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What Material Properties are Useful
• In a climbing rope?
• In a carbon fibre bike fork?
• In a bullet-proof vest?
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
Define the followingKey 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
DurableProperties do not deteriorate with repeated loading and unloading
Bone
Stress and Strain
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.
Working out uncertainty
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.
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?
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?
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
Resistivity
Build this circuit with the wire in the place of your resistor
1. Work out the cross-sectional area of the wire.
2. Mark five equally spaced lengths on the wire.
3. Keep the voltage, and hence the current, really low, no more than 1V ideally.
4. Keeping one of the crocodile clips static, move the other along the wire to take five readings (three repeats at each)
5. Using the V=IR equation and readings at the ammeter and voltmeter, determine a value for resistance.
Length/m Voltage/V Current/I Resistance/Ω Cross-sectional Area/m2
Homework
• Plot resistance against length
• Work out the gradient
• Multiply the gradient by the cross-sectional area to determine the resistivity of the wire
• Plot error bars on the first three points