# why do we need current?

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#### mvenkat3272

##### Newbie level 5
we all know current is the rate of flow of charges.In the maxwell's equations in electromagnetics why do we need current density and why not we use charges instead of currents

#### Mr.Cool

some of us do

guess i should give a better answer...

we "need" current density because of the way in which we construct our circuits. by this i mean, we always feel the need to connect our grounds (reference) together at one point. the circuit then becomes closed loop and it is because of the closed loop that a forceful pushing electrons are required.

do we need to close our loops to shuttle energy between different nodes in a circuit? ahh.. now that is another matter.

see example 2.1 on page 40 of this excellent FREE online book on subject of electromagnetic fields.

you'll read it and realize that the caveat is the need for magnetic monopoles which are commonly believed to be impossible.

often our electromagnetic equations are built on the assumption that magnetic equations are similar to electric equations. but one difference is an electron can pour out a field in one direction only while a magnet pours out two fields.

how to get a magnet to pour out ONE field, such that it then is a true equivalence of the elctron AND can be used in the cited equation? this then is the challenge. a challenge that has been solved (2 unique ways) but not reported.

i guess i am strolling off topic, hopefully i answered your question somewhere in there.

Mr.Cool

### mvenkat3272

points: 2

#### dineshbabumm

mvenkat3272 said:
we all know current is the rate of flow of charges.In the maxwell's equations in electromagnetics why do we need current density and why not we use charges instead of currents
There are currents which are not due to charges too which gets accomodated in Maxwell's equation! For instance,when we add J in maxwell equation,it includes all currents such as Source current,drift current,diffusion current,convection current etc..

So we are accomodating all the currents which are not due to charges as well!

The statement curren is the rate of flow of charges hold good only for those due to particles! Guess you wil be understood by now!

#### rrumpf

##### Full Member level 5
Charge does not move or it is called current. Current cannot be static or it is called charge. You can have current without charge and you can have charge without a current. That is why both are needed in Maxwell's equations.

#### Mr.Cool

is static REALLY static? put a microscope on it, and you see the elctron as being violently energetic even when you thought it was static!!

maybe we ought to start building our machines as nature would prefer it to be operated, rather than as a human prefers it to be operated.

oh.. there i go again, ruining a perfectly good thread ;P

Mr.Cool

#### azaz104

##### Member level 5
see example 2.1 on page 40 of this excellent FREE online book on subject of electromagnetic fields.
Mr.Cool
where is this book ? can you give us the link ?

#### Mr.Cool

good question.

it used to be here:

h**p://www.plasma.uu.se/CED/Book/

but now i try it and the website is not responding. send me a private message if you can't log onto the website and i will post the book

Mr.Cool

ps.. i checked the site again a few days later and its still down.. dunno, so i post the book here:

h**p://www.sendspace.com/file/pcsexq

#### raghuram_msc

##### Full Member level 1
Dear Friends,

we all know current is the rate of flow of charges.In the maxwell's equations in electromagnetics why do we need current density and why not we use charges instead of currents

Obviously for all good reasons we dont talk about maxwellian equations for low frequencies, where things look pretty sweet and simple. All hell breaks loose when the applied signal's wavelength becomes comparable with that of the circuits dimentions. lets take a simple conductor....for low frequency signals (Lambda>>conductor length) though field propagation is still random but there is a net movement of charge over the length of the conductor......where as for HF (ie., Lambda<= conductor Length) the field variation within the conductor is so high that we cannot talk of any net movement. This is where all ugly effects (Like "skin effect") and all also start dominating the scene.......

So our clever Maxwell thought that it would be quite unwise (i mean impractical) to talk of instantaneous voltages (or currents) and rather thought it was wiser (Imean more practical) to talk of fields than mere charges, as there are lot of uncertainities involved.

Please tell me if went wrong....!!

Sai

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