Materials

Flashcards covering all of the materials topic for AS level AQA Physics exams.

What is a compressive force?

A force that ‘squashes’ an object (attempts to reduce its size)

What is a tensile force?

A force that stretches an object (attempts to increase its size)

Stress

σ = force/cross-sectional area of the object (units are Pa or N/m²)

• σ=F/A

• ​If tensile force, positive stress

• If compressive force, negative stress

Strain

= extension/original length (ratio so no units)

• ϵ=ΔL/L​

• If tensile force, positive strain

• If compressive force, negative strain

Elastic strain energy

• The same as elastic potential energy

• Energy supplied to an object when an elastic object is stretched (under a tensile force)

• Equal to the area under a force-extension graph

Breaking stress

• force over unit area required to break a material

Ultimate tensile stress

Maximum force per unit area (tensile stress) an object can withstand.

• Some materials undergo some strain beyond ultimate tensile stress before breaking.

Fractures

Occur when the material completely breaks

Brittle behaviour

• Occurs beyond the object’s elastic limit

• object extends very little. breaks suddenly and/or cracks easily

• exhibits fracture behaviour

• Little plastic behaviour shown

Plastic behaviour

When an object undergoes permanent deformation under stress

• Occurs after the elastic limit

• On a force-extension graph, look to the right of the elastic limit.

What is k in Hooke’s law

constant of proportionality/spring constant, which is a measure of the stiffness of a spring

Total energy in a spring system

The sum of all the kinetic and potential energies of an object

• obeys the conservation of energy laws

• total energy at all points in an oscillation is the same

Elastic potential energy

Energy stored within a material (like springs) when it is fully stretched or compressed

Equal to the area under a force-extension graph

Crumple zones (ethical transport design)

Parts of a vehicle that permanently deform

• show plastic behaviour

• energy from the impact is redirected towards plastically deforming the crumple zones, therefore people are not injured.

Airbags (ethical transport design)

• also absorb the energy from the impact

WANNA ELABORATE?

The Young Modulus

• represents the stiffness of a material

• Stiff materials have a large young modulus

• E = stress/strain (ratio of stress to strain)

• gradient of a stress-strain graph is Young’s Modulus
  

Outline a method to determine the the Young Modulus of a copper wire (RQP)

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Elastic deformation

When the load is removed, the object will return to its original shape/length

Shown in the elastic region of the graph

Plastic deformation

The material is permanently deformed. When the load is removed, the object will not return to its original shape/length.

This is beyond the elastic limit, and is shown in the plastic region of the graph

Where is the elastic region on a force-extension graph?

• Where the extension is proportional to the force applied to the material (straight line)

Where is the plastic region on a graph?

Where the extension is no longer proportional to the force applied to the material (graph begins to curve). This begins at the elastic limit and ends at the point of fracture.

Where is the point of fracture on a load/extension graph?

The point at which the curve ends.

point of fracture = maximum extension of an object before it breaks.

Define ductile

materials with a larger plastic region, material stretches to a new shape/undergoes deformation before breaking.

Draw brittle line vs ductile line on a force-extension graph