# Faraday's law for electrostatics

1. Jul 27, 2014

### Axe199

I recently learned about faraday's law about electromagnetic induction , where you move a piece of magnetic towards and away from a coil of conductor, what if we replaced the magnet with an electret ( electrostatic equivalent to permanent magnet) and we moved it towards and away from a grounded metal disk , there will be current in the wire we used to ground the disk , is there any formula to calculate this current ??
why no one on the internet is mentioning this , except for 1 invention by tesla , which is basically a generator but instead of using magnets he's using plates charge by a wimshurst machine , and the current was produced by the repulsion/attraction between the electrons and the charged plates

2. Jul 28, 2014

### Staff: Mentor

The problem is that the current generated by moving a charged object toward or away from a grounded object is very, very low unless you use exceedingly high voltages that aren't realistically feasible to achieve. It's also very difficult to keep an object charged, as even with extremely good insulators you'd experience a gradual leakage of charge from the object. Magnetically based generators are easier, simpler, and cheaper to use.

3. Jul 28, 2014

### Axe199

so i didn't bother to do it because it's not efficient, or because it's hard to measure?

4. Jul 28, 2014

### Staff: Mentor

I don't understand your question.

5. Jul 28, 2014

### Deco56

Very good post! Yes there is displacement current associated with moving or pulsating electrostatic fields, similar to capacitors.....displacement current is still not well studied! People on this site seem to have a disdain towards Tesla, glad you mentioned it.

6. Jul 28, 2014

### Staff: Mentor

I seriously doubt that.

7. Jul 28, 2014

### Deco56

Doubt is good! I asked MIT prof. Walter Lewin why the displacement current is a misnomer (physicists believe it is NOT a current) when it is specifically measured in AMPERES.

Now, I don't know about you, but if you tell me something is measured in JOULES I will call it energy or work!

8. Jul 28, 2014

### Staff: Mentor

Per wiki:http://en.wikipedia.org/wiki/Displacement_current

In electromagnetism, displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field. Displacement current has the units of electric current density, and it has an associated magnetic field just as actual currents do. However it is not an electric current of moving charges, but a time-varying electric field. In materials, there is also a contribution from the slight motion of charges bound in atoms, dielectric polarization.

I believe the displacement current is measured in amps because it represents the amount of current required to produce a magnetic field equal to the magnetic field produced by the time-varying electric field.

9. Jul 28, 2014

### Deco56

You are free to believe that, but it makes no sense to measure something in AMPERES and for it not to be a current. Again, if I measure something in JOULES it must be energy or work. If it represents the magnetic field then why is it not measured in Teslas?

10. Jul 28, 2014

### Staff: Mentor

It makes perfect sense if doing so allows you to use existing equations and laws to solve a problem.

It doesn't represent the magnetic field itself, but the current required to produce the magnetic field. Do you see the distinction?

11. Jul 28, 2014

### Deco56

You can not have it both ways. Either you measured it in AMPERES and can measure it in an ammeter and call it a current or the theory is flawed. Name me another example where I would measure something in KELVIN but wouldn't call it the temperature! Ridiculous...

12. Jul 28, 2014

### Staff: Mentor

Nonsense. There's no reason we can't use the term current to refer to an imaginary current flow and then measure it in amps. As I said, if it allows us to use existing equations to solve problems then it's fine.

Easy. Many light bulbs are labeled as having a "color temperature", measured in kelvin. This doesn't represent the temperature of the bulbs, but the temperature of a hypothetical black body that would emit approximately the same spectrum of light as the bulb. So a bulb labeled as 5,000 k is not at that temperature.

13. Jul 28, 2014

### Deco56

These are comparative tools for specific application.

Maybe you don't get the question. If I am measuring it in Amperes, what do you suppose we call it? How can we call it a current but then in the same sentence say it is not a current. It's true that it is NOT an electronic current (no electrons flowing), but to not consider it a current is disingenuous. Even Oliver Heaviside saw displacement current as a current!

14. Jul 28, 2014

### Staff: Mentor

To be clear, if we have a physical dielectric between two plates of a capacitor we actually do have a movement of charges in response to this time varying electric field. The charges of the dielectric will move and polarize in response to this field, so there is a type of current. If we have empty space then we don't have a movement of charges.

Yet in both cases a magnetic field will develop between the plates that can be mathematically described as being the result of current flow even if no charges are actually moving.

I think the last sentence is of critical importance. Mathematical terms do not always refer to something in reality, which is both a good thing and a bad thing. As long as the math is giving us the right answer, it doesn't matter if it refers to something real or not. That's why it's important to understand what is physically happening as well as understanding the math.

Historically, displacement current as was originally developed by Maxwell focused on a dielectric medium that could become polarized. This has since been expanded to include free space as well. See here: http://en.wikipedia.org/wiki/AmpĂ¨re's_circuital_law#Displacement_current

Maxwell's original explanation for displacement current focused upon the situation that occurs in dielectric media. In the modern post-aether era, the concept has been extended to apply to situations with no material media present, for example, to the vacuum between the plates of a charging vacuum capacitor. The displacement current is justified today because it serves several requirements of an electromagnetic theory: correct prediction of magnetic fields in regions where no free current flows; prediction of wave propagation of electromagnetic fields; and conservation of electric charge in cases where charge density is time-varying.

15. Jul 28, 2014

### Deco56

".... of magnetic fields in regions where no free current flows;"

Yet, you say :"To be clear, if we have a physical dielectric between two plates of a capacitor we actually do have a movement of charges in response to this time varying electric field. The charges of the dielectric will move and polarize in response to this field, so there is a type of current."

So first you say electrons move and create current flow in capacitor, then your link says there is no free current flow. First you say the displacement current is not a type of current, only measured in amperes then you say it IS a type of current? You can't have it both ways.

You are wrong, this experiment would work if no dieletric is present, vacuum. This is due to the fact the vacuum is not actually empty.

16. Jul 28, 2014

### Staff: Mentor

That is incorrect. It works because a time varying electric field gives rise to a magnetic field.

17. Jul 29, 2014

### Axe199

ok, i have no idea what you guys are talking about , but i watched some videos, and this what i understood so correct me if i am wrong , displacement current is created ( mainly in capacitor) due to electric flux , and it's a different kind of current than the conduction current , in a circuit with a capacitor , it's the displacement current that makes up for the gap

18. Jul 29, 2014

### Staff: Mentor

That's right. Also, for your original question, a capacitor acts in a similar way to your charged object and metal disk. By that I mean that the charging of the plates requires the removal or addition of charges on each plate. This movement of charges onto or off of each plate constitutes a flow of current.

In your original example, we can measure the movement of charges through a section of the wire grounding the disk and therefor get the current flow.

19. Jul 29, 2014

### Axe199

@drakkith thanks , i have one last question , it's about sparks ( discharges ) should i ask it now or make another thread?

20. Jul 29, 2014

### Staff: Mentor

If it's a detailed question, make another thread. If it's something simple, feel free to ask here.

21. Jul 29, 2014

### carrz

a.) moving charges
b.) time-varying electric field

This is vague and ambiguous. What exactly is "electric charge" compared to "electric field" and how are they not one the same thing? Is it not electric field of moving charges "time-varying", in that it changes its position over time? What property of "time-varying electric field" is actually varying, is it magnitude, position, maybe velocity, or what?

22. Jul 29, 2014

### Staff: Mentor

An electric charge is a particle carrying an electric charge of +1 or - 1.
A description of the electric field can be found here: http://en.wikipedia.org/wiki/Electric_field

In this example, the magnitude of the field is changing over time.

23. Jul 29, 2014

### carrz

I believe it is just as valid to say that electric field carries an electric charge of either + or - polarity, in a sense where "charge" is kind of synonym of "magnitude".

Also, it seems to me, "charge" is implicit reference to electric field of some "particle" rather than some kind of little ball in the middle of that field.

How can we even know if in the center of an electron's electric field there is actually some little ball made of something else but just an electric field itself?

24. Jul 29, 2014

Staff Emeritus
Then you believe wrongly.

If you have questions, please ask them. However, this forum is not about personal speculation; it is about physics as it is generally understood.