# Ampere definition limitations (1 Viewer)

#### Bob99

##### Member
Hi all, could someone please explain the limitations of each of the following definitions?

Old definition:
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10^-7 newton per metre of length.

2019 New revised definition: The ampere is a measure of the amount of electric charge in motion per unit time ― that is, electric current. One ampere is the current in which one coulomb of charge travels across a given point in 1 second. One coulomb is equal to about 6.241 x 10^18 electric charges (e).

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#### Run hard@thehsc

##### Well-Known Member
What trial paper did you get that q from if you do not mind?

#### Run hard@thehsc

##### Well-Known Member
What trial paper did you get that q from if you do not mind?
seems like a q I have seen before - so I can help you with that

#### Bob99

##### Member
What trial paper did you get that q from if you do not mind?
I am not sure what trial paper that is from. A friend of mine sent it to me, and asked to post it here. But any help will be greatly appreciated.

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#### Bob99

##### Member
seems like a q I have seen before - so I can help you with that
Mate are you gonna tell me anytime soon?

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#### Bob99

##### Member @vivekjain0908 why are you deleting your messages. I have no clue what you are on about. I just want an answer for the question that my friend had.

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#### Run hard@thehsc

##### Well-Known Member
Mate are you gonna tell me anytime soon?
oh sos - I remember answering that the old definition was based on the accuracy of the length of a metre and that the new one is based on the accuracy on the measurement of time - both of which is not absolute as we now know due to special relativity. Not sure if this is right, but this is what I wrote about. Please correct me if I am wrong!!!

• Bob99

#### wizzkids

##### Member
Rather than re-state the definitions of the fundamental SI units that are readily available on the Internet, I would refer interested students to this article from the United States National Institute of Standards and Technology, about the reason for the change in 2019.
https://www.nist.gov/si-redefinition/ampere-introduction

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#### bruhwth

##### New Member
oh sos - I remember answering that the old definition was based on the accuracy of the length of a metre and that the new one is based on the accuracy on the measurement of time - both of which is not absolute as we now know due to special relativity. Not sure if this is right, but this is what I wrote about. Please correct me if I am wrong!!!
isn't the value of 1m a constant value? 1m = distance light travels in 1/3x10^8 secs? c is constant so defining a metre by a constant should make the value of 1m constant no? Same with time. 1 second is defined as the time taken for roughly 9.2 x10^9 energy transitions of caesium -133 atoms, which is a constant too. IDK correct me if I'm wrong

#### Run hard@thehsc

##### Well-Known Member
isn't the value of 1m a constant value? 1m = distance light travels in 1/3x10^8 secs? c is constant so defining a metre by a constant should make the value of 1m constant no? Same with time. 1 second is defined as the time taken for roughly 9.2 x10^9 energy transitions of caesium -133 atoms, which is a constant too. IDK correct me if I'm wrong
I would assume that to be right - what I meant before was how they had a literal rod/stick which was measured to be 1 m. But this could vary (since length contracts across reference frames based on relativistic speeds). Hence, as you had proposed, they redefined it to be the time that light takes to travel a metre since the speed of light is constant for all observers.