Physics Concepts: Photoelectric Effect, Superconductivity, and More
Photoelectric Effect and Superconductivity
Photoelectric effect: The energy needed to remove an electron from a surface. φ is the minimum energy needed for an electron to escape. Light consists of photons, and one photon is absorbed by one electron. An electron can escape if hf > φ. The kinetic energy of an emitted electron is less than hf – φ. Electrons below the surface do work to reach the surface.
Superconductivity: A material has zero resistance. Resistance decreases with temperature until the material becomes superconducting at a critical temperature.
Atomic Energy Levels and Light
Electrons in an atom occupy certain energy levels. The ground state is the lowest energy state an electron can occupy. Electrons collide with orbital electrons, gaining the energy necessary to move to a higher level. Electrons later return to the ground state, losing energy.
Monochromatic Light and Electron Emission
Increasing the frequency of light increases the kinetic energy of released electrons. Increasing the frequency of photons increases their energy. Increasing the intensity of light increases the number of electrons emitted because more photons strike the metal surface. If the energy of the photon is less than the work function, no electrons are emitted.
Particles and Antiparticles
Antiparticles and particles have the same rest mass but different quantum states.
Excited Atoms and Coating Purpose
Metal atoms are excited in a tube. Electrons flow through the tube and collide with metal atoms, raising electrons to higher levels. Photons emitted from metal atoms are in the UV spectrum. These photons are absorbed by a powder, and the powder emits photons in the visible spectrum.
Conservation Laws and Nuclear Forces
Conservation law: Lepton number is conserved. The lepton number before decay equals zero.
Threshold frequency (f0): Below this frequency, no electrons are emitted. The photoelectric effect is not observed because light travels as photons, and the energy of a photon depends on its frequency. Below f0, there is not enough energy to liberate electrons.
Strong nuclear force: The force between two nucleons varies with the separation of the nucleons. It is repulsive to attractive at short ranges (0.1 – 1.0 fm).
Kinetic energy of emitted electrons: The kinetic energy of emitted electrons has a maximum value because the energy from photons is the same, but the energy required to remove electrons varies. Therefore, the kinetic energy of electrons varies.
Electrical Components and Pair Production
Non-ohmic component: Does not have a constant resistance.
A large current is needed to start a car. Internal resistance limits the current.
Pair production: A photon interacts with a nucleus. The energy of the photon is used to create a particle-antiparticle pair. After conversion, the kinetic energy of the photon is zero. The energy of the photon needs to provide at least the rest masses of the electron and positron.
Particle Characteristics and Nuclear Forces
A characteristic of a strange particle is a strange quark and a long half-life.
The strong nuclear force is short-range and has no effect at distances larger than 3 fm. It has no effect on alpha particles outside the nucleus.
Ionization energy (IE): The energy required to remove an electron from an atom in its ground state.
Terminal potential difference (pd): Decreases as current increases. Electromotive force (emf) > V. The potential difference across internal resistance increases with current.
Nuclear Properties and Battery Characteristics
Specific charge of a nucleus: The ratio of charge to mass of the nucleus (Ckg–1).
Rechargeable battery: Has low internal resistance. Internal resistance limits current, so it can provide a higher current.
Electron capture: An atomic electron interacts with a proton in the nucleus (weak interaction) to form a neutron.
A positron, after creation in pair production, meets an electron and annihilates, converting into two or more photons.
Fundamental Particles and Wave Properties
Hadrons and leptons: Leptons are fundamental. Similarities include that all have rest mass, are affected by weak interactions, and experience electromagnetic interactions.
Superposition: Demonstrates the wave properties of electrons.
Filament lamp: A non-ohmic conductor as current is not proportional to voltage.
Light Refraction and Ladder Forces
Light refraction: The paths followed by red and blue rays immediately after light is incident on a glass-liquid interface. Blue light undergoes total internal reflection (TIR), and red light is refracted because the critical angle for red light is greater.
Ladder forces: When someone climbs a ladder, the force from the ground increases, the direction of the resultant force from the ground changes, and the friction force between the ladder and the ground increases.