materials Flashcards
what is density?
a measure of the compactness of a substance. it relates to the mass of the substance to how much space it takes up. its mass per unit volume
for a solid object to float on a liquid, what does its density need to be?
lower density than the fluid
whats the density of water?
1g cm^-3
or
1000kg m^-3
what is Hooke’s law?
the extension of a stretched wire is proportional to the load of force F
what is the elastic limit?
when the force becomes great enough, this is reached which causes a force-extension graph to plateau, increasing the force past this will lead to a permanent stretch of the material
how long do metals obey Hooke’s law for?
up till the limit of proportionality which is just before the elastic limit
what is the limit of proportionality?
the point beyond which the force is no longer proportional to the extension.
how could you investigate how the extension of an object varies with the force used to extend it?
use a clamp stand and a clamp the object at a measured height
then add weights one at a time to the object
calculate the extension by new length - original length
plot a graph to see result
how can you tell if a deformation is elastic?
the material will return to its original shape once forces are removed, for a metal this occurs as long as Hooke’s law is obeyed
how can you tell if a deformation is plastic?
the material is permanently stretched, occurs for metals stretched past their elastic limit
what is tensile stress?
the force applied divided by the cross sectional area. this causes strain
what are tensile forces?
if the forces stretch a material
what are compressive forces?
if the forces squash the material
what is tensile strain?
the change in length
what is the breaking stress?
when stress becomes so large that atoms separate completely and the material breaks
what is ultimate tensile stress?
the maximum stress that a material can withstand
what is elastic strain energy?
before the elastic limit, all the work done in stretching is stored as potential energy in the material. this stored energy is called elastic strain energy
what does the area under a force-extension graph give you?
elastic strain energy
how is energy conserved in elastic deformation?
work is done while stretching which is stored as elastic strain energy this then gets transformed into other forms eg. kinetic in an elastic band
how is energy conserved in plastic deformations?
work is done to separate atoms
mostly dissipated as heat
how is energy conserved in a vertical spring?
elastic strain energy is stored, when mass is released this transfers to kinetic and gpe. spring then starts to compress and KE is transferred back to elastic strain energy
change in KE= change in potential energy (elastic+gpe)
what is the Youngs modulus?
a measure of how stiff a material is
the Youngs modulus experiment RP4
-set up a bench and clamp a wire fixed at one end with a pulley at the other which you will hang weights off
-test wire should be thin and as long as possible
-first find cross sectional area of the wire using a micrometer for the diameter in several places and take an average x pi r^2
-start with the smallest weight necessary to straighten the wire
-measure the distance between the marker and fixed end of wire- this is the un stretched distance
-then increase weight in 100g intervals recording the marker each time to get extension
-then use results to calculate stress and strain and plot a stress-strain graph
how can you find the Youngs modulus from a stress-strain graph?
its the gradient
what does the area under a stress-strain graph give you?
the strain energy (energy stored) per unit volume
if hooke’s law is obeyed, what should a stress-strain graph look like?
straight line
how do you find the limit of proportionality on a stress-strain graph?
the point where the graph starts to bend
how would you find the elastic point on a stress-strain graph?
after the limit of proportionality, past this the material will no longer be able to go back to its original size and shape
how do you find the yield point on a stress-strain graph?
point just before a sudden drop in stress- material starts to stretch without any extra load
what are force-extension graphs dependant on?
dimensions
what does the area between loading and unloading line on force-extension graph give you?
work done to permanently deform the wire
how is a stress-strain graph different for brittle materials?
it doesn’t curve, just straight lines
straight line till fractures
what is brittle fracture?
when stress is applied to a brittle material any tiny cracks at the materials surface get bigger and bigger until it completely breaks
