PHYS157 - 1b Thermal expansion, stress, and strain

Explain why the change of length of an object due to thermal expansion is proportional to its initial length

why do materials expand when heated?

The expansion (or contraction) of any material is due to the kinetic energy of its atoms. When a material is heated,

• increase in energy =atoms and molecules move more + take up more space (EXPAND)

• electrons repel each other, vibrate more

why does water freeze from top-down?

density of ice is less than the liquid

• ice layer at lower temperature floats ABOVE the water, so it freezes top to bottom

what is tensile stress?

when an object is stretched by forces acting at the ends (eg. guitar strings) (change in length = POS)

what is compressive stress?

object is compressed at the ends (change in length=NEG)

what is strain?

• dimensionless

• fractional change in size

• strain=change in length/original length

what is stress?

• force per area applied (same formula as pressure)

• stress=force/area

• has units of pressure (Pa, N/m², kgm^-1s^-1)

what is deformation?

a change in shape due to an application of a force

what is young’s modulus?

Y= basic property of a material

• resistance to stretching or compressing

• how easily it can be deformed

• THE STIFFNESS OF A MATERIAL**

• elastic modulus that shows how small tensile/compressive stress/strain are LINEARLY PROPORTIONAL

is a small youngs modulus or greater youngs modulus more resistant to change?

A greater youngs modulus is more resistant to change (eg. a rock). (Low YM = rubber)

how is a youngs modulus of a material defined?

ratio of stress over strain

how to calculate youngs modulus from a graph?**

• select two points on the linear part of the slope

• calculate the slope (rise-over-run)

• from stress divided by strain

Why does Young’s modulus depend only on the material and not on the shape or size

• it is an intrinsic** property of the material

• it depends on the inter-molecular bond strength of the material

• weight/area/length/shape does NOT affect!

does young’s modulus change with temperature?

young’s modulus decreases when temperature increases

what is newtons second law?

f=ma

if force acting on an object increases, the acceleration of an object increases

what is newtons third law?

• for every action (force) in nature there is an equal and opposite reaction.

• If object A exerts a force on object B, object B also exerts an equal and opposite force on object A