Explain Topic of A2 Physics - A2 物理原理性知识点.md

14 Temperature

1. Density of a liquid: As temperature increases, typically the density of a liquid decreases due to thermal expansion.

2. Volume of a gas at constant pressure: According to Charles' Law, the volume of a gas increases with an increase in temperature when pressure is held constant.

3. Resistance of a metal: The resistance of metals generally increases with temperature because the atomic lattice vibrates more intensely, hindering the flow of electrons.

4. E.M.F. of a thermocouple: A voltage is generated in a circuit of two different metals joined at two junctions at different temperatures.

15 Ideal gases

1. Molecular collisions: Gas molecules move randomly and collide with the walls of their container.

2. Change in momentum: Each collision involves a change in momentum, which exerts a force on the walls.

3. Average force leads to pressure: Although individual collisions vary in force, the average of many collisions over time results in a constant pressure exerted by the gas on the container walls.

17 Oscillations

• In simple harmonic motion (SHM), there is a continuous transformation between kinetic and potential energy.

• At maximum displacement, the kinetic energy is zero and potential energy is at its maximum. At the equilibrium position, potential energy is minimal (or zero), and kinetic energy is at its maximum.

• The total mechanical energy remains constant if there is no damping.

• Light damping: The system slowly loses energy, oscillating with decreasing amplitude.

• Critical damping: The system returns to equilibrium as quickly as possible without oscillating.

• Heavy damping: The system returns to equilibrium slowly without oscillating.

18 Electric fields & Magnetic fields

• Velocity selector devices use perpendicular electric and magnetic fields to filter particles by velocity. Particles with a velocity (where E is the electric field strength and B is the magnetic field strength) will pass through undeflected. Particles with other velocities will be deflected and filtered out.

• Changing magnetic flux induces EMF: According to Faraday's Law, a change in magnetic flux through a loop induces an electromotive force (EMF).

• Lenz's Law: The induced EMF acts in a direction to oppose the change in magnetic flux that produced it.

• Factors affecting induced EMF: These include the rate of change of magnetic flux, the number of turns in the coil, and the area of the coil.

• The Hall effect occurs when a conductor with current flowing perpendicular to an applied magnetic field develops a voltage across it. This is due to the Lorentz force acting on the moving charge carriers, which causes them to accumulate on one side of the conductor, creating a voltage known as the Hall voltage.

• The force between two current-carrying conductors arises due to their magnetic fields interacting.

• If the currents are in the same direction, the conductors attract each other; if the currents are in opposite directions, they repel each other.

• This is due to the Ampère's force acting between the magnetic fields and the currents.

21: Alternating Currents

• Half-Wave Rectification: A single diode allows current to pass only during one half of the AC cycle, effectively "clipping" the negative part of the waveform.

• Full-Wave Rectification (Bridge Rectifier): Uses four diodes arranged in a bridge to convert both halves of the AC input into DC output.

22: Quantum Physics

• Photoelectric emission occurs when photons hitting a material have enough energy to overcome the work function of the material, which is the minimum energy needed to liberate an electron from the surface. If the photon energy exceeds the work function, the excess energy is converted into the kinetic energy of the emitted electron.

• The maximum kinetic energy of the emitted photoelectrons

• depends only on the energy of the incident photons (determined by their frequency)

• independent of the light’s intensity, which just determines the number of photons hitting the surface per unit time.

• Hence, while the maximum energy of each electron remains constant, the total number of electrons and the current increases with intensity.

• Electron diffraction experiments show that when electrons are fired at a crystalline material, they produce a diffraction pattern (similar to that produced by waves passing through a slit).

• This pattern can only be explained if electrons have wave-like properties such as diffraction and interference, thus supporting the wave nature of particles.

23: Nuclear Physics

• Nuclear Fusion: The process where two light atomic nuclei combine to form a heavier nucleus, releasing a large amount of energy.

• Nuclear Fission: The process where a heavy nucleus splits into two lighter nuclei, (along with the emission of neutrons) and a significant amount of energy.

• (often triggered by the absorption of a neutron.)

• Graph Trend: The curve of binding energy per nucleon versus the number of nucleons generally increases up to nucleon number 56, which is the most stable nucleus, and then gradually decreases for heavier nuclei.

• When nucleon number < 56 (fusion): Lighter nuclei have relatively lower binding energy per nucleon. During fusion, when light nuclei combine to form a heavier and more stable nucleus, the binding energy per nucleon increases. That energy is released in the process.

• When nucleon number > 56 (fission): Heavier nuclei have but lower binding energy per nucleon compared. In fission, when a heavy nucleus splits into two or more smaller and more stable nuclei, the resulting new nucleus have a higher binding energy per nucleon than the original nucleus. That energy is released in the process.

24: Medical Physics

• generated:
1. An alternating p.d. is applied across a piezo-electric crystal, causing it to change shape
2. The alternating p.d. causes the crystal to vibrate and produce ultrasound waves

• detected：

1. When the ultrasound wave returns, the crystal vibrates which produces an alternating p.d. across the crystal

2. This received signal can then be processed and used for medical diagnosis

• When ultrasound waves encounter boundaries between different types of tissues (like muscle and bone), some of the waves are reflected back to the transducer.

• By measuring the time it takes for the echoes to return and their Intensity, an image of the internal structure can be created for diagnostic informations.

• X-rays are produced in an X-ray tube where high-energy electrons are accelerated and then hit the metal target. The sudden deceleration of electrons and energy change emitting photon which produces X-rays.

• In PET scanning, a radioactive tracer emits positrons, which almost immediately annihilate upon interact with electrons in the body tissues.

• This annihilation produces two gamma-ray photons that travel in opposite directions. These photons are detected by the scanner to provide images that indicate the tracer distribution within the body, which can be used to diagnose diseases.

25: Astronomy and Cosmology

• Redshift refers to the phenomenon where the light from distant galaxies is observed to be shifted towards longer wavelengths (red end) of the spectrum.

• According to the Doppler effect, this shift occurs because the galaxies are moving away from us.

• leading to the conclusion that the universe is expanding:

• All parts of Universe moving away from each other in every direction.

• the farther away a galaxy is, the faster it appears to be moving away.

• This leads to the Big Bang theory, which predict that the universe started from an extremely dense and close in the past.

• Age of the Universe: