two infinitely long parallel wires?

[Vay]

I was reading the book "General Chemistry" by Linus Pauling and i stumbled upon a confusing part where it explains the IS units of coulomb.

The book explained Coulomb; "The coulomb is one-ampere second, and the ampere is defined as the current in each of two infinitely long parallel wires 1 meter apart that causes an electromagnetic force of 2 x 10^-7 newton per meter of its length to act on each wire." as quoted from the book.

 

This definition i did not just get from the book, i searched up online and the other sources also said infinitely long parallel lines.

 

I am confused at the "infinitely" long parallel wire part.

What is this infinitely long parallel wire? Are infinitely long parallel wires even possible?

[hermanntrude]

they may or may not be possible but you don't have to have some to be able to measure the size of an Ampere experimentally.

 

What you do is measure the current in long wires, then longer wires, then the longest wires you can find, and plot a graph of length of wire vs current. Hopefully if you took enough points you will see that the current tends toward a maximum. You can then find the value of the current at the aymptote.

 

However it's much easier to use another definition of an amp, which is one coulomb per second. Also using Faraday's constant you can define it in terms of the number of electrons per second.

[Kaeroll]

The theoretical limit of infinity comes up here and there in definitions. e.g. plots of potential energy between particles being measured from infinite separation. It is, as hermanntrude said, simply an extrapolation from values that we can measure.

[Vay]

they may or may not be possible but you don't have to have some to be able to measure the size of an Ampere experimentally.

 

What you do is measure the current in long wires, then longer wires, then the longest wires you can find, and plot a graph of length of wire vs current. Hopefully if you took enough points you will see that the current tends toward a maximum. You can then find the value of the current at the aymptote.

 

However it's much easier to use another definition of an amp, which is one coulomb per second. Also using Faraday's constant you can define it in terms of the number of electrons per second.

 

It is found that the work that must be done to bring two unit charges (each with 1 Stoney or 1.054830935 x 10^-5 coulomb) from infinity to the distance 'r' is 1/rJ. Considering that force of attraction is equal to:

K x e1 X e2 divided by r^2

K=unit of measurement for electric charge

e1= positive charge

e2= negative charge

r= distance

J= Joules

 

And work = force X distance

 

is this implication or whatever assumption of actions needed to bring from infinity meters away between two charges to the distance 'r' apart is 1/rJ, in context the same as what you said?

 

It seems that this is the gap between two charges from infinity to 'r' and the one i fore mentioned is the length of the infinitely parallel lines with the charges.

Edited February 20, 2009 by Vay

[hermanntrude]

You seem to be saying that it is the distance to be vaired, rather than the length of the wires. I don't claim to know much about the definition of an ampere but the decription you give in the original post suggests the opposite: that the wires are kept 1 meter apart but are infinitely long. There is no mention of infinite distance between them.

[Vay]

You seem to be saying that it is the distance to be vaired, rather than the length of the wires. I don't claim to know much about the definition of an ampere but the decription you give in the original post suggests the opposite: that the wires are kept 1 meter apart but are infinitely long. There is no mention of infinite distance between them.

 

this is the whole description from the book:

"With use of coulomb's law for the force as a function of distance, as represented in this figure, we can calculate the amount of work that must be done to bring the two unit charges from an infinite distance apart to the distance 'r' apart."

 

the figure is a diagram illustrating the inversely square law of electrostatic repulsion: force is inversely proportional to the square of the distance between the charges. The numbers correspond to two 1 stoney charges.