Limit of proportionality vs elastic limit?

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Al4stair
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#1
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#1
https://image.ibb.co/b1uFH7/20180328_122936.jpg
That link is a image of my Physics revision guide. The book shows two different points and states one is the limit of promotionality (P) and the point futher along the curve is the elastic limit (E).
The limit or proportionality: 'the greatest stress that can be applied to an elastic body without causing permanent deformation.'

Elastic limit: 'the maximum extent to which a solid may be stretched without permanent alteration of size or shape.'

Therefore these seem to be the same thing. So does the spring not return to it's original shape when the forces are removed at P or E????
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uberteknik
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(Original post by Al4stair)
https://image.ibb.co/b1uFH7/20180328_122936.jpg
That link is a image of my Physics revision guide. The book shows two different points and states one is the limit of promotionality (P) and the point futher along the curve is the elastic limit (E).
The limit or proportionality: 'the greatest stress that can be applied to an elastic body without causing permanent deformation.'

Elastic limit: 'the maximum extent to which a solid may be stretched without permanent alteration of size or shape.'

Therefore these seem to be the same thing. So does the spring not return to it's original shape when the forces are removed at P or E????
Yes, in both cases the spring returns to it's original shape. The book wording is misleading.

The difference is stated in the words:

Limit of proportionality is the point after which further applied force is not supported by the Hookes law linear relationship where extension is directly proportional to the applied force. i.e. up to that point, Hookes law applies, after that point it does not. It is. however, still elastic. NB when the force is removed, the spring will still return to it's original shape.

Elastic limit is the applied force beyond which permanent deformation results after removal of the applied force. The spring is no longer elastic but exhibits plastic behaviour.
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Al4stair
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(Original post by uberteknik)
Yes, in both cases the spring returns to it's original shape. The book wording is misleading.

The difference is stated in the words:

Limit of proportionality is the point after which further applied force is not supported by the Hookes law linear relationship where extension is directly proportional to the applied force. i.e. up to that point, Hookes law applies, after that point it does not. It is. however, still elastic. NB when the force is removed, the spring will still return to it's original shape.

Elastic limit is the applied force beyond which permanent deformation results after removal of the applied force. The spring is no longer elastic but exhibits plastic behaviour.
Hi. Thanks for the response and sorry for the extreme delay.
So when the forces are removed from the string after the limit of proportionality has been reached (but before the elastic limit) has been reached, the spring will still go back to its original shape? Unlike the definition I said above which claimed it would have 'permanent deformation'. So a more accurate definition of the limit of proportionality would be this?...
'The limit of proportionality is the is the point beyond which Hooke’s law is no longer true when stretching a material.'
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uberteknik
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(Original post by Al4stair)
Hi. Thanks for the response and sorry for the extreme delay.
So when the forces are removed from the string after the limit of proportionality has been reached (but before the elastic limit) has been reached, the spring will still go back to its original shape? Unlike the definition I said above which claimed it would have 'permanent deformation'.
Correct

(Original post by Al4stair)
So a more accurate definition of the limit of proportionality would be this?...
'The limit of proportionality is the is the point beyond which Hooke’s law is no longer true when stretching a material.'
You need to spell it out and state both force and extension in the answer and not simply reference Hooke's law.

A better answer would be:

The limit of proportionality is the point beyond which extension is no longer a linear function of the applied force as defined by Hooke's law.
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Dr Keith
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This answer is incorrect. Once you stretch the material past its elastic limit, it will not return to its original length/shape. If you stretch it past the limit of proportionality, but not past the elastic limit, it will remain elastic (return to its original configuration).Beyond the elastic limit there will be plastic behaviour - deformation.
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Rhys_M
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#6
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Well, at A-level then the difference depends on the exam board.For AQA you must say limit of proportioality, but OCR mark schemes specifically state that you must say elastic limit - both in the same context.
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foofung
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#7
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(Original post by Rhys_M)
Well, at A-level then the difference depends on the exam board.For AQA you must say limit of proportioality, but OCR mark schemes specifically state that you must say elastic limit - both in the same context.
I'm doing OCR currently and it's quite weird- OCR definitely favours the elastic limit. The limit of proportionality only seems to brought up when specially referring to a stress-strain graph to which materials always follow Hooke's law, while for the elastic limit "materials may follow the elastic limit up to this point but not always.". Slightly contradicts what was stated before but that was specifically for force-extension graphs.
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