P6 Flashcards
Hookes law
Force is proportional to extension until limit of proportionality (slightly after elastic limit) - then permanent structural changes
Interpreting force extension graphs
gradient = spring constant (F = kx)
area underneath = energy stored (E = 0.5kx2 or 0.5Fx)
line drawn from curve point parallel to initial straight line - x intercept shows permanent extension
Breaking point is where the line ends
Ultimate tensile strength is the highest y value
Upper and lower yield points show rapid changes of extension/ gradient e.g turning points
Investigating springs
Attach spring to clamp, boss and clamp stand secured to bench, set up a metre rule close to spring straightened with set square
attach objects (weighed with balance) to spring and measure extension (new - initial) - then multiple readings, draw a graph
Spring constant of springs in series
1/k1 + 1/k2 = 1/k (total)
Energy and work
work = F(delta)x
for uneven loading unloading graphs - area underneath is work done, area between the loading/unloading line is thermal energy lost and added together is epe - e.g shopping bags and bungee cords
loading unloading graphs
metal - straight line up, then curve, line back parallel to original
rubber - sideways s curve up, sideways s curve down but lower leading s shape in middle
polythene - sideways s loading, very steep unloading down so high thermal energy dissipation
Stress strain graphs of materials
Strong materials have high max strain
Brittle materials are a straight line stopping abruptly
Ductile materials plateau and continue
Young modulus
= Stress/ strain
or force/csa / extension/og length
Y = FL/Ae
or AeY = FL
Wire behaviour
1st yield point - atoms slip so weakened temporarily, 2nd yield point is permanent weakened so plastic flow
then work hardening, where more atoms dislocate making movement harder
after UTS starts to rapture then at breaking point bonds snap and break
elastic behaviour - more tension so more inter-ionic spacing as ions move, but then return
plastic behaviour - planes of atoms slide over each other - stress bonds broken one at a time
alloys can strengthen materials as atoms of different sizes slip less
Elastic and plastic deformation
Elastic returns to original length
Plastic doesnt
Loading and unloading
metal wire - straight line up then curve - straight line back to x intercept
rubber - loading is sideways s curve, unloading again it lower leading sideways S in middle
Polythene - loading sideways s curve, unloading is very steep line to x axis so high energy lost
