Summary of "Лекция 5 2 2"
Summary of Lecture 5 2 2
This lecture covers two main topics: the properties of magnetic fluids and the structural and thermal properties of crystals.
1. Properties of Magnetic Fluids
Levitation Effect
- A non-magnetic body placed in a magnetic fluid outside a uniform magnetic field experiences an additional Archimedean force.
- This force pushes non-magnetic bodies (e.g., non-ferrous metals, glass) out of regions with stronger magnetic fields.
- This phenomenon is called levitation.
Viscosity of Magnetic Fluids
- Magnetic fluids have viscosity similar to liquids but differ from suspensions with micron-sized particles.
- The viscosity changes slightly (about 1.3 times) under an applied magnetic field.
- The slight increase in viscosity is due to the alignment of magnetic particles’ axes along the magnetic field, reducing rotational freedom.
Speed of Sound and Compressibility
- Speed of sound decreases nearly linearly with increasing temperature (example given for distilled water).
- Compressibility (β) of magnetic fluids decreases as the concentration of solids increases.
- Elastic properties of magnetic fluids are close to those of their carrier fluids.
2. Crystalline Solids: Structure and Properties
Crystalline Structure
- Most solids, including metals and minerals, have a crystalline structure characterized by long-range order.
- Particles are arranged in a regular pattern that maintains order over long distances.
- Crystals exhibit anisotropy: physical properties (mechanical, thermal, optical) depend on direction.
- Non-direction-dependent bodies are called amorphous solids (e.g., glasses, supercooled liquids).
Crystal Morphology
- Crystals have flat faces intersecting at specific angles.
- Crystals tend to split along certain planes.
- Large single crystals can be grown under controlled conditions.
Crystal Lattice Parameters
- Defined by three vectors (a, b, c) and angles (α, β, γ) forming a parallelepiped unit cell.
- The lattice has translational symmetry (periodicity).
- Other symmetries include rotational axes and mirror planes.
- Only rotational symmetries of order 1, 2, 3, 4, and 6 are possible in crystals.
Crystal Systems (Syngony)
- Ordered by increasing symmetry:
- Triclinic
- Monoclinic
- Orthorhombic
- Tetragonal
- Trigonal
- Hexagonal
- Cubic
Types of Crystal Lattices Based on Particle and Bonding
- Ionic Crystals: Composed of oppositely charged ions (e.g., NaCl). Bonding is ionic (electrostatic attraction).
- Atomic Crystals: Neutral atoms bonded covalently by shared electron pairs (e.g., silicon, germanium).
- Metallic Crystals: Positive metal ions surrounded by a “sea” of delocalized electrons providing cohesion.
- Molecular Crystals: Molecules held together by weak van der Waals forces (e.g., ice, dry ice).
3. Thermal Motion and Heat Capacity of Crystals
Thermal Oscillations
- Particles oscillate around their average lattice positions.
- Oscillation amplitude increases with temperature, leading to thermal expansion due to increased average inter-particle distances.
Heat Capacity
- Near absolute zero, heat capacity depends strongly on temperature (proportional to T³).
- Near room temperature, heat capacity approaches classical Dulong-Petit limit (y = 3RT).
- Classical theory only partially matches experiments.
- Einstein’s theory of heat capacity better fits data by quantizing vibrational energy and assuming independent oscillators.
Lattice Vibrations and Waves
- Strong particle interactions cause vibrational disturbances to propagate as traveling waves in the crystal lattice.
Conclusion
- The lecture concludes the review of Elements of Continuum Mechanics, covering magnetic fluids and crystalline solids.
- Appreciation was expressed to the audience for their attention.
Speakers / Sources
- The lecture appears to be delivered by a single unnamed lecturer.
- No other speakers or external sources are explicitly mentioned.
Category
Educational
