Chapter 11 - Materials Flashcards
What is the definition of the density of a substance?
Mass per unit volume
What is the equation for density? Give the units.
density = mass/volume
Kgm^-3
How could you measure the density of a regular solid?
- ) Measure its mass using a balance
- ) Measure its dimensions using a vernier calipers or a micrometer and calculate its volume using the appropriate equation.
- ) Then calculate density by density =mass/volume
How could you measure the density of a liquid?
- ) Measure the mass using a volume of an empty measuring cylinder and measure the volume of the liquid directly. (use as much liquid as possible to reduce percentage error)
- ) measure mass of the cylinder and liquid to enable the mass of the liquid to be calculated.
- ) measure density from mass/volume
How could you measure the density of an irregular solid?
- ) measure the mass of the object
- ) immerse the object on a thread in liquid in a eureka can, collect volume of water dispersed in a measuring cylinder - this is the volume of the object.
- ) calculate density of the object from it’s mass/volume
What is an alloy?
Solid mixture of two or more metals
How do you work out the density of an alloy?
- Find mass of each metal in alloy using mass = volume x density
- use equation density = mass/volume
What is Hooke’s law?
Hooke’s law states that the force needed to stretch a spring is directly proportional to the extension of the spring from its natural length - up to the limit of proportionality
F = k x e force = spring constant x extension
What is the elastic limit?
Point where spring does not return to original length after force applied is removed
Whats the equation for work done to stretch a spring?
= 1/2 x change in length x force
Using equation for work done to stretch a spring, find the equation for elastic potential energy stored in a stretched spring?
F = ke
so, Ep = 1/2 x k x e^2
What is the limit of proportionality?
Point to which a material obeys Hooke’s law
What is elastic deformation?
When a material to regains it’s shape after it has been deformed and forces are released.
What is plastic deformation?
When a material is permanently stretched after forces are removed. Work is done to separate atoms
What are the forces that stretch a material called?
Tensile forces
What are the forces that squash a material called?
Compressive forces
What is tensile stress? give the equation the equation and give the units.
Force applied divided by the cross sectional area
stress = force/area
units - Nm^-2 or Pa
What is tensile strain? Give the equation and the units
Change in length (extension) divided by the original length of the material. strain caused by stress.
strain = extension/ length
No units as it’s a ratio
What is breaking stress?
The stress at which the atoms in the material are separated completely, and the material breaks
What is the ultimate tensile stress?
Maximum stress a material can withstand
Describe what happens, in terms of energy transfers, when a spring is stretched vertically down then released?
Elastic strain energy is stored in the spring as it’s stretched down. When released the elastic strain energy is transferred to kinetic energy and gravitational potential energy (as it gains height). The spring then begins to compress and the kinetic energy is transferred back to stored elastic strain energy.
change in kinetic energy = change in potential energy
How is plastic deformation ideas used in transport design?
Dissipation of energy is used to design safer vehicles. Crumple zones deform plastically in crashes - so some of the cars kinetic energy goes into changing the shape of the vehicles’s body, so less is transferred to the people inside
What is young modulus?
What are the units?
Measure of how stiff a a material is.
Young modulus = tensile stress/ tensile strain
= F/A x L/E
Units - Nm^-2
How do you find young modulus from a stress - strain graph?
Gradient - stress/strain
What is the area under a a stress - strain graph?
Strain energy per unit volume
What is the yield point on a stress- strain graph?
Point at which a material suddenly starts to stretch without any extra load
What is the difference between force-extension graphs and stress-strain graphs?
Force-extension graphs depend on the dimensions of material.
Stress-strain graphs describe the general behaviour of a material, because stress and strain are independent of the dimensions.
Describe the stress-strain graph of a brittle material
Doesn’t curve.
Straight until it reaches a point where the material fractures
What would a typical stress-strain graph look like?
Give some examples of brittle materials?
Ceramics or chocolate bars
What is a brittle fracture?
When stress is applied to a brittle material any cracks at the material’s surface get bigger and bigger until the material breaks completely.
Why are brittle materials brittle?
As they have a rigid structure that allow cracks to be able to grow.
Describe an experiment that tests young modulus
- ) Find the cross-sectional area of the wire. Use a micrometer and measure the diameter of the wire in several places. Then use formula for area of a circle for finding cross sectional area.
- ) Clamp the wire to the bench so you can hang weights off one end
- ) Measure the distance between the fixed end of the wire and the marker - this is the un-stretched length
- ) Then if you increase the weight, the wire stretches and the marker moves
- ) Increase the weight in steps, recording the marker reading each time - the extension is the difference between this reading and the un-stretched length.
- ) You can use your results from this experiment to calculate the stress and strain of the wire and plot a stress-strain curve
What does area between loading and unloading graph on force-extension graph represent?
Work done to permanetly deform the material and thermal energy store
