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lionely
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Homework Statement
This may be a stupid question but... I don't understand why the length of the wire has to be infinite in defining the ampere... could someone explain this to me?
lionely said:Homework Statement
This may be a stupid question but... I don't understand why the length of the wire has to be infinite in defining the ampere... could someone explain this to me?
lionely said:My textbook says... One ampere is that steady current which when flowing in each of two infinitely long, straight parallel wires of negligible cross-section placed one metre apart in a vacuum causes each wire to exert a force of
2 x10^-7 Newton on each metre length of the other wire.
That makes the "end effects" vanishingly small. You can ignore them because they are not there.lionely said:I don't understand why the length of the wire has to be infinite in defining the ampere... could someone explain this to me?
phinds said:An amp is simply a flow of one coulomb per second.
That's been the definition for a long, long time. Not an infinitely long time, but quite long nonetheless. The end effects are getting vanishingly small. (You have to be over 80 to remember the old definition.) The ampere currently is a base unit in the metric system and the coulomb is a derived unit, an ampere-second.phinds said:Weird. I've never seen that one before. An amp is simply a flow of one coulomb per second. Are you quoting this exactly? That is, are you sure that it is presented as "THE" definition of the amp, or as simply a result of the definition of the amp?
That's right. There's an ongoing effort to completely overhaul the International System of units (SI).f95toli said:The definition WILL change to "flow of X number of electrons per second"
D H said:That's been the definition for a long, long time. Not an infinitely long time, but quite long nonetheless. The end effects are getting vanishingly small. (You have to be over 80 to remember the old definition.) The ampere currently is a base unit in the metric system and the coulomb is a derived unit, an ampere-second.
No, it wasn't. Fifty years ago was 1963. The current definition of the ampere and coulomb had already been in effect for 15 years. Your teacher at that time was old enough to remember the old definition and wasn't playing attention in 1948 when the ampere and coulomb were redefined.phinds said:Bolded part is not true. I took EE 50 years ago and coulomb/sec was the definition then.
Not just to 13/14 year olds, but also to the teachers of 13/14 year olds. I don't know about Poland, but here in the US, primary school science teachers do not have to take the classical electricity and magnetism (E&M) class that vexes so many students of physics.Borek said:My bet is C/s was easier to swallow for 13/14 years old than two infinite wires attracting each other with 10-7N per meter.
D H said:No, it wasn't. Fifty years ago was 1963. The current definition of the ampere and coulomb had already been in effect for 15 years. Your teacher at that time was old enough to remember the old definition and wasn't playing attention in 1948 when the ampere and coulomb were redefined.
epenguin said:this question gives me the chance to point out something not often mentioned that I ever saw. The laws according to Wiki are:
Faraday's 1st Law of Electrolysis - The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode. Quantity of electricity refers to the quantity of electrical charge, which measured in coulombs.
Faraday's 2nd Law of Electrolysis - For a given quantity of D.C electricity (electric charge), the mass of an elemental material altered at an electrode is directly proportional to the element's equivalent weight.
In a manner of speaking you could say that Faradays laws have nothing to do with electricity, and that the first law is superfluous.
To explain - the quantity of charge transferred is measured in Coulombs. For a long time the coulomb was defined electrochemically. If you look in old textbooks you will find the coulomb defined as the amount of charge that causes some ridiculous number of grams of silver to be deposited in an electrochemical cell.* If that that defines the quantity of electricity then the first law is contained in the second.
And together they could be restated something like “chemical equivalents in electrochemistry are the same as in ordinary chemistry”.
The historical fact (I do not know the history very well) is that the same Faraday who pioneered electrochemistry also pioneered electromagnetism. So no doubt he measured the currents in his electrochemical experiments with an electromagnetic device, and formulated the laws that way. He was deep into the unity of all these things. But someone else might have done just electrochemistry alone quite well and could have formulated the law chemically without ever seeing an ammeter, and we would probably be to this day teaching the subject slightly differently.
The silver deposition lends itself to fairly accurate measurement, that is why it became the standard. But at some point it proved possible to measure better, more precisely I suppose, by electromagnetic force of a current, and they made that the standard for the ampere and the coulomb that multiplied by a time. That is what it is now.
But don’t invest too much into it because next year they are going to change it into something more chemical again! http://en.wikipedia.org/wiki/New_SI_definitions
*then the ampere that per second or maybe at one time per hour
An ampere is a unit of electric current that measures the rate of flow of electric charge in a circuit. It is named after the French physicist André-Marie Ampère.
The length of wire is one of the factors that determines the amount of electric current that can flow through it. A longer wire will have more resistance, which will limit the amount of current that can pass through it.
The definition of ampere in terms of length of wire helps us understand the relationship between current and wire length. It also allows for accurate measurement and standardization of electric current.
Yes, the length of wire can affect the accuracy of ampere measurement. Using a longer wire with higher resistance can lead to a decrease in current flow, resulting in a less accurate measurement.
Scientists use mathematical formulas and calculations, as well as different types of equipment such as voltmeters and ammeters, to determine the appropriate length of wire for a specific ampere measurement. They also take into account factors such as the type of wire and its resistance.