Chapter 11 - Materials Flashcards
What is the definition of the density of a substance?
The mass per unit volume
Why are gases much less dense that liquids and solids?
As the average separation between the molecules in a gas is much greater that in a liquid or solid.
How to measure the density of a regular solid?
1) Measure its mass using a balance
2) Measure its dimensions using a vernier caliper or a micrometer
3) Calculate volume
4) Density=mass/volume
How to measure the density of a liquid?
1) Measure a mass of an empty measuring cylinder
2) Pour some liquid into this measuring cylinder and measure the volume exactly
3)Make sure to use as much liquid as possible to reduce the % error
4) Measure the mass of cylinder and liquid
5) Density=mass/volume
How to measure the density of an irregular solid?
1) Measure the mass of an object
2) Put the object in an Eureka can and measure the volume of water displaced using a measuring cylinder
3) Density=mass/volume
What is an alloy?
A solid mixture of two or more metals
What does Hooke’s law state?
The force needed to stretch a spring is directly proportional to the extension of the spring from its natural length.
What happens when a spring is stretched beyond its elastic limit?
It does not regain its initial length when the force applied to it is removed
What is the effective spring constant of springs in series?
1/k = 1/k1 + 1/k2 …..
What is the effective spring constant of springs in parallel?
k= k1 + k2 +…..
What is the area under a force extension graph?
Elastic energy stored in a stretched string
What does the gradient of a force extension graph tell us?
The spring constant
What is the definition of the elasticity of a solid material?
Its ability to regain its shape after it has been deformed or distorted and the forces that deformed it have been released.
What do we call deformation that stretches an object?
Tensile
What do we call deformation that compresses an object?
Compressive
What happens at the yield point of a stress- strain?
The wire weakens temporarily
What is the ultimate tensile stress sometimes called?
The breaking stress
What happens beyond the maximum tensile stress, the ultimate tensile stress?
The wire loses its strength, extends and becomes narrower at its weakest point. The increase of tensile stress occurs due to the reduced area of cross-section at this point until the wire breaks.
What can you use to compare the stiffness of different materials?
The young modulus
What type of material snaps without any noticeable yield and what is an example of this kind of material?
A brittle material such as glass (breaks without any give)
What do you call a material that can be drawn into a wire?
A ductile material
Does a stronger material have a higher or lower maximum tensile stress?
It has a higher maximum tensile stress
What does a stress - strain graph
look like for a rubber band?
Change in length during unloading is greater than during loading for a given change in tension. It returns to the same unstretched length, but the unloading curve is below the loading curve except at zero and maximum extensions. Has a low limit of proportionality, but remains elastic.
What does a stress - strain graph look like for a polyethene strip?
The extension during unloading is also greater than during loading, but the strip does not return to the same length when it is unloaded as it has a low limit of proportionality and suffers plastic deformation.
What is the strain energy?
The work done to deform an object
What does the area between the loading and unloading curve on a stress - strain graph for a rubber band represent?
It represents the difference between energy stored in the rubber band when it is stretched and the useful energy recovered from it when it is unstretched.
What does the area between the loading and unloading curve on a stress - strain graph for a polyethene strip represent?
As it does not regain it’s original length, it represents the work done to deform the material permanently, as well as the internal energy retained by the polyethene when it unstretches.
