Chapter 11 Materials ( completed )

Chapter 11 Materials ( completed )

1. 11.1 Density

   • The density of a substance is defined as its mass per unit volume.
   • density = mass / volume
   • 1. how to measure a regular solid's density, first measure its mass using a balance. Next measure its volume by using measurement of its dimensions taken using vernier calipers. Then calculate the density from the mass/ volume
   • 2. how to measure a liquids density, measure the volume of some liquid in a pre-weighed measuring cylinder. Use as much liquid as possible to reduce % error. Measure the mass of the cylinder and liquid to enable the mass of the liquid to be calculated. Calculate the density from mass/ volume
   • 3. Irregular solids, Measure the mass of the object. Immerse the object on a thread in liquid in a measuring cylinder, observe the increase in liquid level. This is the volume of the object. Calculate density from the mass/ volume.

2. 11.2 Springs

   • Hooke's law
   • The force needed to stretch a string is proportional to the extension of the spring.
   • If a spring is stretched beyond its elastic limit, it does not regain it's initial length when the force applied is removed.
   • The area under a force/extension graph is equal to 1/2 F x extension

3. 11.3 Deformation of solids

   • Deformation that stretches an object is TENSILE. Deformation that compresses an object is COMPRESSIVE.
   • Tensile stress and Tensile strain
     • TENSILE STRESS - the tensile force applied to a material per unit area of the material.
     • TENSILE STRAIN - the extension per unit length a material undergoes when under a specific tensile force.
   • YOUNG'S MODULUS
     • E - the elastic limit. Y1 is the yield point, where the wire weakens temporarily.
     • For a wire of uniform diameter d, the area off cross section = pie x diameter ( squared ) / 4
   • The yield point is beyond the elastic limit and it is where the wire weakens temporarily. Beyond the elastic limit it where the material is permanently stretched and suffers plastic deformation. After the yield strength a small increase in the tensile stress = a large increase in tensile strain as the material undergoes plastic flow. Beyond the ultimate tensile strength, the wire loses its strength, extends and becomes narrower at its weakest point. An increase in tensile stress occurs due to the reduced area of cross section at this point until the wire breaks at the 'fracture'. The ultimate tensile stress is called the breaking stress.
     • The stiffness of the material can be compared by using the gradient of the stress - strain line.
     • A brittle material snaps without any significant yield. e.g glass
       • A ductile material can be drawn into a wire. eg copper

4. 11.4 More about stress and strain

   • Permanent extension - when the wire is slightly longer when unloaded because it has been stretched past its elastic limit.
   • A rubber band has a low limit of proportionality which means that it's unloading curve is below the loading curve.
   • For a polythene strip, the extension during unloading is greater than during loading. The strip does not return back to it' s original length when it is completely unloaded. It has a low limit of proportionality and it also suffers plastic deformation.
   • The elastic energy stored in a stretched wire = 1/2 x Tension x extension

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