Lesson_Text_4.6 - 1 Lesson 4.6 Properties of Matter 1...

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1 Lesson 4.6 Properties of Matter 1. Elastic Properties of Materials. When a material is subjected to an external force, it either gets stretched, compressed or simply changes it shape. In other words, an external force produces deformation on an object. When the external force is removed, if the object regains its original dimensions and shape, we say that the object is elastic. Elasticity is the ability of an object to regain its original shape after being deformed by an external force . Steel is more elastic than rubber because steel can withstand large amount of external force before being permanently deformed. Deforming force per unit area is called stress F stress A = Stress is measure in Nm -2 As a result of stress, an object can change its length, volume or shape. A stress that produces a change in length is called a tensile stress . A stress that produces a change in volume is called compressional stress or volume stress . A stress that simply deforms the object at a certain angle is called the shear stress . When an object is subjected to a stress, it suffers a strain . The ratio of change in length to original length is called tensile strain. If L is the increase in length for an original length L, then: L Tensile strain L = The ratio of the change in volume ( v) to the original volume (v) is called the bulk strain.
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2 v Bulk strain v -∆ = Bulk strain is also called volume strain. We use the negative sign here because a bulk stress causes a decrease in volume. When the deforming force is removed, if the object does not regain its original shape, the object is said to gave crossed the elastic limit . If the object is within the elastic limit, it will return to its original shape and dimension when the deforming force is removed. The law of elasticity is valid only within elastic limit of an object. Within elastic limit, the ratio of stress to strain will remain a constant, and this ratio is called the elastic modulus of the material. stress Elastic modulus = strain Since strain has no unit, the unit of elastic modulus Nm -2 , the unit of stress. (i) Young’s Modulus. The ratio of tensile stress to tensile strain is called the Young’s modulus of the material. We represent this by the letter Y. ( / ) ( / ) Tensile stress F A Y Tensile strain L L = = ….(i) The unit of Young’s modulus is N.m -2 . Equation (i) above can be solved for L as 1 FL L A Y = This means that the increase in length of a rod or wire under tensile stress is inversely proportional to the young’s modulus of the material.
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3 (ii) Bulk Modulus The ratio of volume stress to volume strain is called the bulk modulus of the material. We use B to represent the bulk modulus. ( / ) ( / ) Volume stress F A B Volume strain V V = = -∆ Just as in the case of Young’s modulus, it can be shown that the change in volume V is inversely proportional to the bulk modulus B of the material.
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