Unit 4 Physical and Chemical Properties of Materials
Physical properties are tho that can be obrved without changing the identity of the substance. The general properties of matter such as color, density, hardness, are examples of physical properties. Properties that describe how a substance changes into a completely different substance are called chemical properties. Flammability and corrosion/oxidation resistance are examples of chemical properties.
The difference between a physical and chemical property is straightforward until the pha of the material is considered. When a material changes from a solid to a liquid to a vapor it ems like them become a difference substance. However, when a material melts, solidifies, vaporizes, condens or sublimes, only the state of the substance changes. Consider ice, liquid water, and water vapor, they are all simply H高考英语试题及答案2O. Pha is a physical property of matter and matter can exist in four phas: solid, liquid, gas and plasma.
In general, some of the more important physical and chemical properties from an engineering material standpoint include pha transformation temperatures, density, specifi完形填空
c gravity, thermal conductivity, linear coefficient of thermal expansion, electrical conductivity and resistivity, magnetic permeability, and corrosion resistance, and so on.
Pha Transformation Temperatures
When temperature ris and pressure is held constant, a typical substance changes from solid to liquid and then to vapor. Transitions from solid to liquid, from liquid to vapor, from vapor to solid and visa versa are called pha transformations or transitions. Since some substances have veral crystal forms, technically there can be solid to another solid from pha transformation.i词霸
Pha transitions from solid to liquid, and from liquid to vapor absorb heat. The pha transition temperature where a solid changes to a liquid is called the melting point. The temperature at which the vapor pressure of a liquid equals 1 atm (101.3 kPa) is called the boiling point. Some materials, such as many polymers, do not go simply from a solid to a liquid with increasing temperature. Instead, at some temperature below the melting point, they start to lo their crystalline structure but the molecules remain linked in chains, which results in a soft and pliable material. The temperature at which a solid, glassy mate
rial begins to soften and flow is called the glass transition temperature.
Density
Mass can be thinly distributed as in a pillow, or tightly packed as in a block of lead. The space the mass occupies is its volume, and the mass per unit of volume is its density
Mass (m) is a fundamental measure of the amount of matter. Weight 手续费英语(w) is a measure of the force exerted by a mass and this force is produced by the acceleration of gravity. Therefore, on the surface of the earth, the mass of an object is determined by dividing the weight of an object by 9.8 m/s2 (the acceleration of gravity on the surface of the earth). Since we are typically comparing things on the surface of the earth, the weight of an object is commonly ud rather than calculating its mass.
The density (r) of a material depends on the pha it is in and the temperature (the density of liquids and gas is very temperature dependent). Water in the liquid state has a density of 1g/cm3 =1000g/m3 at 4 °C. Ice has a density of 0.917 g/cm3 at 0 °C, and it s
betrothhould be noted that this decrea in density for the solid pha is unusual. For almost all other substances, the density of the solid pha is greater than that of the liquid pha. Water vapor has a density of 0.051 g/cm3.
Some common units ud for expressing density are grams/ cubic centimeter, kilograms/cubic meter, grams/milliliter, grams/liter, pounds for cubic inch and pounds for cubie inch and pounds per cubic foot; but it should be obvious that any unit of mass per any unit of volume can be ud.
Specific Gravity
Specific gravity is the ratio of density of a substance compared to the density of fresh water at 4 °C. At thisjava教育培训中心 temperature the density of water is at its greatest value and equal to 1 g/mL. Since specific gravity is a ratio, so it has no units. An object will float in water if its density is less than the density of water and sink if its density is greater than that of water. Similarly, an object with specific gravity less than 1 will float and tho with a specific gravity greater than one will sink. Specific gravity values for a few common substances ar
e: Au, 19.3; mercury, 13.6; alcohol, 0.7893; benzene, 0.8786. Note that since water has density of 1 g/cm3, the specific gravity is the same as the density of the material measured in g/cm3.
haymitchMagnetic Permeability
Magnetic permeability or simply permeability is the ea with which a material can be magnetized. It is a constant of proportionality that exists between magnetic induction and magnetic field intensity. This constant is equal to approximately 1.257 x10bearded-6 Henry per meter (H/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized μ0.
Materials that cau the lines of flux to move farther apart, resulting in a decrea in magnetic flux density compared with a vacuum, are called diamagnetic. Materials that concentrate magnet flux by a factor of more than one but less than or equal to dunhillten are called paramagnetic; materials that concentrate the flux by a factor of more than ten are called ferromagnetic. The permeability factors of some substances change with rising or f
alling temperature, or with the intensity of the applied magnetic field.
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