4 TESTING MATERIALSProperties of materials
o Classify and explore mechanical and electrical properties of materials
o Explain the meaning of the term intensive property, with examples
o Appreciate the need for log scales for representation of properties
Classifying materials
• Look at the vast range of solid materials used in objects around you.
• What general classes of material do they fall into?
• Can you describe the internal structure of materials in these classes?
Investigating breaking stress
• Measure the breaking force and breaking stress for a material
• Use “plot and look” to identify outliers
Starter
• How can we compare how strong/stiff different metals are when the wire samples we have are of different diameters for different metals?
• Similarly, how can you compare how elastic/stretchy different metals are when the wire samples we have are of different lengths for different metals?
Force extension graphs and Young’s modulus
• Record and interpret a load-extension graph for copper
• Make an accurate and precise measurement of the Young’s Modulus of a material, estimating the uncertainties involved
StarterA wire of Material A fractures under a load of100 N, while a wire of Material B fracturesunder a load of 10 N.
Q1. What, if anything, can you conclude about the strengths of the materials A and B?
Q2. What further information would you need in order to make a valid comparison between materials A and B?
Measuring Young’s Modulus
• Make an accurate and precise measurement of the Young’s Modulus of a material, estimating the uncertainties involved
Stress strain curves
• Describe and explain the stress-strain curves for metals, ceramics and polymers
These are microscope images of the fracture cross sections of rods of two different materials.
A
Which one has undergone ductile fracture, and which one brittle fracture? Explain your answer.
AA
B
Electrical properties of materials
• Investigate the relationship between material dimensions and resistance
• Derive equations linking resistance, conductance, resistivity and conductivity
Measuring resistivity
• Make an accurate and precise measurement of the resistivity of a material, estimating the uncertainties involved
Starter: Write down the equation for working out the resistance R of a wire of length L and area A. Compare with the equation of a straight line. What should you measure and plot in order to determine the resistivity of the metal?
Uncertainty and Error How do they apply in this experiment?
Random uncertainties• Identify sources of
measurement uncertainty and quantify them.
• Which contributes the most to the uncertainty in the final answer?
Systematic errors• Identify limitations in
the design of the experiment and explain how they could either be eliminated or reduced in effect.
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