# Ampere and charge density

1. May 7, 2004

### Mica

Hi all,

I cann't figure out the relationship between a current and the charge density. I have a current Io which circulated in a hollow cylindrique, how can I related with the charge density?

Something like charge density = Io * ....

Mica

2. May 7, 2004

### drag

A conductor has no net charge, current is just the flow
of electrons inside it. If you do have a net charge then
you need to devide it by the volume (C/ft^3). If, perhaps,
you meant current density - then you need to devide the
value of the current through a cross section area by the
size of that area (Amperes/square inch).

Live long and prosper.

3. May 7, 2004

### Mica

Well, I'm trying to find the potentiel of an circular ring. The formula is :

V = lamda * a /(2*eo*(a2 + d2)1/2)

all the parametres are known expect one is the lamda. Lamda is the lineaire charge density so, if I know the current and should know the charge density?

Thanks,

Mica

4. May 7, 2004

### drag

Is your ring open or closed, and connected to something
else - a circuit or something ? If you just have charge
on a ring then there's NO current, and if there's just current
in a circuit there's NO charge to create an external potential
outside the ring. Perhaps if you describe the whole problem

Live long and prosper.

5. May 10, 2004

### Mica

Hi,

I have a coil which I will applied a current into. I want to know the voltage which circuled between the conductor rings. So, I think that if I can calculed one ring, it will be the same for other rings. Is it possible?
Thanks,

Mica

6. Jul 8, 2004

### eJavier

Current= charge density * velocity of the charges

7. Jul 8, 2004

### eJavier

Perhaps this will help:
1.- Voltage = Inductance * d (Current)/dt

2.- Voltage = d (flux) / dt

If you calculate for one ring and want to calculate for the whole coil, you'll have to assume that in the coil all the current flows in a circular path, which may be a good aprox. and then integrate the whole thing

8. Jul 13, 2004

### Mica

Thanks for your reply. I have found something in electronic circuit. I have saw the term that you have provided to me which is:
Voltage = Inductance * d(current)/dt but I have found the whole equation.

Inductance * d(Current)/dt + Current * d(Inductance)/dt + Resistance * Current = 0

If I decompose the terms,

Voltage = Inductance * d(current)/dt for one ring
Voltage = Resistance * Current for lost in Ohmic
Voltage = Current * d(Inductance)/dt , what is this term for or means ?

Thanks,

Mica

9. Jul 14, 2004

### eJavier

The equation above is for time variant Inductances. It comes from noting these:

$$V= \frac{d\phi}{dt}$$

where V= voltage and $$\phi$$ is the flux of magnetic field. For linear inductors:

$$\phi= L i$$ where L is the inductance and i is the current

therefore

$$V= \frac{d\phi}{dt} = L\frac{d i}{dt}+ i \frac{d L}{dt}$$

But in most situations the inductance is time invariant, therefore the second term vanishes

Last edited: Jul 14, 2004
10. Jul 14, 2004

### Mica

Thanks for the details. If the inductance is time variant, then how can I calculated the voltage between two rings of the coil?

Mica

11. Jul 14, 2004

### eJavier

I think you should have some extra information to get the $$\frac{dL}{dt}$$. For instance, some info like $$L(t)$$

Last edited: Jul 14, 2004
12. Jul 15, 2004

### Mica

I mean to calculed the voltage between two rings, if the inductance is time variant, I have to add this term $$\frac{dL}{dt}$$?

Mica