why ampere is fundamental unt in S.I rather than charge

[krates]

why ampere is fundamental unt in S.I rather than charge

I = Q/T

( Q= Charge ; T= time ; I= current)

So why current is a fundamental unit ??

it should be a derived unit man

Please give me a proper explanation.

[slugger]

the only electrical paper i had to scrap through was held eons ago, but i'll take a shot

Current gives a more accurate representation of electrical quantitiy flowing through a body. It would be a better indicator of weather a body is in any electrical danger or not.

Lets consider charge of 50 coulombs flows through a body. It can mean many things

100 amperes curent flows through the body in 0.5 seconds [hazard]
OR
0.1 ampere current flows through the body in 500 seconds [benign]

if one were to design an insulator for such a body, simply stating the charge that flows through the body, would not be sufficient

On the other hand if you state that a current of 100A flows through the body, then it gives you a much clearer idea of the electrical activity in the body

100 coulombs charge flows in 1 second
OR

300 coulomb charge flows in 3 second

So an insulator that is designed to handle 100 couombs charge in 1 second will also handle 300 coulomb charge in 3 seconds, thus stating much clearer design parameters

due to the insuffciency of data provided by stating charge, it has been demoted to a derieved unit ampere-second

you might want wait for sum Electrical/E&TC person to give u an explanation - i quite hated the subject

[Psychosocial]

OMG......I am fainting .

[krates]

slugger easy words plz

like i have to write in my physics project

[dheeraj_kumar]

I'll use a different example to explain slugger's post.

Think of the hottest girl you can think of, standing under the shower.

She can turn the shower valve to the full, for the fastest bath ever, using lots of water in a short time. Or she can turn the valve to minimum and enjoy a nice calm long bath, using lots of water in a long time.

Both ways cause lots of water to be used, but what's different is the flow of water - fast or slow. She can experience a pummeling forceful water or soft tendrils of soothing water.

Same way, amount of water = charge. Flow of water = current (literally too!)

Another example would be using a full petrol tank to drive say 20 km very fast, or 40 km slowly. Cooking gas at minimum cooks the food, maximum burns it.

[mehra.rakesh]

Well ,, from what i remember 1 ampere was not defined as 1C/second ya it is same but actual defn for ampere is ..

Its defn is rather a constant current flowing thru two straight conductors & creating a force of some mag derived 4m ampere's force law per m of length .. obviously in vacuum ..

So using this ^^ defn ampere becomes free of any other units namely charge ...

Simply put its easier to measure current using above method rather than counting each electron passing in a sec

[sreevirus]

From the Electrical Engineer's Reference Book by Laughton and Warne:

In general, charges may be at rest, or in motion, or in acceleration. At rest, charges have around them an electric (or electrostatic) field of force. In motion they constitute a current, which is associated with a magnetic (or electrodynamic) field of force additional to the electric field.

This is just a layman's definition which shows the difference between a charge at rest (a coulomb) and a charge in motion (an ampere). An ampere really is Coulombs per second. But for a current to flow, there has to be a potential difference.

To be frank, I really can't say why the Ampere is taken as a base fundamental SI unit like the kg or meter, but I guess the people who did it might have had good reasons for doing so. Convenience of measurement and ease of reproduction could be two. And then again, you'd have to change your perception of a base unit. It does not have to be something given like the kg or mole. The base unit can be used to define others. Like 1C = 1 A-sec. Thus, the coulomb could also be defined as the charge carried by a current of 1 amp for one second. Even the candela is lumens per steradian, but the candela is a base unit. I hope you got it.

This may not be really related to this topic, but if people are serious to know more, watch MIT's Prof. Walter Levin's lectures on Electricity and Magnetism. Damn interesting, engaging and definitely not boring, the way they usually teach in Indian colleges.
Here's the link to the 1st lecture: http://www.youtube.com/watch?v=3omwHv3Cmog

Edit: Here's the full list, just in case: http://ocw.mit.edu/OcwWeb/Physics/8-...ions/index.htm

[krates]

thanks