Physics 2 PDF Study Notes

## Structure of Solids - **Amorphous, crystalline and liquid crystalline states.** - **Mechanical deformations** - **Hooke's law** - **Strength and hardness of materials** - Scale of Mohs - Method of Brinell - **Amorphous solids:** Solid that lacks crystalline structure, e.g. glass. Int...

## Structure of Solids - **Amorphous, crystalline and liquid crystalline states.** - **Mechanical deformations** - **Hooke's law** - **Strength and hardness of materials** - Scale of Mohs - Method of Brinell - **Amorphous solids:** Solid that lacks crystalline structure, e.g. glass. Internal structure made of structural blocks - **Crystalline solids:** Solid where atoms, molecules or ions are highly ordered forming a crystal lattice in all directions. - **Liquid crystalline states:** Liquids with highly ordered molecules, orientated in a crystalline way. - Solids is one of the three common phases: Solid, liquid and gas. A solid maintains a fixed shape and a fixed size; even if a large force is applied to a solid, it does not readily change in shape or volume. - The constituent of solids can be arranged in two ways: - In a repeated three-dimensional pattern called crystal lattice producing a crystalline solid. - They can aggregate in no specific order called an amorphous solid. - **Mechanical deformations** from contraction and stretching. In stretching the elastic forces act in perpendicular direction to the cross section. - **Deformations, Hooke's law:** Deformation is change of shape due to force, can be compression, tensility (both linear deformations), shear, torsion or bending. Solids are elastic or plastic. Hooke's law states stress is proportional to relative strain: - Stress (sigma unit Pascal, pa) = E x change in length/length, - E = young's modulus (measured in Pa) - All solids have a limit of proportionality where the strain will increase within Hooke's law, the elastic limit, the point at which deformation is elastic, the yield point, where there is an increase in strain and stress, and the breaking point. - Hooke's law is only a first-order linear approximation to the real response of springs and other elastic bodies to applied forces. It must eventually fail once the forces exceed some limit, since no material can be compressed beyond a certain minimum size, or stretched beyond a maximum size, without some permanent deformation or change of state. Hooke's law is an accurate approximation for most solid bodies, as long as the forces and deformations are small enough. - According to Hooke's law, the relative stress is proportional to the deformation. T = Εε ## Brinell Hardness Test - **The Brinell test** was one of the most used hardware tests during World War II. It tests the strength of a material by forcing a diameter of it into a surface and measuring the indentation. ### Method of Brinell: - Characterizes indentation hardness, another measure of hardness of materials. A 10mm steel ball is fired at a material at 3,00 kgf force, the indent is then measured. This is then used to calculate the hardness of the substance. ### Strength and hardness of materials, scale of Mohs: - **hardness** = resistance to plastic (permanent) deformation, - **strength** = measure of elastic and plastic range (can be compressive, shear or tensile strength). - **Mohs scale** is qualitative and measures scratch resistance of minerals, one of the measures of hardness of materials. It categorises based on what can visibly scratch what, if a can scratch b, a is harder. ## Structure of liquids - **Molecular structure of bodies.** - **Surface tension of liquids, coefficient of surface tension, molecular pressure.** - Trickling. Wetting, non-wetting, meniscus, capillary phenomena. La Place pressure. Gas embolism. Water - structure, properties and importance for living organisms. - **Molecular structure of bodies:** High density, viscosity, low compressibility. Molecular structure unordered like amorphous solids, properties the same in all directions. - The total of the forces acting towards the interior of a liquid is called surface tension, it is what is causing water to appear in drops. The tension arises from the attractive forces between the molecules. The force is defined as follows: y = F/L (N/m) - Temperature has a considerable effect on surface tension. - The surface tension makes it possible for objects to float if their weight is less than the buoyant force. Soaps and detergents lower the surface tension of water and enables it to penetrate fibers of materials. - In tubes with small diameters, liquids are observed to rise or fall relative to the level of the surrounding liquid. This phenomenon is called capillarity and the thin tubes are called capillaries. - **Surface tension of liquids, coefficient of surface tension, molecular pressure:** - Surface tension is the force acting toward the interior of the liquid, attempting to reduce the free surface area. The coefficient of surface tension is the force per meter (N/m) acting perpendicular on the surface. It can be defined as: - **Surface tension (N/m) = Force/length of liquid area (m)** - Relating to molecular pressure, this will be evenly distributed to every surface of the container, increasing with depth. If the liquid is at rest, we can define depth as: - **Pressure = density x gravitational acceleration x depth** - Assuming pressure at free surface = 0 - **Trickling:** is formed from drops of liquid, the formation of the drop can be calculated as: - **Force of gravity = mass x acceleration of g = density x volume x acceleration of g** - **Volume drop = 2 x pi x coefficient surface area x radius/density x acceleration of g** - Whether a liquid rises or falls depends on the relative strengths of the adhesive and cohesive forces. The amount of rise and fall depends on the surface tension. - The coefficient of surface tension is given in the formula: σ = δΑ/dS, δA = work - With the recorded forces of surface tension, it is possible to decide the weight of a drop passed out through a pipette. The drop detaches when the surface tension is equal to the gravity of the drop. - **V = 2.σ.π.r/p.g, m = σ.l/g** - The coefficient of surface tension depends on: - Type of liquid - Temperature - Impurities - With increasing temperature, the forces of surface tension decrease as can be seen in the case of boiling water. - Compounds that reduces surface tension are called positive surface active substances and compounds that increase surface tension are called negative surface active substances. ## Wetting - Liquids can be wetting and nonwetting, where the angle between the surface and the tangent of the liquid surface is less than 90 it is a wetting liquid. - **Capillary phenomena:** The change in level of liquids in narrow tube caused by surface tension is called capillary. The free surface of the liquid in capillaries is called meniscus. The method of determining surface tension is by measuring the change in pressure caused by distortion of the surface of a liquid. - It can be calculated in the law of Laplace: **P = 2.0/r** - A manometer can be used to record the change in pressure when it rises to Pmax where the bubble overcomes the additional pressure p and breaks. - **Pressure = 2. surface tension coefficient/radius** - **σ = r.p.g.H/2 = k.H, k =r.p.g/2** - p = density of liquid, g = acceleration of gravity, k = constant of the instrument, r = radius of the drop (capillary tube), - **Δρ = H.g.ρ = 2.σ/г** - **Meniscus:** This is the crescent curve in the upper surface of a liquid caused by surface tension. - **Gas embolism:** Is an obstruction caused by an air blot. When the liquid is stationary the internal pressures of the menisci on either side are equal, when the liquid is moving the pressure of the menisci on the side behind the flow of liquid is greater. - Water is composed of two oxygen and one hydrogen molecules, bounded by covalent bonds. It can act as a base or acid and is important for the human organism's survival in terms of metabolism and gas exchange.