Physics Unit 1 Section 3

Physics Unit 1 Section 3

1. Hooke's Law

   • Extension is proportional to force
     • F=ke
     • k is a constant depending on material
     • Stiffness constant
   • Applies to springs
     • Tensile forces (stretching)
     • Compressive forces (squashing)
   • Stops working when load is too great
     • Linear graph starts to curve at elastic limit
     • After elastic limit, material permanently stretched
   • Straight line relationship graphically
   • Stretches
     • Elastic
       • Returns to original shape
       • Atoms pulled apart under tension
       • Return to equilibrium after load removed
     • Plastic
       • Permanently stretched
       • Atoms move relative to each other
       • Do not return to place

2. Stress

   • Stress = Force/Area
   • Nm^-2 OR Pa
   • Causes a strain
   • Pulls atoms apart
   • UTS = atoms separate completely, material breaks

3. Strain

   • Strain = Extension/Original length
   • No units

4. Young's Modulus

   • Stress and strain are proportional to a limit
   • Ym = Stress/Strain
   • Ym = Fl/eA

5. Metals

   • Crystalline structure
     • Sea of free electrons
     • Lattice (repeating regular pattern)
     • Good conductors as electrons are charge carriers
   • Strong metallic bonds
     • Makes metal stiff
   • Tough due to lattice
   • Ions can move make it ductile

6. Ceramics

   • Pottery/Brick/Glass
   • Crystalline or poly crystalline structure
     • Atoms in each grain of structure line up in different direction
   • Glass = amorphous
     • Atoms arranged at random
   • Ionic or Covalent bonds in rigid structure
     • Strong bonds make it stiff
     • Brittle materials

7. Polymers

   • Monomers joined together
   • Covalent bonds
     • Hard to separate
     • Strong
   • Chains are scrunched up or folded
     • Unfold by rotating about bonds
     • Flexible

8. Composites

   • Combination of two diff materials
   • Get properties you want from both materials

9. Mechanical properties

   • Brittle
     • Wont deform plastically when force is applied
     • Will snap
     • can be weak if there are cracks
   • Ductile
     • Can be drawn in to wire without loosing strength
     • Copper used for electric wires
   • Malleable
     • Can change shape may loose strength
     • Can be bent or stretched into complex shapes
     • Ed. Gold/Brass
   • Hard
     • Resistant to cutting/indentation
     • Cutting tools need to be hard to dent another surface without getting dented
     • Diamond = hardest material
   • Stiff
     • High resistance to bending or stretching
     • Measured by YM
     • Safety helmets need to be stiff so they don't crush you
   • Tough
     • Difficult to break
     • Energy material can absorb before it breaks

10. Electrical properties

    • Resistance
      • Affected by length, area and resistivity
      • Resistivity
        • rho = Resistance X Area / Length
    • Metals
      • Charge carriers are free electrons
      • Good consuctors
      • Increase in temperature
        • Lattice vibrates more
        • Charge carrier amount stays same
        • Less free to move
        • conductivity will decrease
    • Semi conductors
      • Fewer free electrons
      • Lower conductivity
      • Increase in temperature
        • More electrons freed
        • Conductivity increases