OCR A-level Physics Notes (New Spec) Full & Revised with Complete Solutions for Final Exam.

OCR A-level Physics Notes (New Spec) Full & Revised with Complete Solutions for Final Exam. Particles and Radioactivity Alpha Particle Scattering Experiment Plum Pudding model of the atom ● Atoms made up of a large mass of positive matter with a few negatively charged electrons spread within it Rutherford’s experiment disproved the Plum Pudding model Rutherford's alpha scattering experiment: ● Stream of alpha particles from a radioactive source was fired at a thin gold foil (only a few atoms thick) ● The alpha particles were scattered by the foil and detected on a screen, which produced a speck of light Conclusions: ● Most of the alpha particles went straight through the foil ○ So the atom is mostly empty space ● Some alpha particles were deflected through large angles, so the centre of the atom must have a large positive charge to repel them (the nucleus) ● Some particles were reflected angles larger than 90 degrees, so the nucleus must be very small and has a high mass The nucleus Proton number = number of protons in the nucleus - atomic number, Z Nucleon number = the number of protons and neutrons - the mass number, A Isotopes have the same proton number but a different number of neutrons. They will also have the same charge and the same chemical properties. The number of neutrons affects the stability of the nucleus, unstable nuclei may be radioactive Nuclear radius is proportional to the cube root of the nucleon number ● Diameter of an atom is 0.1nm and the diameter of a nucleus is around 1fm The nucleus of an atom is very small, massive and is therefore very dense ● Nuclear density is higher than atomic density The strong nuclear force The strong nuclear force is an attractive force that overcomes the electrostatic repulsive force between the positive charges of protons ● Has a short range, can only hold nucleons together when they are separated y a few femtometers ● The strength of the strong nuclear force falls quickly beyond this distance ● The strong nuclear force works equally between all nucleons ● At very small separations, the strong nuclear force must be repulsive otherwise it would crush the nucleus to a single point 1) Strong nuclear force is repulsive for very small separations (below 0.5fm) 2) After 0.5fm, the force becomes attractive and reaches a maximum, and then falls to zero 3) After around 3fm, it can no longer hold the nucleons together Fundamental forces: ● Strong nuclear ○ Experienced by nucleons ○ Range around 1fm ● Electromagnetic ○ Experiences by static and moving charged particles ○ Infinite range ● Weak nuclear ○ Responsible for beta decay ○ Range around 10^-18 ● Gravitational ○ Experienced by all particles with mass ○ Infinite range Fundamental Particles: A fundamental particle is a particle that cannot be divided into smaller parts. Quarks, electrons and neutrinos are all fundamental particles Hadrons are particles that are affected by the strong nuclear force ● They aren’t fundamental particles ● Made of quarks ● Protons and neutrons are hadrons ● The proton is the only stable hadron ● Most hadrons will decay into other particles ● Experience gravitational force Leptons aren’t affected by the strong nuclear force: ● Interact with other particles using the weak nuclear force and gravitational force (and electromagnetic force is they are charged) ● 2 types; electrons and neutrinos ● Neutrinos have zero mass, zero charge Antiparticles ● Each particle has a corresponding antiparticle with the opposite charge and same mass Particle symbol charge antiparticle symbol charge proton 1 antiproton -1 neutron 0 antineutron 0 electron -1 positron 1 neutrino 0 antineutrino 0 Quarks make up hadrons, antiparticles of hadrons are made of antiquarks Quark symbol charge antiquark symbol charge up +2/3 anti-up -2/3 down -1/3 anti-down +1/3 strange -1/3 anti-strange +1/3 Protons and neutrons are a type of hadron called a baryon, which is made of three quarks, hadrons made up of a quark and antiquark are called mesons Proton = uud neutron = udd The weak nuclear force can change the quark type ● Hadrons Decay into other particles via the weak nuclear force ● In beta-minus decay, a neutron is changed into a proton ○ A down quark turns into an up quark, an electron and an antineutrino ● In beta plus decay, a proton changes into a neutron ○ Up quark changes to a down quark, a positron and a neutrino ● Charge is always conserved Individual quarks are bound together by the strong nuclear force Radioactive decay If a nucleus is unstable, it will break down to become more stable This could be because: ● Too many neutrons ● Too many nucleons in total (too heavy) ● Too few neutrons (too many protons) ● Too much energy in the nucleus The nucleus decays by releasing energy and particles until it reaches a stable form Radioactive decay is spontaneous and random ● Random ○ Cannot predict when a nucleus will decay ○ Each nucleus has the same probability of decaying per unit time ● Spontaneous ○ Not affected by the presence of other nuclei ○ Not affected by external factors like pressure or temperature Although the decay of an individual nucleus cannot be predicted, with a large number of nuclei, their behaviour shows a pattern A cloud chamber can be used to detect the presence of these types of radiation Types of nuclear radiation: ● Alpha ○ A helium nucleus ○ 2 protons and 2 neutron ○ Highly ionising due to their larger mass ○ Can be stopped by a few cm of air or paper ● Beta ○ Fast moving electrons ○ Charge e ○ A few mm of aluminium or 1m of air can stop it ● Gamma ○ High energy photons of EM radiation ○ No charge ○ Travel at c ○ Can be stopped by a few cm of lead ○ Least ionising Charged parallel plates can be used to distinguish between the different types of radiation The nucleus before decay is the parent nucleus, after decay it is called a daughter nucleus. The half life of an isotope is the average time it takes for the number of radioactive nuclei to halve When calculating half life, measure several times and find an average The activity of a source is the rate at which nuclei decay, and it is much higher than the observed count rate ● Measured in Bq ● Depends on ○ Half life of isotope ○ The number of undecayed nuclei present Decay constant is the probability of decay of an individual nucleus per unit time Determining half life in reality is easy; measure the number of nuclei using a mass spectrometer, and measure the activity using a geiger-muller tube The number of undecayed nuclei decreases exponentially with time By taking logs of the exponential graph, the half life can be determined from -ln2/gradient Carbon Dating ● Organisms take in carbon when alive ● They stop taking in carbon when they die ● The ratio of carbon-14 to carbon-12 nuclei for the relic sample is determined ● The current ratio of carbon-14 to carbon-12 nuclei is determined ● The age of the relic is found using the exponential equation ● Other elements with a longer half life than carbon can be used to date ancient rocks ● Limitation: ○ The ratio of carbon-14 to carbon-12 is assumed to be constant ○ Count-rate from relic may be comparable to background count Decay Alpha Decay ● Helium nucleus released ● Mass number - 4 ● Atomic number - 2 Beta Decay ● Beta Minus decay ○ Caused by weak nuclear force ○ Too many neutrons for stability ○ Neutron turns into a proton and an electron, and an anti-electron neutrino is released ○ QUARKS; udd to uud ● Beta Plus decay ○ Caused by weak nuclear force ○ Too many protons for stability ○ Proton turns into a neutron and a positron, and an electron neutrino is released ○ QUARKS uud to udd Einstein’s mass-energy equation 2 interpretations 1) Mass is a form of energy a) Illustrated by pair production where annihilation causes the entire mass of the particles to be transformed into 2 gamma photons 2) Energy has mass a) A moving object has kinetic energy, implying it has more mass than its est mass.