The Young Modulus (E)  the modulus of elasticityThe Young Modulus is named after Thomas Young (1773 to 1829) who was a British polymath, contributing to our understanding of optics, physiology, and Egyptology, among other fields The Young Modulus is also known as Young's Modulus or the elastic modulus or tensile modulus. It is a measure of the stiffness of a material that is independent of the particular sample of a substance. That means a generic value can be given for a material without its dimensions being known (like the values given for resistivity). It is the ratio of tensile stress (force/cross sectional area) to tensile strain (extension/ original length)  don't forget to put the word 'tensile' into any definition you give in an exam  it will lose you marks! N.B. AQA no longer expect you to know the symbols for these values. The unit will therefore be the same as the unit of pressure (Pa or N/m^{2})
Strain values can be expressed in several ways: as a percentage  %, or in decimal form. E.g. a 5% strain = 0.05 The Young Modulus can be experimentally determined from the slope of a stressstrain curve created during tensile tests conducted on a sample of the material. The value of the Young's Modulus is quoted in scientific data manuals for various materials but the value given is only approximate. This is because Young's Modulus can vary considerably depending on the exact composition of the material. For example, the value for most metals can vary by 5% or more, depending on the precise composition of the alloy and any heat treatment applied during manufacture.
Click here to find out about the difference between brittle, plastic and ductile behaviour. Click here to see notes on an experiment to determine the Young Modulus. 
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