Use these quick notes to help you revise each topic from the Chapter.
1.1 Inside the Atom
NucleonsProtons and Neutrons make up the nucleus. They have a mass of 1 relative to each other but an actual mass of 1.67 x 10-27kg. The proton carries a charge of +1.6 x 10-19C. In certain calculations you must use the actual values.
ElectronsAre negatively charged -1.6 x 10-19C and have a opposite charge to that of a proton.
IsotopesIsotopes are simply elements with more or less neutrons. This means that they can be more unstable than the ones usually found in the periodic table. Often Isotopes appear in small % so effect the relative mass of a sample. Carbon is a good example with many isotopes
- C14 is b- emitter 5730 years T1/2)
- C13 is stable
- C12 is stable
- C11 is b+ emitter (20.3min T1/2)
- C10 is b+ emitter (19.2s T1/2)
IonsOf course ions will be more tricky at A level. Instead of thinking of the removal of an electron as – -1 or the atoms charge becomes +1 to form a +1 ion i.e. Na+.
Now we think about ions as….
- 1.6 x 10-19C -> +1
- 3.2 x 10-19C -> +2
- 4.8 x 10-19C -> +3
Specific ChargeWhen we think about an ion, atom or particle it has what is called a specific charge. This is a simple idea of the charge C/mass kg so should come out in Ckg-1. So the specific charge on a magnesium 24Mg ion is found by adding all nucleon masses which is 24 x 1.67 x 10-27kg. Then the overall ion charge which is 3.2 x 10-19C and dividing to produce a value of 8.04 x 106CKg-1. You don’t add all the charges as they cancel out.
Periodic TableIn the periodic table all atoms have a symbol A = mass or protons + neutrons in relative form i.e. 1+2 = lithium. Z = proton number. You must learn these terms!
Probing the NucleusRutherford did an experiment to investigate the nucleus. He fired alpha radiation at the nucleus and found that most went through and a few returned deflected at 180° or near to. The conclusion was that the nucleus was very small and very positive with a lot of empty space around it
1.2 Stable and unstable nuclei
Strong Force Acts on nucleons only as they contain quarks. It keeps the nucleus stable; short-range attraction to about 3 fm, very-short range repulsion below about 0.5 fm. This balance causes nucleons to be happy at the distance to make a stable atom.
Electrostatic ForceAll charged particles i.e. protons, electrons, positrons, muons etc.. either attract or repel each other. The force gets very large at small separations.
Alpha Decay This is where 4 nucleons (2p &2n) split from the nucleus of an atom to make the atom more stable. Z & N become Z-2 & A-4. Alpha always have the same energy for a particular atom i.e. 5MeV.
Beta (β-) DecayAn atom has a nucleon decay via the weak interaction. A neutron converts to a proton and emits a β– particle (or e-) and an electron anti-neutrino. There is symmetry for charge before and after. The particle and anti-particle share the energy of the decay differently for each emission.
Beta (β+) DecayThis is the same process as Beta (β–) but opposite as a proton converts to a neutron and emits a beta + particle (or e+ positron) and an electron-neutrino. There is symmetry for charge before and after. The particle and anti-particle share the energy of the decay differently for each emission.
Gamma DecayA Gamma ray (γ) is emitted from the atom which has no charge or mass.
NeutrinosThese are produced in the Sun by the weak interaction (β+ or β– decay). They have no charge or mass and are not affected by strong or electromagnetic force. They are fundamental particles. There are three types, or “flavours”, of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Each type also has a corresponding antiparticle, called an antineutrino with an opposite chirality. You will not need the tau for AS Physics.
Electromagnetic WavesThis is energy in the form of waves. The formula to express the speed that an EM wave travels in a vacuum is c = fλ. The speed is always 3.00 x 108ms-1. We often use the suffix of nm for the wavelength visible light i.e. 500nm or 500 x 10-9m
PhotonsElectromagnetic waves are produced when a charged particle such as an electron collides with atomic electrons to make electrons change shells or very fast electrons are slowed down in matter to produce x-rays. Photons have zero mass but carry both energy and momentum. We call these waves or short bursts “photons” or wavepackets. We can think of them like particles or packets of energy containing an energy E = hf (where f is the frequency of the wave, h = the plank constant 6.63 x 10-34Js). Or as c=fλ. E = hc/λ
A simple example would be what is the frequency of a 600nm EM wave….
c/λ = 3.00 x 108 ms-1 / 600 x 10-9m = 5.00 x 1014Hz
What is the Energy of the wave…..
E = hc/λ = (6.63 x 10-34Js x 3.00 x 108 ms-1)/ 600 x 10-9m = 3.32 10-19J or 2.07eV
The Electron Volt The electron volt is a very simple way of expressing a little quantity of energy. It saves us using tricky figures i.e. 3.32 10-19J = 2.07eV. What we do is simply divide the energy in Joules by the value of the charge on the electron or 1 x 10-19JeV-1.
Laser Beams A laser beam is simply a lot of photons all discharged at the same time and in phase with each other. This is a property called coherence. We mean all the ups and down happen at the same time and they are of the same frequency. The power of a laser beam of energy E = hf…… P = nhf where n is the number of photons.
1.4 Particles and antiparticles
Matter & Anti MatterYou should know that for every type of particle, there is a corresponding antiparticle. The positron, the antiproton, the antineutron and the antineutrino are the antiparticles of the electron, the proton, the neutron and the neutrino respectively. They have the same mass but opposite charge (if charged). In the case of hadrons this is because they are made up of anti-quarks. When matter and antimatter meet they annihilate.
PET Scanning“Positron Emission Tomography” is a process where a radioactive tracer is injected into the body. The body then metabolises the isotope at a certain rate. The isotope then decays and producing a position which decays into two gamma ray photons when it hits an adjacent electron. These photons are picked up and mapped to produce 3D images.
Rest Energy Sometimes we talk of the rest energy of a particle. This is the energy it takes to form a particle. The units we usually use to express this are Joules or electron volts. However, we often use MeV or GeV which are 1 x 106eV or 1 x 109eV respectively.
Annihilation In which a particle and a corresponding antiparticle collide and annihilate each other, producing two photons of total momentum and total energy equal to the initial momentum and energy of the particle and antiparticle, including their combined rest energy 2mc2.
Pair Production In which a high-energy photon produces a particle and its antiparticle. This can only occur if the photon energy E= hf = hc/λ is greater than or equal to 2mc2, where m is the mass of the particle, with rest energy mc2 for each particle of the pair produced. More generally, particles are always created in particle–antiparticle pairs. The masses of particles and their antiparticles are identical. This often happens when a photon passes near a nucleus.
Cloud ChambersThese chambers show us charged particles by creating droplets in a saturated gas. The particles can be deflected by a magnetic field and show momentum and charge +/- to an observer. Carl Anderson found the positron by this method.
1.5 How particles interact
The Electromagnetic ForceThe Electromagnetic force acts only between charged particles and is transmitted by the mass-less particle THE PHOTON.
The Strong InteractionActs between the nucleons in atoms (protons and neutrons) and is transmitted by the Gauge Boson called the GLUON. Theory has predicted that there are 8 of them but you don’t need to know this. Can effect mesons (explained later in unit) as they contain quarks as well (explained later in unit)
The Weak InteractionActs over an even shorter range than the strong interaction. It acts on both Leptons and Hadrons and is transmitted by 3 bosons called; W+, W- and Z Bosons. It is often called “decay” as the particles which are effected by it change into something else.
Gravity The gauge boson that transmits the gravitational force is the GRAVITON. This has never as of yet been discovered and is predicted to have zero mass.
W+ & W– Bosons The W Boson is an exchange particle which has a very short life time 10-27s so it does not travel very far.
It operates at a distance of 0.001fm.
W+ & W– Bosons are exchanged during interactions or decays
Feynman DiagramsThe eminent Physicist Richard Feynman invented a graphical method to represent interactions of particles. The only thing that really means anything is the time and direction of the arrows. The angles are not significant. You need to know about several main diagrams for weak interactions AS Physics…
- β+ Decay
- β– Decay
- Electron Capture
- Neutron-neutrino interaction
- Proton-antineutrino interaction
All these interactions obey conservation of charge and mass laws. You should look for symmetry across the diagrams to help you remember them.
Now try the quiz….