AQA GCE Physics A Unit 1 – Revision notes

18 Pages·2011·0.54 MB·English

by  Joe Gibbins

PHY1 – AQA GCE Physics A Unit 1 – Revision notes
Chapter 1 – Base Physics! 4
Base units:! 4
Derived units:! 4
Prefixes:! 4
Chapter 2 – Particles! 5
The Atom:! 5
Specific charge:! 5
Strong nuclear force:! 5
Unstable nuclei:! 5
Alpha decay:! 5
β– decay:! 5
Photons – particles of light:! 6
Antimatter:! 6
Annihilation:! 6
Pair production:! 7
Quarks:! 7
Hadrons:! 7
Leptons:! 7
Properties of quarks and leptons:! 7
Fundamental forces:! 8
Exchange particles:! 8
Feynman Diagrams:! 9
Weak interaction Feynman diagrams:! 9
Chapter 3 – Quantum Phenomena! 11
Photoelectric effect:! 11
Einstein’s photoelectric equation:! 11
Wave particle duality:! 11
The electron volt:! 12
Line spectra:! 12
Electron energy levels:! 12
page 2 of 18
Fluorescent tubes:! 13
NB:! 13
Chapter 4 – Electricity! 14
Current and charge:! 14
EMF and potential difference:! 14
Circuit rules:! 14
Variation of current and potential difference:! 14
Resistivity:! 15
Resistivity of metals and superconductors:! 15
Superconductors:! 16
Resistors in series and parallel:! 16
Electrical energy and power:! 16
Internal Resistance:! 17
Potential Dividers:! 17
Potentiometer:! 17
Alternating currents:! 17
Root mean square values:! 18
The oscilloscope:! 18
Chapter 1 – Base Physics
Chapter 1 – Base Physics
Base units:
• International system of units (SI system) is based on 7 fundamental units:
Basic Quantity Unit Symbol
Mass Kilogram kg
Length Metre m
Time Second s
Electric current Ampere A
Temperature Kelvin K
Amount of substance Mole mol
Luminous intensity Candela cd
Derived units:
• Some common derived units:
Derived Unit Measures Derivation SI system derivation
Joule (J) Energy or work N·m kg·m2·s-2
Watt (W) Power J/s kg·m2·s-3
Coulomb (C) Electric charge A·s A·s
Volt (V) Ampere W/A kg·m2·s-3·A-1
Ohm (Ω) Electric resistance V/A kg·m2·s-3·A-2
Prefixes:
10-15"10 -12"10 -9"10 -6"10 -3"10 0"10 3"10 6"10 9"10 12
femto" pico" nano" micro" milli" " kilo" mega" giga" terra
F! p! n! μ! m! ! k! M! G! T
4
Chapter 2 – Particles
Chapter 2 – Particles
The Atom:
• Protons and neutrons are referred to as nucleons.
Particle Relative mass Relative Charge Mass / kg Charge / C
Proton 1 +1 +1.67 x 10-16
11..6677 xx 1100--2277
Neutron 1 0 0
Electron 0.00005 -1 9.11 x 10-31 -1.67 x 10-16
• Isotopes are atoms of the same element – so same no. of protons – but with differing no.s of neutrons.
Specific charge:
• Charged particles are deflected in electric or magnetic fields.
• Specific charge = charge / mass
C kg-1" C kg
• Greater the spec. charge of a particle, the greater will be its deflection in electromagnetic fields.
Strong nuclear force:
• Reason nucleus does not break apart is because there is an attractive force between all nucleons holding them
together known as the strong nuclear force.
• ~100 times stronger than electrostatic repulsive force but very short range – less than 3fm.
• Also, when separation of 2 nucleons decreases to less than ~0.5fm, the strong force becomes repulsive, thus the
nucleons are kept at a certain average separation.
Unstable nuclei:
• Small stable nuclei have near enough equal numbers of protons and neutrons.
• However, for larger stable nuclei, there must be more neutrons to protons.
• This is because the strong force is very short range and only attracts nucleons which are nearest neighbours
whereas the electrostatic force has a much larger range and all the protons in the nucleus repel each other. So,
more neutrons are needed to hold the nucleus together.
Alpha decay:
• An α particle is a helium nucleus 42He, so when an unstable nucleus decays by emitting an α particle, it loses 2
protons and 2 neutrons.
• E.g.: 20884Po ➞ 42He + 20482Pb
β– decay:
• The emission of an electron from an unstable nucleus – there are, of course, no electrons in the nucleus of an
atom.
• Therefore, β– decay occurs when a neutron changes into a proton inside the nucleus and an electron is also
formed (to conserve charge)
• n ➞ p + e
• The electron is immediately emitted from the nucleus as a β– particle.
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Chapter 2 – Particles
• It is also found that the range of energies released in β– decay can only be explained if another particle is emitted
in addition to the electron. This particle has no charge and negligible mass (hence very hard to detect). It is called
the antineutrino, ṽ
e.
• So… n ➞ p + e + ṽe
• Following the decay, the nucleus therefore has one fewer neutron and one additional proton – so the nucleon
number does not change, just the proton number gains 1.
Photons – particles of light:
• Some experiments of the 20th century couldn’t be explained by considering electromagnetic radiation as waves.
• Energy of a photon depends on the frequency of the radiation:
• E = hf = hc / f (h is Planks constant, = 6.63 x 10-34)
• Photons have no mass or charge but many aspects of a photon’s behaviour is very similar to that of particles
such as electrons. For example, photos do have momentum.
Antimatter:
• Every particle has its equivalent antiparticle.
• A particle and its antiparticle have the same mass, have the same but opposite charge and spin in the opposite
direction.
• E.g. The electron and the positron both have a rest mass of 9.11 x 10-31kg, so both have a rest energy of 0.511
MeV. The electron has a charge -1.6 x 10-16C and the positron +1.6 x 10-16C.
Annihilation:
• As soon as a particle meets its antiparticle, the 2 destroy each other. The mass of the particles is converted into
energy.
• For example, when an electron and a positron collide, the annihilate producing 2 gamma ray photons of energy;
" "e – + e+ ➞ 2γ
• 2 photons are produced since momentum (as well as charge and mass-energy) must be conserved in the
interaction.
• When sufficient energy is available, the energy released in annihilation can be converted back into matter. This is
how particles are created in accelerator experiments.
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Chapter 2 – Particles
Pair production:
• Pair production is the opposite process to annihilation i.e. the creation of a particle and an antiparticle usually
from the energy of a proton.
• E.g. γ ➞ e– + e+
• This is only possible if the photon has sufficient energy to provide at least the rest mass/energies of the 2
particles i.e. in this case, the gamma ray photon must have an energy exceeding twice the rest energy of an
electron, so greater than 1.022 MeV.
Matter
Hadrons Leptons
Baryons Mesons
Quarks:
• In the past 80 years, experiments using particle accelerators have revealed there are more than 200 subatomic
particles. In 1963, Murray Gell-Mann simplified matters by suggesting that many of the new particles are made
up of different combinations of smaller particles which he called quarks.
• Experimental evidence that quarks exist came just a few years later in 1969.
• There are 6 types of quark – up, down, strange, (top, bottom, charm).
Hadrons:
• Particles which are made up of quarks are called hadrons.
• Hadrons are subject to the strong nuclear force.
• There are two types of hadron:
• Baryons:
• Made up of 3 quarks – q q q, (or 3 x anti-q)
• Protons contain 2 up quarks and 1 down – u u d
• Neutrons contain 1 up and 2 downs – u d d
• Mesons:
• Made up of a quark and an antiquark.
• E.g. Pions:
• π0 = (u, u-) or (d, d-)
• π+ = (u, d-)
• π– = (u-, d)
• Kaons:
• K0 = (d, s-)
• K+ = (u, s-)
• K– = (u-, s)
Leptons:
• Leptons are fundamental particles – they have no internal structure.
• Leptons do not feel the strong nuclear force, they are subject to the weak interaction.
• There are 6 types of lepton, including the electron, the muon, the electron neutrino and the muon neutrino.
Properties of quarks and leptons:
• We know charge and mass-energy are conserved in any interaction. However, some particle interactions never
occur even though these are conserved.
• To explain this, it is found that there are other properties which must be conserved for a particle interaction to
occur.
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Chapter 2 – Particles
• Baryon number:
• All quarks have a baryon no. of +⅓ and all antiquarks of -⅓.
• Therefore:
• All baryons have baryon number +1.
• All anti-baryons have baryon no. -1.
• All mesons have baryon no. 0.
• All leptons have baryon no. 0.
• Baryon number is conserved in all interactions.
• Lepton number:
• All leptons have a lepton no. +1.
• All anti-leptons have lepton no. -1.
• All hadrons have lepton no. 0.
• Can be considered in generations – i.e. electron lepton number, muon lepton number, and so forth.
• In any interaction, lepton number of any generation is conserved.
• Strangeness:
• Particles that contain strange quarks are called strange particles. Strange particles are unusually long lived.
• The s quark has a strangeness of -1, and the s- antiquark has a strangeness of +1.
• So:
• K0 = (d, s-) = has strangeness +1.
• K+ = (u, s-) = has strangeness +1.
• K– = (u-, s) = has strangeness -1.
• Strangeness is conserved in interactions involving the strong nuclear force – in weak interactions,
strangeness can either be conserved or change by ±1
• E.g. K+ ➞ μ+ + vμ this interaction is allowed since the
• Strangeness: +1 0 0 kaon decay is a weak interaction.
Fundamental forces:
• There are 4 fundamental forces or nature:
• Gravitational:
• Weakest of the four, but acts over infinite distances and is the force which holds stars and galaxies
together.
• Electromagnetic:
• Also has an infinite range and is the force which holds atoms and molecules together.
• It is responsible for the chemical, mechanical and electrical properties of matter.
• Strong nuclear:
• Strongest force but has a very short range and so only acts between neighbouring nucleons.
• It binds quarks and antiquarks to hold nucleons together.
• Weak nuclear:
• Even short range.
• Does not cause attraction or repulsion like the other forces, it changes particles from one type to another.
• It is responsible for β decay and for nuclear fusion reactions in the sun.
Exchange particles:
• When an interaction occurs between particles, there is a change in the momentum and the energy of each
particle. We know that mass and energy are interchangeable so we can explain this transfer of energy between
the particles as being caused by them exchanging particles called exchange particles.
• For example, 2 protons repel each other by exchanging virtual photons. The photons are described as virtual
because they exist for such a limited time and space that they cannot be detected.
Force Acts upon Range Relative strength Exchange particle
Strong nuclear Quarks 10-15m 1 Gluons
Electromagnetic Charged particles ∞ 10-2 Photons
W bosons
Weak nuclear Quarks and leptons 10-18m 10-5
(and Z bosons)
Gravity Anything with mass ∞ 10-38 Gravitons (not yet detected)
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Chapter 2 – Particles
Feynman Diagrams:
• Richard Feynman devised a shorthand way of describing particle interactions called Feynman diagrams.
• Particles are represented by straight lines with arrows on. The precise directions of the lines are not significant
and do not show the directions of the particles.
• Exchange particles are represented by wavy lines with no arrows.
• The “time axis” usually points upwards.
• Each point at which lines come together are called vertices. At each vertex, charge, baryon number and lepton
number are all conserved.
• E.g. electromagnetic attraction between a proton and an electron:
p
e
p γ p h oto n e
"
Weak interaction Feynman diagrams:
• β– decay:
• Involves the change of a neutron into a proton in the nucleus – therefore involves the change in character of
a quark (from d to u).
e– e–
p u
(uud)
ṽe ṽe
W– W–
d
n
(udd)
" " " "
• β+ decay:
• Involves the change of a proton in a neutron in the nucleus – therefore the change in character of a quark
(from u to d).
e+ e+
n d
(udd)
ve ve
W+ W+
u
p
(uud)
" " " "
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Chapter 2 – Particles
• Electron capture:
• One of the innermost electrons in an atom travels very close to the nucleus and interacts with a proton by the
weak force.
• e.g. 3718Ar + 0-1e– ➞ 3717Cl + 00ve
• i.e. p + e– ➞ n + ve
• The electron is effectively captured by the proton to form a neutron.
v
e
n
W+ e–
p
" "
• Electron proton collisions:
• The same “electron capture” interaction as above can occur when a proton and an electron collide at very
high speed.
• If the electron has sufficiently high energy then the change can occur as a W– exchange from the electron to
the proton.
n
v
e
W– p
e–
" "
• Neutron neutrino interaction:
e–
p
W+ ve
n
" "
• Proton antineutrino interaction:
e+
n
W+ ṽe
p
" "
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