4.2 materials

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Cards (11)

Density
density = mass / volume
Hooke’s law
Force is directly proportional to the extension of the spring
f = kx
spring constant, k
The measure of the stiffness of a spring
Series 1/k = 1/k1 + 1/k2
Each spring is extended by the same length
Parallel k = k1 + k2
Each spring takes the weight so the coils add up
Tensile stress
All springs have a surface area
When a load is attached to a spring it experiences pressure - stress (pascals)
The ultimate tensile stress is the maximum force per original cross-sectional area a wire can support until it breaks
stress = force / area
Tensile strain
The deformation of a solid due to stress in the form of elongation or contraction
Dimensionless unit - the ratio of lengths
strain = extension / original length
Elastic strain energy
E=1/2 Fx = $E=$ $1/2kx^2$
Energy stored = area under force-extension graph
Regions that obey hooke’s law - work done is the area of a right angled triangle under the graph
Regions that dont obey hooke’s law - the area is the full region under the graph
Elastic limit - the maximum length a material can be stretched and still return to its original length

Breaking stress
The maximum stress a material can withstand before fracturing
Ductile - a material with high breaking stress
Can extend more before breaking because of plastic deformation
E.g. copper - ductile and good electrical conductor
Ultimate tensile stress (UTS) - the maximum stress a material can withstand
Plastic behaviour
Elastic deformation
When the load is removed the object will return to its original shape
Occurs in the elastic region of the graph - extension is proportional to the force applied to the material
Plastic deformation
The material is permanently deformed
When the load is removed the object will not return to its original shape iorr length
Beyond the elastic limit and occurs in the plastic region of the graph - extension is no longer proportional to the force applied to the material
Brittle - material breaks with little elastic and insignificant plastic deformation
Have very little to no plastic region
E.g. glass, concrete
Ductile - material stretches into more shape before breaking
Have larger plastic region
E.g. rubber, copper
young modulus =stress / strain E= Fl / Ax
The measure of a material’s ability to withstand changes in length with an added load
How stiff a material is
Informs about the elasticity of a material
Defined as the ratio of stress and strain
Measured in pascals
Core practical
method
Measure the original length of the wire using a metre ruler and mark this reference point with tape
Measure the diameter of the wire with a micrometre screw gauge or digital callipers
Measure or record the mass or weight used for the extension
Record the initial reading on the ruler where the reference point is
Add mass and record the new scale reading from the metre ruler
Record the final reading from the new position of the reference point on the ruler
Add another mass and repeat the method
Improving experiments and reducing uncertainties
Reduce uncertainty of the cross-sectional area by measuring the diameter in several places along the wire and calculating an average
Remove the load and check wire returns to the original limit after each reading
Take several readings with different loads and find the average
Use a vernier scale to measure the extension of the wire
Energy conservation
When a material is stretched work has to be done
If deformation is elastic all work done is stored as elastic strain energy
When the stretching force is removed this energy is transferred to other forms
If deformation is plastic work is done to separate atoms and energy isn’t stored as strain energy - mostly dissipated as heat
Transport design - crumple zones designed to deform plastically in a crash with energy going into changing the shape of the vehicle