The resistivity of a material depends on: - Its atomic/molecular structure - Temperature - Presence of impurities Some key points: - Metals have low resistivity due to free electrons - Non-metals have high resistivity due to lack of free electrons - Resistivity increases with temperature for most materials - Adding impurities increases resistivity by scattering electrons The resistivity, ρ, of a material is a constant for a given temperature and purity. It is measured in Ωm. To calculate resistance, R, we use: R = ρ * l / A Where l is the length of the material and A is

1. What Material Properties are Useful
• In a climbing rope?
• In a carbon fibre bike fork?
• In a bullet-proof vest?

2. What Material Properties are Useful
• In a climbing rope? Elasticity and strength
• In a carbon fibre bike fork? Strength and
stiffness
• In a bullet-proof vest? Toughness and
plasticity

3. Define the following
Key Word Definition Example
Stiff Does not easily change shape when force is applied Glass
Elastic Returns to original shape when force is removed. Copper
Plastic Remains deformed when force is removed Blu-tac
Ductile Can be readily pulled out into a thinner shape. Copper
Malleable Can be deformed under compression Copper
Strong Requires a large force to break it Steel
Brittle Easily cracks Glass
Tough Needs a large force to deform it Kevlar
Smooth Low friction surface PTFE
Properties do not deteriorate with repeated loading
Durable Bone
and unloading

4. Stress and Strain

6. Using the results from last lesson
• Draw a graph
• Plot a line of best fit
• Determine the gradient of the
straight line section (Young’s
Modulus)
• Work out the area under this
section. This is the energy stored
in the material.

7. Working out uncertainty

8. Working out uncertainty 2
• Whether you are multiplying or dividing units, you will
always add the uncertainties together.
• I measure the side of a cube to be 10±0.5cm
• What is its volume if all dimensions are the same?
• What is the uncertainty with that volume?
• Now place error bars on your first and last two plots on
the graph. Assume mass has no uncertainties.

9. Necking
As the metal wire experiences plasticity, it becomes narrower
at one point.
This is called necking.
1. What happens to the stress experienced at that point in
the wire as it begins to narrow?
2. Work out the stress on a 1mm diameter section of wire
with a experiencing a force of 10N.
3. What is the stress if it narrows to 0.9mm?

10. Resistance
• Write a description of resistance in no more
than 9 words
• What is the energy transfer for resistance?
What type of energy does it convert, and into
what?
• What is the equation that relates Voltage,
Current and Resistance?

11. Resistance
• Three things determine the resistance of a
wire:
1. Length (l) - The longer the wire the more
difficult it is for current to flow
2. Area (A) – The wider the wire the easier it is
for electrons to pass along it
3. Resistivity (ρ) – This is a property of the
material

12. Resistivity