Particles for OCR A-level Physics
This page covers the following topics:
2. The Photon Model
3. Annihilation and pair production
4. Particle interaction
5. Weak interactions
Every particle has a corresponding antiparticle. An antiparticle is a subatomic particle which has the same mass and rest energy, and the opposite charge as the corresponding particle. Some common antiparticles are given with their corresponding particle in the diagram. The rest energy in MeV of neutrons and antineutrons is 938.3, of protons and antiprotons is 939.6, of electrons and positrons is 0.51 and of neutrinos and antineutrinos is 0.
The Photon model states that electromagnetic radiation is released in energy packets at discrete energy values called photos. The energy of a photon can be calculated using the following equation: Energy = hf = hc/λ, where h is the Planck constant 6.63 × 10⁻³⁴ Js, f is the frequency, c is the speed of light in a vacuum and λ is the wavelength.
Pair production is the process through which a photon is converted into matter and antimatter. Since a particle and its antiparticle are formed, charge is conserved in the reaction. A photon has no mass, therefore the energy of the photon is what will be converted into mass. For this to be possible, the photon must have more energy than the masses of the particles formed in pair production. The minimum energy needed from the photon for pair production to occur is the sum of the rest masses of the particles. The most common pair of particles produced in pair production is an electron and a positron, due to the fact that they have low masses and thus a low energy photon is required. Annihilation occurs when a particle collides with its antiparticle. The particles will disappear and release energy in the form of two photons.
There are four fundamental forces through which particles interact: gravity, the electromagnetic force, the strong nuclear force and the weak nuclear force. All particles with mass attract each other through gravitational forces. This is done through exchanging particles called gravitons. Gravitons have not yet been detected. Electically charged particles will attract or repel each other depending on their charge. The exchange particle responsible for this is the virtual photon, which carries the electromagnetic force of attraction or repulsion between them. The strong nuclear force interaction occurs through exchange particles called gluons, whereas the weak nuclear force one occurs through exchange particles called W bosons.
Atoms will radioactively decay through the weak interaction, which occurs through the exchange particle of W bosons, which come in the forms of W⁻ and W⁺ bosons. β⁻ and β⁺ decay occurs through the weak interaction. In β⁻ decay, a neutron will decay into a proton, an electron and an anti-electron neutrino through the W⁻ boson as the exchange particle. In β⁺ decay, a proton will decay into a neutron, a positron and an electron neutrino through the W⁺ boson as the exchange particle. Two more examples of the weak interaction are electron capture and electron-proton collisions. Electron capture occurs when a proton absorbs an electron and releases a neutron and an electron neutrino through the W⁺ boson exchange particle. Electron-proton collisions also result in the same products of a neutron and an electron neutrino through the W⁻ boson exchange particle.
Draw a table to show the rest energies, in MeV, of an antineutrino, an antiproton, a positron and an antineutrino.
Give two examples of weak interactions which occur through the W⁺ boson.
Two examples of weak interactions are β⁺ decay and electron capture.
State the four exchange particles for the fundamental forces and which one of them has not been detected yet.
The four exchange particles are gravitons, the virtual photon, gluons and W bosons. Gravitons have not been detected yet.
Describe what annihilation is.
Annihilation occurs when a particle collides with its antiparticle, where the particles will disappear and release energy in the form of two photons.
A proton is formed in pair production. State what the second particle formed is and why.
The second particle formed is the antiproton. A particle and its antiparticle are formed in pair production to conserve charge.
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