Materials (topic 11) Flashcards
Density (definition)
Mass per unit volume
Why are gases much less dense than solids or liquids?
Average separation between molecules is greater
How to measure mass/volume for a regular solid
mass => top pan balance
Volume => dimensions using vernier calipers or micrometer
How to measure mass/volume for a liquid
mass => measure total mass in a cylinder and then take away mass of cylinder
Volume => measure directly from measuring cylinder
How to measure mass/volume for an irregular solid
mass => top pan balance
Volume = use thread to fully emerge in eureka can setup
Density of alloys equation
(mass of A + mass of B)/total volume
How is the force needed to extend an object related to it’s tension
Equal and opposite
What is Hooke’s law?
The force needed to stretch a spring is proportional to the extension of the string
F = k x (delta L)
What is k in F = kL?
Spring/stiffness constant; larger k means stiffer spring
What can Hooke’s Law sometimes be represented as in exam questions?
F = (lambda x delta L) / L
where lambda is the spring modulus
What happens to k when springs are in parallel?
Effective k = k(1) + k(2)
What happens to k when springs are in series?
Effective 1/k = 1/k(1) + 1/k(2)
In terms of energies, what happens when a stretched spring is suddenly released?
Elastic energy (transferred from the work done to stretch the spring) is suddenly transferred to kinetic energy
What does the area under a force/extension graph represent?
Elastic potential energy
Elasticity (definition)
Ability to regain shape after being deformed or distorted
Tensile deformation (definition)
Stretching of an object
Compressive deformation
Compressing of an object
How to test elasticity of materials?
- hold at upper end and load with hanging weights
- use set square to measure extension
- draw tension-extension graph
How to measure extension of strained wire? (Searle’s apparatus)
Micrometer attached to control wire and levelled with test wire. Test wire is loaded to extension so that “spirit level” drops and extension can be measured.
Tensile stress (definition)
Tension force per unit area (N/m^2)
Tensile strain (definition)
Extension per unit of original length (ratio - no units)
What happens between 0 and the yield point on the stress/strain graph for a wire.
Limit of proportionality = stress/strain no longer constant
Elastic limit = wire will no longer return to original shape
Yield point = wire temporarily weakens; local maxima on curve
What happen between the yield point and breaking point on the stress/strain graph of a wire?
Y2 = plastic flow changes graph shape; local minima
Ultimate tensile stress (UTS) = local maxima; wire loses strength and narrows at weakest point
Breaking stress = wire breaks
What is the gradient on a stress/strain curve
stiffness (Young’s modulus)
Brittle (definition)
snaps without noticeable yield
Ductile (definition)
Can be drawn into a wire, large plastic flow before breaking
Strength of material (definition)
The maximum tensile stress it can reach (UTS)
How does the loading/unloading curve look for a METAL WIRE?
Unloading line is parallel to loading line due to permanent extension
How does the loading/unloading curve look for a RUBBER BAND?
Change in length during loading is greater than unloading for any one tension point. Low limit of proportionality.
How does the loading/unloading curve look for a POLYETHENE STRIP?
extension during loading is greater than unloading. low limit of proportionality AND A LOT OF PLASTIC DEFORMATION.
Strain energy (definition)
Work done to deform an object
Strain energy for metal wire equation
1/2(T x extension)
As long as limit of proportionality is not reached
Calculating strain energy for a RUBBER BAND
Area between curves - useful energy when unloading
Total area under curve - work done to stretch
Calculating strain energy for POLYETHENE
Does not regain initial length, hence area between curves represents work done AND internal energy retained
