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aqa a level physics > materials > Flashcards

materials Flashcards

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1
Q

what is density

A

measure of compactness of a substance
mass per unit volume

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2
Q

state what is meant by centre of mass [1]

A

the point through which (the line of action of) a force has no turning effect

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3
Q

State two necessary conditions for the beam to be in equilibrium [2]

A

resultant force = 0
sum of anticlockwise moments = sum of clockwise moments

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4
Q

State the law of conservation of energy [2]

A

energy cannot be created or destroyed
it can only be transferred from one form to another

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5
Q

density {}

A

ρ = m/v

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6
Q

Hooke’s law {}

A

F = K * change in L

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7
Q

what is hooke’s law

A

extension is directly proportional to force applied given that environmental conditions are kept constant

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8
Q

what is k

A

spring constant (Nm-1)
the stiffness of an object being stretched

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9
Q

tensile force

A

stretching

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10
Q

compressive force

A

squashing

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11
Q

what does a typical force-extension graph look like for a typical metal wire

A

straight line through origin
limit of proportionality
curves off

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12
Q

limit of proportionality

A

the point beyond which hooke’s law is no longer obeyed

the force is no longer proportional to the extension

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13
Q

elastic limit

A

the point beyond which the material suffers plastic deformation

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14
Q

In a force - extension graph what is the gradient
what is the area under the graph

A

k
since F = ke
spring constant (Nm-1)

work done to deform the material

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15
Q

how to investigate extension

A

set up clamp with spring
take original length
place load on spring and take final length
subtract to find extension
take repeats
plot a graph

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16
Q

what does it mean to say a deformation is elastic.
what does the graph look like?

A

the material returns to its original shape once the forces have been removed
aquafresh looking graph. returns to original length after extension

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17
Q

what does it mean to say a deformation is plastic

A

the material is permanently stretched after the load has been removed
materials stretched past their elastic limit show plastic deformation

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18
Q

what is meant by tensile stress

A

force applied per unit cross-sectional area resulting in stretch of the material

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19
Q

what is meant by tensile strain

A

a measure of how the material stretches

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20
Q

tensile stress {}
what are the units of tensile stress

A

force / cross-sectional area
Nm^-2 or Pa

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21
Q

tensile strain {}
what are the units of tensile strain

A

ΔL/L
extension / original length
no units - it’s a ratio

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22
Q

how would use a force extension graph to find the elastic strain energy stored by a stretched material

A

area under the graph
up to extension required

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23
Q

how is energy conserved as a stretched wire undergoes plastic deformation

A

some of the kinetic energy transferred to elastic potential energy
some of it is used to separate atoms and is dissipated as heat (thermal energy)

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24
Q

describe the energy changes as a spring is stretched and released

A

kinetic energy is transferred to elastic potential as the spring is stretched
elastic strain energy is stored in the spring until the spring is released when it its transferred back to kinetic energy

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25
Q

the greater the tensile force applied to a material …

A

the greater the stress on it

26
Q

what is the effect of stress

A

atoms are starting to be pulled apart from one another

27
Q

what is breaking stress on an atomic level

A

as stress is applied atoms are pulled apart from one another
eventually the stress becomes so great the atoms separate completely and the material breaks
(stress big enough to break a material)

28
Q

what is ultimate tensile stress
and what is it also known as

A

maximum stress that a material can withstand before it fractures
breaking stress

29
Q

what is elastic strain energy

A

stored energy in a material after work has been done to stretch it

30
Q

work done {}

A

wd = 1/2 F * ΔL
(elastic strain energy - area under straight line graph)

31
Q

Energy{}

A

E = 1/2 k * e^2
since F = ke
and E = 1/2 * F * e

32
Q

why isn’t all work done stored as elastic strain energy when the deformation is plastic

A

some work is done to change the position of the atoms
not stored as strain energy

33
Q

change in kinetic energy =

A

change in potential energy
( elastic + gravitational )

34
Q

what are the units for young’s modulus

A

Nm^-2 or Pa

35
Q

young’s modulus {}

A

tensile stress / tensile strain
gradient of a stress-strain graph

36
Q

what are the four things you would have to measure in an experiment to find the young’s modulus of a wire

A

diameter of the wire
extension of the wire
original length of the wire
weight (load)

37
Q

what does the area under a stress-strain graph tell you?

A

strain energy stored per unit volume in the material

38
Q

how are stress and strain related

A

they are proportional to one another
( first cousin, twice removed on their mother’s side )

39
Q

describe the young’s modulus experiment

A

wire
place varying loads and measure extension
measure diameter of wire to calculate cross-sectional area
plot stress strain graph
gradient = young’s modulus

40
Q

REQUIRED PRACTICAL
what should the wire be like and why

A

thin and long
meaning it would extend more for the same force
decreasing uncertainty in measurements

41
Q

REQUIRED PRACTICAL
what is assumed when calculating the stress

A

The cross-sectional area is circular

42
Q

what is the difference between a material at its elastic limit and its limit of proportionality

A

past limit of proportionality the material no longer obeys hooke’s law but will still behave elastically and return to its original shape
past the elastic limit the material behaves plastically and will not return to its original shape when stress is removed

43
Q

what is the yield point of a material

A

the point at which plastic deformation begins to occur

44
Q

what is shown by the area between the loading and unloading curves on a f-e graph showing plastic deformation

A

the area is the work done to permanently deform the material

45
Q

what does the f-e graph look like for a metal wire that has been stretched beyond its limit of proportionality

A

straight line through origin
curves off after limits (elastic and of proportionality)
unloading line parallel to loading ( since k remains constant )
returns to a longer length than original since permanent deformation has taken place

area between these lines is the work done to permanently deform the wire

46
Q

what does the f-e graph look like for a brittle material

A

straight line
and then the material fractures

same for stress-strain

47
Q

what is brittle fracture

A

when stress is applied to a material
cracks at the materials surface get bigger and bigger until the material breaks completely

48
Q

what is a brittle material

A

one which doesn’t deform plastically but snaps when the stress acting on it reaches a certain amount
this is due to their rigid structure, atoms won’t move to prevent cracks getting bigger

49
Q

do brittle materials obey hooke’s law, explain why

A

yes.
stress strain graph shows straight line through origin

50
Q

give an example of brittle material

A

ceramics

51
Q

what is the unit for density

A

Kgm^-3

52
Q

using experimental methods to find the density of regular, irregular objects and liquid

A

regular
measure the mass
take appropriate measurements to calculate volume
density = mass / volume
irregular
measure the mass
immerse object on a thread in liquid in a measuring cylinder ( eureka can )
displacement = volume of object
density = mass / volume
liquid
measure mass
measure volume

53
Q

how do you calculate the density of an alloy

A

use the densities of each material and the corresponding volumes of each to find a mass value for each metal
add these mass values up and divide by the total volume to find an overall density

54
Q

1m^3 in cm^3

A

10^6 cm^3

55
Q

if a weight is supported by the means of two springs in parallel to one another how is the extension of the springs affected
and the effective spring constant?

A

the weight is shared between the two springs so the overall extension is halved
the springs constant doubles as it has become more “stiff”

56
Q

if a weight is supported by the means of two springs in series to one another how is the force needed affected
and the effective spring constant?

A

Force needed for the same extension would half
both springs experience the same tension it would singularly
therefore they both extend by the same amount
extension doubles
springs constant halves

57
Q

what is young’s modulus a measure of

A

the stiffness of different materials

58
Q

what is ultimate tensile stress a measure of

A

the strength of a material

59
Q

what is a ductile material

A

a material that can be drawn into a wire

60
Q

what is the difference between elastic and plastic deformation

A

elastic - when force is removed the object will return to its original shape
plastic - after load is removed it will not return to its original shape

61
Q

how is the work done to stretch or compress a material stored

A

elastic strain energy

62
Q

how is dissipation of energy in plastic deformation used to design safer vehicles

A

crumple zones
- kinetic energy transferred to plastic deformation
seat belts
stretch to convert passenger’s kinetic energy to elastic strain energy

aqa a level physics (10 decks)

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