (C) Mass (kg) Charge-Mass Ratio (C kg-1) or (C/kg)
Proton 1.6 x 10-19 1.673 x 10-27 1.6 x 10-19 ÷ 1.673 x 10-27 9.58 x 107
-27 -27
Neutron 0 1.675 x 10 0 ÷ 1.675 x 10 0
-19 -31 -19 -31
Electron (-) 1.6 x 10 9.1 x 10 1.6 x 10 ÷ 9.11 x 10 (-) 1.76 x 1011
We can see that the electron has the highest charge-mass ratio and the neutron has the lowest.
Ions (Also seen in GCSE Physics 2)
An atom may gain or lose electrons. When this happens the atoms becomes electrically charged (positively or
negatively). We call this an ion.
If the atom gains an electron there are more negative charges than positive, so the atom is a negative ion.
Gaining one electron would mean it has an overall charge of -1, which actually means -1.6 x 10-19C.
Gaining two electrons would mean it has an overall charge of -2, which actually means -3.2 x 10-19C.
If the atom loses an electron there are more positive charges than negative, so the atom is a positive ion.
Losing one electron would mean it has an overall charge of +1, which actually means +1.6 x 10-19C.
Losing two electrons would mean it has an overall charge of +2, which actually means +3.2 x 10-19C.
Antimatter
British Physicist Paul Dirac predicted a particle of equal mass to an electron but of opposite charge (positive).
This particle is called a positron and is the electron’s antiparticle.
, Every particles has its own antiparticle. An antiparticle has the same mass as the particle version but has
opposite charge. An antiproton has a negative charge, an antielectron has a positive charge but an antineutron
is also uncharged like the particle version.
American Physicist Carl Anderson observed the positron in a cloud chamber, backing up Dirac’s theory.
Anti particles have opposite Charge, Baryon Number, Lepton Number and Strangeness.
If they are made from quarks the antiparticle is made from antiquarks
Annihilation
Whenever a particle and its antiparticle meet they annihilate each other.
Annihilation is the process by which mass is converted into energy, particle
and antiparticle are transformed into two photons of energy.
Mass and energy are interchangeable and can be converted from one to
the other. Einstein linked energy and mass with the equation:
E mc 2
You can think of it like money; whether you have dollars or pounds you would still have the same amount of
money. So whether you have mass or energy you still have the same amount.
The law of conservation of energy can now be referred to as the conservation of mass-energy.
The total mass-energy before is equal to the total mass-energy after.
Photon
Max Planck had the idea that light could be released in ‘chunks’ or packets of energy. Einstein named these
wave-packets photons. The energy carried by a photon is given by the equation:
hc
E hf Since c f we can also write this as: E
How is there anything at all?
When the Big Bang happened matter and antimatter was produced and sent out expanding in all directions. A
short time after this there was an imbalance in the amount of matter and antimatter. Since there was more
matter all the antimatter was annihilated leaving matter to form protons, atoms and everything around us.
Pair Production
Pair production is the opposite process to annihilation, energy is
converted into mass. A single photon of energy is converted into a
particle-antiparticle pair. (This happens to obey the conservation laws)
This can only happen if the photon has enough mass-energy to “pay for the mass”.
Let us image mass and energy as the same thing, if two particles needed 10 “bits” and the photon had 8 bits
there is not enough for pair production to occur.
If two particles needed 10 bits to make and the photon had 16 bits the particle-antiparticle
pair is made and the left over is converted into their kinetic energy.
If pair production occurs in a magnetic field the particle and antiparticle will move in circles of
opposite direction but only if they are charged. (The deflection of charges in magnetic fields
will be covered in Unit 4: Force on a Charged Particle)
Pair production can occur spontaneously but must occur near a nucleus which recoils to help
conserve momentum. It can also be made to happen by colliding particles. At CERN protons are accelerated and
fired into each other. If they have enough kinetic energy when they collide particle-antiparticle pair may be
created from the energy.
The following are examples of the reactions that have occurred:
p p p p p p p p p p p p p pnn
In all we can see that the conservation laws of particle physics are obeyed.
Rutherford Also seen in GCSE Physics 2
Rutherford fired a beam of alpha particles at a thin gold foil. If the atom had no inner structure the alpha
particles would only be deflected by very small angles. Some of the alpha particles were scattered at large
angles by the nuclei of the atoms. From this Rutherford deduced that the atom was mostly empty space with
the majority of the mass situated in the centre. Atoms were made from smaller
particles.
Smaller Scattering
In 1968 Physicists conducted a similar experiment to Rutherford’s but they fired
a beam of high energy electrons at nucleons (protons and neutrons). The results
, they obtained were very similar to Rutherford’s; some of the electrons were deflected by large angles. If the
nucleons had no inner structure the electrons would only be deflected by small angles. These results showed
that protons and neutrons were made of three smaller particles, each with a fractional charge.
Quarks
These smaller particles were named quarks and are thought to be fundamental particles (not made of anything
smaller). There are six different quarks and each one has its own antiparticle.
We need to know about the three below as we will be looking at how larger particles are made from different
combinations of quarks and antiquarks.
Charge Baryon Strangeness Anti Charge Baryon Strangeness
Quark
(Q) Number (B) (S) Quark (Q) Number (B) (S)
d -⅓ +⅓ 0 d̄ +⅓ -⅓ 0
u +⅔ +⅓ 0 ū -⅔ -⅓ 0
s -⅓ +⅓ -1 s̄ +⅓ -⅓ +1
The other three are Charm, Bottom and Top. You will not be asked about these three
Quark Charge Baryon No. Strangeness Charmness Bottomness Topness
d -⅓ +⅓ 0 0 0 0
u +⅔ +⅓ 0 0 0 0
s -⅓ +⅓ -1 0 0 0
c +⅔ +⅓ 0 +1 0 0
b -⅓ +⅓ 0 0 -1 0
t +⅔ +⅓ 0 0 0 +1
The Lone Quark?
Never! Quarks never appear on their own. The energy required to pull two quarks
apart is so massive that it is enough to make two new particles. A quark and an
antiquark are created, another example of pair production.
A particle called a neutral pion is made from an up quark and an antiup quark.
Moving these apart creates another up quark and an antiup quark. We now
have two pairs of quarks.
Trying to separate two quarks made two more quarks.
Particle Classification
Now that we know that quarks are the smallest building blocks we can
separate all other particles into two groups, those made from quarks and
those that aren’t made from quarks.
Hadrons – Heavy and made from smaller particles
Leptons – Light and not made from smaller particles
Made from Smaller Stuff
Hadrons, the Greek for ‘heavy’ are not fundamental particles they are all made from smaller particles, quarks.
The properties of a hadron are due to the combined properties of the quarks that it is made from.
There are two categories of Hadrons: Baryons and Mesons.
Baryons Made from three quarks
Charge Baryon Strangeness Charge Baryon Strangeness
Proton Neutron
(Q) Number (B) (S) (Q) Number (B) (S)
u +⅔ +⅓ 0 d -⅓ +⅓ 0
u +⅔ +⅓ 0 u +⅔ +⅓ 0
d -⅓ +⅓ 0 d -⅓ +⅓ 0
p +1 +1 0 n 0 +1 0
The proton is the only stable hadron, all others eventually decay into a proton.
Mesons Made from a quark and an antiquark
Pion Charge Baryon Strangeness Pion Charge Baryon Strangeness
Plus (Q) Number (B) (S) Minus (Q) Number (B) (S)
u +⅔ +⅓ 0 ū -⅔ -⅓ 0
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