Bulk Properties of Solids

When it comes to materials, they can do a lot of different things depending on their properties. These properties can include things like how well they conduct electricity, or how much they can stretch before they break. These properties are determined by how the atoms bond together in groups. That's why we call them "material properties."

Bulk properties of matter

When atoms or molecules bond together, their behaviour changes. The mechanical characteristics, which arise from how molecules or atoms respond to forces or energy exchanges when bonded, are known as bulk properties. One example of mechanical properties in solids is when you extend a rubber band or scratch a diamond.

A rubber band consists of molecules forming long chains known as polymers. The chains are entangled and disordered, as shown in the figure below.

*The long chains, known as polymers, which make up a rubber band, are entangled when no force acts to make the band tense*

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When stretched, the chains expand, allowing the band to increase its length without breaking. In this case, the force applied untangles the chains, as shown in the next figure.

When the rubber band is made tense, the chains that make up the rubber elongate and reorder

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The chains can resist a certain amount of force without breaking or deforming. If the force applied to the band is small enough to not deform it permanently, it will go back to its initial length.

Here is another example:

Carbon atoms have four electrons in their outer shell that can bond with other elements. When carbon atoms bond together in a diamond, they share these electrons with each other. Under high pressure and heat, the carbon atoms are compressed to form a solid crystal structure. This structure results in a very hard diamond that does not react with acids due to its lack of free electrons. Some important properties of solids include hardness, electrical conductivity, thermal conductivity, elasticity, density, and tensile strength.

Bulk properties of matter in mechanics

Bulk properties of materials can tell us a lot about how they will behave under pressure and force. For example, hardness and elasticity are important mechanical properties to consider. We need to know how materials will respond to different loads and forces. Some materials, like the frames of glasses, need to be flexible so they don't break under pressure. Other important properties include density, tensile strength, and bulk modulus.

These properties are partly determined by the chemical bonds between particles. Metals have metallic bonds, polymers usually have covalent bonds, and ceramics have ionic bonds. These bonds not only affect the mechanical properties of the material but also its structure. You can think of bonds like springs between atoms. Metallic bonds are stiffer than the bonds in a rubber band.

The elasticity of a material can be represented using the analogy of springs between the elements that make up the material. Each material features springs with different stiffness

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Molecules in a rubber band are made of long entangled chains. When they are pulled by a force, their arrangement changes and they elongate.

When a rubber band tenses, its molecules untangle, allowing the band to expand

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The structure of diamonds and metals is very different from that of a rubber band, as they have a tight, linearly packed structure that does not allow for much elongation. The type of bond also plays a role in other bulk properties, such as the melting point. Materials with ionic, covalent, or metallic bonds tend to have higher melting points, while materials with weaker bonds, such as hydrogen bonds or van der Waals' forces, have lower melting points. This is because stronger bonds require more energy to be broken, so they have a higher melting point. Understanding the type of bond in a material can tell us a lot about its properties and behavior under different conditions.

Elasticity

Elasticity is a crucial property of materials in both scientific and industrial applications. It refers to the ability of a material to stretch when a force is applied to it and then return to its original shape when the force is removed. Elastic materials are used in many different applications, such as in the production of airbags. During a crash, the airbag must be able to withstand powerful impacts and rapid deformation while still retaining its shape and structure. Materials with high elasticity are essential to ensuring that the airbags can perform their life-saving function in these critical situations. Understanding the elasticity of materials can lead to the development of improved products and technologies that are safer, more efficient, and more effective.

The elasticity of a material is measured in Pascals. To perform the measurement, a mechanical test is needed, which is usually conducted by elongating a material using a force ‘F’, as shown in the image below.

Material properties, such as elasticity, can be measured by conducting mechanical tests, in which objects are exposed to forces that elongate them. The elongation causes a deformation δl by a force F

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When a material is subjected to a force, it experiences what is known as mechanical stress, which causes it to change in length. However, if the force is not too strong, the material can revert back to its original length. This is known as elasticity, and the more force a material can resist without deforming, the more elastic it is. An important property of elastic materials is Young's modulus, which is measured in Pascals. Young's modulus is a measure of the proportion between the strain or deformation of a material and the mechanical stress it is subjected to. The relationship between stress and strain can be visualized on a stress versus strain graph. This graph shows how much a material can stretch before it reaches its elastic limit, beyond which it begins to deform permanently. Understanding the behavior of materials under stress and strain is essential for designing and engineering products that can withstand a variety of forces and loads.

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A material in a tensile test presents a deformation (strain), which depends on the force applied (mechanical stress). The resulting slope is equal to Young’s modulus

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The stress versus strain graph shows that in the elastic zone, the stress and strain are proportional to each other and the material will revert to its original shape after being deformed. The slope of the red line in the elastic zone is known as the elastic modulus or Young's modulus, which is different for different materials. To calculate the Young's modulus from a stress versus strain plot, you need to calculate the slope of the curve in the elastic zone using the formula: Young's modulus = (stress/strain).

Let's use an example to illustrate how to calculate the Young's modulus. Suppose we have a metal bar with an area of 4.2 x 10^-8 square meters and an initial length of 2 meters. The bar is subjected to a tension test, and the data collected is shown in the table below:

Force (N) Extension (mm)
0 0
10 0.365
20 0.732
29 1.454
2.9N is the maximum force that can be applied before the material enters the plastic zone. Therefore, we can use the data from 0 to 2.9N to calculate the Young's modulus.

For F = 0, the stress is 0, and the strain is also 0.

For F = 2.9N, the stress is force/area = 2.9N/(4.2 x 10^-8 m^2) = 69047619.

The strain is the change in length/original length = (1.454 - 0)/2 = 0.727.

Therefore, the Young's modulus is (69047619/0.727) = 9.5 x 10^10 Pa.

Beyond the elastic zone, the material enters the plastic zone, where it starts to deform irreversibly as the force keeps pulling it. This is indicated by the blue line in the stress versus strain graph. Hooke's law can be used to model the elastic zone of a material, where the force is proportional to the elongation and the spring constant.

zone of a material can be modelled using Hooke’s law. In this case, an analogy is useful. The force applied to the material is analogous to a weight pulled by gravity, and the material elasticity is a spring deformed by the weight.

In this case, the force is proportional to the elongation and the spring constant k.

When materials are elongated beyond their elastic zone, they enter their plastic zone, which indicates the area in which materials start to deform irreversibly as the force keeps pulling them. See the blue line in the following image.

A material in a tensile test presents a deformation (strain) that depends on the force applied (mechanical stress)

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In summary, bulk properties of solids are mechanical properties that arise from the atoms or molecules acting together after bonding. Some important bulk properties include elasticity, tensile strength, density, hardness, electrical conductivity, and thermal conductivity. Elasticity and hardness respectively describe how bodies can be deformed and revert back to their original shape (elasticity) and how easy it is to penetrate or scratch them (hardness). The elastic limit is the point at which a material starts to deform so that it can no longer revert back to its original shape. When the material leaves its elastic zone, it enters its plastic zone, where it deforms irreversibly. Young's modulus describes the ratio of mechanical stress and the deformation of the material. Density is another important property of materials, which is defined as the amount of mass existing in a unit of volume. The density of a material varies depending on its atomic structure and the element of which it consists. Materials with low elasticity, such as metals and diamonds, are not elastic, as the strength of their bonds and structures resists penetration by a force trying to push them. Understanding these bulk properties is critical for designing and engineering materials that can withstand specific forces and loads.