chapter 4 Flashcards
state Hooke’s law.
the extension of a stretched wire, e, is proportional to the load or force, F : F = ke k is the stiffness of the constant
how does Hooke’s law apply to springs?
because the extension or compression of a spring is proportional to the force applied. For a spring , k has the same value whether the forces are tensile or compressive
define tensile forces and compressive forces
tensile F: stretch the spring
compressive F: squash the spring
what is meant by the elastic limit?
the load gets great enough and past the elastic limit the material will be permanently stretched. When all the force is removed, the material will be longer than at the start
what is elastic deformation?
where the material returns to its original shape once the forces are removed
1) when the material is put under tension, the atoms of the material are pulled apart from one another
2) atoms can move small distances relative to their equilibrium positions, without actually changing positions in the material
3) once the load is removed , the atoms return to their equilibrium distance apart
for a metal elastic deformation happens as long as Hooke’s law is obeyed
what is plastic deformation?
the material is permanently stretched
1) some atoms in the material move position relative to one another
2) when the load is removed the atoms don’t return to their original positions
a metal stretched past its elastic limit shows plastic deformation
if the forces stretch a material then they are?
tensile
if the forces squash the material then they are?
compressive
define tensile stress
the force per unit area acting perpendicular to the area stress= F/A
what are the units of tensile stress?
Nm ^-2 or Pa
define tensile strain
the change in length (extension) divided by the original length of the material
strain = e/ l
why does strain have no units?
because it is a ratio so it is just the number of something
what is the breaking stress?
a stress big enough to break the material
how does a material break?
the effect of stress is to start to pull the atoms apart from one another
eventually the stress becomes so great that atoms separate completely, and the material breaks
what is fracture stress?
the stress at the point on the graph where it ends
what is the UTS?
ultimate tensile stress- the maximum stress that a material can withstand
what does a material experience if you apply a load?
tensile stress and tensile strain
when are stress and strain proportional to eachother?
at the limit of proportionality
below the limit of proportionality stress/ strain is a constant, what is it called?
the Young modulus, E, how much a material deflects under a given load
what are the units for the Young modulus?
Nm^-2 or pascals
young modulus=
tensile stress/ tensile strain = (F/A) / (e/l)= Fl/ eA
experiment to find the young modulus of a wire:
1) long thin wire- longer and thinner means it extends more for the same force
2) start with the smallest weight necessary to straighten the wire
3) measure the distance between the fixed end of the wire and the marker- unstretched length
4) if you increase the weight, the wire stretches and the marker moves
5) increase the weight by steps, recording the marker reading each time- the extension is the difference between this reading and the unstretched length
6) once you’ve taken all your readings, use a micrometer to measure the diameter of the wire in several places. take an average of your measurements and use that to work out the average cross-sectional area of the wire
behaviour of a brittle material:
doesn’t undergo plastic deformation and breaks soon after its elastic limit
. they can also be quite weak if they have cracks in them
behaviour of a ductile material:
change the shape by drawing it into wires or other shapes. They keep their strength when they’re deformed
example of a ductile material:
copper- high electrical conductivity so ideal for electric wires
behaviour of a malleable material:
shape can be changed but may lose their stength: hammered or rolled into shape
example: gold, brass
behaviour of a hard material:
resistive to dents and scratches, unit is pascals example: diamond
behaviour of a stiff material:
resistance to deformation. high resistance to bending and stretching. measured by the young modulus- the higher the value the stiffer the material
Changes it’s shape only slightly under elastic loads
behaviour of tough materials:
unit= J m^-2 undergo considerable plastic deformation before they break, it is a measure of the energy a material can absorb before breaking
how is toughness defined?
either a) the energy required to create new surface area: J/m^2
b) the energy absorbed per unit volume : J/m^3
define strong
a large stress is needed to cause failure
define strength
the maximum stress a material can withstand before failure
why do some materials have 2 values for strength?
because there are two types of failure: yielding and breaking/ fracture
draw a stress-strain graph for a ductile material
4 points: straight line- obeying hooke’s law
limit of proportionality- material stops obeying hooke’s law but would still return to its original shape
elastic limit= material starts to behave plastically- from this point onwards, the material would no longer return to its original shape
point y- yield point- the material starts to stretch without any extra load
what is the yield point?
it is the stress at which a large amount of plastic deformation takes place with a constant or reduced load
what 3 things determine resistance?
1) length- the longer the wire the more difficult it is to make a current flow
2) area- the wider the wire the easier it will be for the electrons to pass along it
3) resistivity depends on the material. the structure may make it easy or difficult for charge to flow
in general what does resistivity depend on?
environmental factors eg temperature or light intensity
define resistivity
the ability of a material to resist a current: the resistance of a 1 m length with a 1 m^2 cross- sectional area. it is measure in ohm-metres
what is the equation for resistivity?
p= RA/ l or R= p x l/A
what is the inverse of resistivity and define it
measure a material’s ability to conduct an electric current. conductivity is the conductance of a 1 m length with a 1 m^2 cross-sectional area. it’s measure in siemens per metre S m^-1.
what is the equation for conductivity?
conductivity= Gl/ A or G= conductivity x A/l
experiment to find the resistivity of a wire
1) measure out a length of wire made from the material you want to test using a millimetre ruler
2) use a micrometer to measure the diameter of a piece of wire in several places. Take an average of your measurements, and use that to work out the average cross-sectional area of the wire
3) connect a length of wire into an electrical circuit as shown, ensuring that current can flow through the entire length of wire.
4) record the resistance of the wire using a micrometer, and take lots of readings for different lengths of wire
5) plot a graph of R against l- the gradient = p/A so times by A to get the resistivity
in a metal what are the charge carriers?
free electrons
why are metals good conductors?
because they have a sea of delocalised electrons so the charge carrier density is high
how does temperature effect the conductivity of a metal?
increase the temp: the no. of charge carriers stays about the same. As the electrons move, they scatter from the metallic lattice. As the temp increases, the lattice vibrates more, increasing the electron scattering, so the electrons are slight less free to move. This means that as the temp increases, the conductivity of a metal will slightly decrease.
how does temperature effect the conductivity of a semiconductor?
have much lower charge carrier density than metals so have a lower conductivity. as u increase temp, more electrons are freed to conduct. this means that as temp increases, the conductivity of a semiconductor rapidly increases.
why can’t insulators conduct?
a perfect insulator wouldn’t have any charge carriers so wouldn’t be able to conduct at all
what does a conductive metal depend on?
it’s charge carrier density- how many charge carriers there are per unit volume of material
what is energy?
the capacity of a body to do work
work done =
force x distance
in a force distance graph, the area underneath the graph is?
the workdone
for a spring what increases with extension?
force
the area under a force extension graph is ?
energy
what is the energy stored in a stretched spring and why?
1/2 Fe = 1/2 ke^2 since F=ke and k is the spring constant because the work done is stored in the spring as elastic energy
what is the tension in a stretched wire related to?
the extension of the wire
examples of conductors
steel, bronze, nickel,iron, silver, copper, gold
examples of insulators
polystyrene, perspex, lead glass, aluminium,
examples of semi conductors
pure silicon, pure germanium, doped germanium
what is a zero error?
haven’t properly zeroed your instruments- end up with a systematic error so every reading is wrong by the same amount
what are random errors?
can’t get rid of them, get a random error in any measurement. might be because you can’t keep controlled variables exactly the same throughout the experiment
What is tensile yield strength?
A stress at which plastic deformation begins. From this point onwards the material won’t go back to it’s original length
What is the structure of metals?
Crystalline lattice where the metal atoms are arranged in a regular repeating pattern. They have a sea of delocalised electrons that make them good conductors
The electrostatic attraction between the ion lattice and the free electrons forms the metallic bond. It’s these strong bonds that make metals stiff
What makes a metal tough?
The strongly bonded lattice structure
What makes a metal ductile?
The ions within the lattice can move when you apply a force to the metal
How are ceramics made?
By melting certain materials and letting them cool
What is the structure of a ceramic?
Arrangement of atoms can be crystalline or polycrystalline where there are many regions or grains of crystalline structure. the atoms in each grain line up in a different direction
Structure of glass
Amorphous- no overall pattern; the atoms are arranged at random
What makes ceramics stiff and what makes them brittle?
Atoms in a ceramic are either ionically or covalently bonded in a giant rigid structure. The strong bonds between the atoms makes ceramics stiff and the rigid structure makes them brittle
What is the structure of a polymer
Molecular chain made up of a single repeating unit called a monomer. The monomers are covalently bonded together, so are very hard to separate so makes them STRONG
Natural polymer
Rubber
Man made polymer
Polythene
What’s makes polymers flexible
By rotating about their bonds when u pull them- The more easily the monomers can rotate, the more flexible the polymer will be
What affects the polymers flexibility
The strength and number of bonds between the chains
What makes a polymer more rigid
The stronger the cross-linking bonds and the more cross-linking bonds you’ve got the more rigid the material
What are composites
You combine two different materials to get material with the properties you want
What is one of the most common composites
Reinforced concrete- normal concrete is strong when u compress it but is brittle under any sort of tension force like being bent. To increase the strength of the concrete under tension, you embed steel rods into it
What makes a material flexible
Large strain for a little stress
What makes a material stiff
Little strain for large stress
If u double the area what happens to resistance
You half the resistance R/2
If u double the area what happens to conductance
It doubles
If u double the length what happens to resistance
It doubles
If u double the length what happens to conductance
It halves
