Why is the Ampere a base unit?

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    Ampere Base Unit
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Discussion Overview

The discussion revolves around the rationale for the Ampere being designated as a base unit in the International System of Units (SI), as opposed to the Coulomb. Participants explore the fundamental nature of current versus charge, measurement practicality, and the implications of technological considerations.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question why the Ampere is a base unit instead of the Coulomb, suggesting that charge may be more fundamental than current.
  • Others argue that it is easier to measure current than charge, which may influence the designation of the Ampere as a base unit.
  • One participant notes that the SI system is practical and that base units are chosen based on technological and commercial importance, rather than purely philosophical considerations.
  • There is mention of a proposed redefinition of SI base units, indicating that while the Ampere may be redefined, it will remain a base unit for electrical measurements.
  • Some participants highlight that charge is often used to describe static effects, which can be variable without proper isolation, making current a more stable measurement in practical applications.
  • The concept of "Amp hour" is discussed as a specification for batteries, indicating a relationship between charge and current in practical usage.
  • One participant suggests that if electric charge is not used for static effects, it can be easily measured by multiplying current by time.

Areas of Agreement / Disagreement

Participants express differing views on the fundamental nature of current versus charge and the reasons for the Ampere's status as a base unit. There is no consensus on the superiority of one perspective over the other.

Contextual Notes

Participants mention the practical challenges in measuring charge and the variability of static charge without proper conditions, which may affect the discussion's conclusions.

Isaac0427
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Just out of curiosity, why is the Ampere a base unit and not the Columb? It seems to me that charge is more fundamental than current. Thanks!
 
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It's easier to measure a current than a charge.
 
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axmls said:
It's easier to measure a current than a charge.

And there are also a number of other reason. One very important property of the base units is that they are technologically/commercially important.. People forget that the SI is mainly a practical system for use in our everyday lives (which is why the Candela is a base unit) ; meaning practical considerations are more important that what is most satisfying from a philosophical point of view.

The re-definition of the SI will see the Ampere (as well as a few other units) be re-defined, but it will still be the base unit for the electrical units,
 
Ampere is simply related to Coulomb, although not as unity. In a real life and especially in techniques, charge is used to describe static effects, which without strong isolation and shielding are not constant, because of a various forms of leakages. Much easier and cheaper is to stabilize dynamic situation of charge flow, ie electric current measured in Amperes. Additionally, charge flow generates most effects of electrodynamics, including energetic and "electronic" we are surrounded.
 
zbikraw said:
...charge is used to describe static effects, which without strong isolation and shielding are not constant,
The amount of charge a battery can energize is an important specification. It basically tells you how big the battery is or how long it can last.
 
David Lewis said:
The amount of charge a battery can energize is an important specification. It basically tells you how big the battery is or how long it can last.

And usually you get this specification in an albeit convoluted form, the "Amp hour", all though on its own this is quite useless, typically you also want to know how many joules you can get out of each of those charges...
 
If electric charge (the physical quantity) is not used to describe static effects then it's easy to measure. Multiply current by time.
 

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