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# Young’s modulus 2 and resistivity

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### Young’s modulus 2 and resistivity

1. 1. What Material Properties are Useful• In a climbing rope?• In a carbon fibre bike fork?• In a bullet-proof vest?
2. 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. 3. Define the following Key Word Definition ExampleStiff Does not easily change shape when force is applied GlassElastic Returns to original shape when force is removed. CopperPlastic Remains deformed when force is removed Blu-tacDuctile Can be readily pulled out into a thinner shape. CopperMalleable Can be deformed under compression CopperStrong Requires a large force to break it SteelBrittle Easily cracks GlassTough Needs a large force to deform it KevlarSmooth Low friction surface PTFE Properties do not deteriorate with repeated loadingDurable Bone and unloading
4. 4. Stress and Strain
5. 5. 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.
6. 6. Working out uncertainty
7. 7. 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.
8. 8. NeckingAs the metal wire experiences plasticity, it becomes narrowerat 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?
9. 9. 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?
10. 10. Resistance• Three things determine the resistance of a wire:1. Length (l) - The longer the wire the more difficult it is for current to flow2. Area (A) – The wider the wire the easier it is for electrons to pass along it3. Resistivity (ρ) – This is a property of the material
11. 11. Resistivity
12. 12. Build this circuit with the wire in the place of your resistor1. 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.
13. 13. Length/m Voltage/V Current/I Resistance/Ω Cross-sectional Area/m2
14. 14. 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