Will you get an AC current in this situation?

[mrmojorising]

Say you have a conductor (a single straight wire) which goes from left to right across your computer screen and moves up and down in periodic motion in a constant magnetic field that goes into your computer screen (see diagram below).

(note: you can ignore the computer screen, and you can ignore the Earth's magnetic field. i am just trying to set up a situation where a wire moves orthogonally to a constant magnetic field and am using the computer screen, etc. for reference to make the situation more explicit)

The charges in the wire will be subject to a force from the formula F=qvxB, and this force will create periodic motion of the charges since the wires motion is period. This motion will cause a current since the force will cause the charges to move and by definition current is dq/dt.

My questions:

Will a typically current meter/anemometer connected to both ends of the wire with leads record an rms value for the current? (i think no)

Will a touch-less current clamp anemometer (http://en.wikipedia.org/wiki/Current_clamp) placed around the wire record an rms value for the current? (my guess is yes)

Say you connect a stationary conductor to the moving conductor described above (a stationary single wire connected to the moving single wire by a piece of flexible conductor). Does that electric field created by the charges in periodic motion on the moving conductor prorogate to the stationary conductor? If the electric field does propagate to the stationary single wire does it induce periodic motion of the charges and thus an AC current in the stationary single wire?

more explicitly:

---------- <----- wire


^
|
| <------- direction of wires motion (periodically up and down)
|
v


x x x x x x <----- direction of constant magnetic field into which wire is placed (into computer screen)

 

[dlgoff]

I'm wondering if this might be homework questions and what level of math you are at. Anyway, take a look at http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/coursenotes/index.htm", to see if it might answer some of your questions.

 

[sophiecentaur]

It could depend upon the sort of screen you're using, for a start. Or you could be using the Earth's magnetic field, perhaps? What exactly is the scenario?

 

[mrmojorising]

It could depend upon the sort of screen you're using, for a start. Or you could be using the Earth's magnetic field, perhaps? What exactly is the scenario?

i wasn't clearn in my original statement below. i don't mean literally in front of your computer screen, and you can ignore the Earth's magnetic field. I am just trying to set up a situation where a wire is moving orthogonally to a constant magnetic field, and i was just referring to computer screen, and up down to make it more explicit (easier to imagine). This isn't homework, just something i don't get.

Say you have a conductor (a single straight wire) which goes from left to right across your computer screen and moves up and down in periodic motion in a constant magnetic field that goes into your computer screen (see diagram below).

 

[sophiecentaur]

When you reach the end of your travel and go back the other way, then the direction of induced emf will reverse - and so on. That's AC.
Consideering the whole loop, if the return length is not moving, then no contra- emf is generated in that leg and a current will flow because there will be a net emf. You could also look upon it as changing the area of your loop of wire and, hence, the total flux through it - that will also generate an emf. (a dφ/dt)

Sorry for not catching on to the setup. I thought the screen was an integral part of things durrr.

 

[mrmojorising]

When you reach the end of your travel and go back the other way, then the direction of induced emf will reverse - and so on. That's AC.
Consideering the whole loop, if the return length is not moving, then no contra- emf is generated in that leg and a current will flow because there will be a net emf. You could also look upon it as changing the area of your loop of wire and, hence, the total flux through it - that will also generate an emf. (a dφ/dt)

Sorry for not catching on to the setup. I thought the screen was an integral part of things durrr.

I never mentioned anything about a loop -- i just mean a straight length of wire, not a loop.

 

[sophiecentaur]

If you are to measure an "Alternating Current" then you need to attach a meter between the two ends of your wire. This will form a loop. You can't avoid completing the circuit if you want to make a measurement.

 

[mrmojorising]

If you are to measure an "Alternating Current" then you need to attach a meter between the two ends of your wire. This will form a loop. You can't avoid completing the circuit if you want to make a measurement.

not if you use a current clamp -- it can measure the current without forming a loop, using induction.

http://en.wikipedia.org/wiki/Current_clamp

 

[I_am_learning]

Will a typically current meter/anemometer connected to both ends of the wire with leads record an rms value for the current? (i think no)

Will a touch-less current clamp anemometer (http://en.wikipedia.org/wiki/Current_clamp) placed around the wire record an rms value for the current? (my guess is yes)

Q1: Typical meters won't record for the current will be too small. But for the purpose of theoretical analysis, There will be some current through the meter. Why do you think 'no'?

Q2: Anemometer won't record current for there will be none*.
*Only for a very short time, during, reversing direction.
Because, once the charge are pushed to the sides (it is only duing pushing that current flows), they accumulate to the side,and then create their own Electric Filed, that opposes further charge being pushed there.

 

[sophiecentaur]

Yes you will induce an emf due to the movement and this will polarise the wire whilst you are moving. Once you stop or reverse the motion, the charges will flow back. (Same as in an antenna, which has AC induced in it).
But there will be no current actually at the ends of the wire - due to the boundary conditions (an open circuit at each end).
You could put sensitive ammeters in series with the wire, along its length and they would each give different readings, with a maximum value in the middle.

 

[mrmojorising]

Q1: Typical meters won't record for the current will be too small. But for the purpose of theoretical analysis, There will be some current through the meter. Why do you think 'no'?

Q2: Anemometer won't record current for there will be none*.
*Only for a very short time, during, reversing direction.
Because, once the charge are pushed to the sides (it is only duing pushing that current flows), they accumulate to the side,and then create their own Electric Filed, that opposes further charge being pushed there.

Then why does an AC circuit with a capacitor have AC current in it? The charges accumulate on the plates of the capacitor and create their own electric fields which oppose further charge being pushed. Is it the fact that capacitors have 2 oppositely charged plates which effictevly cancel out any E field from a single plate which would oppose the movement of the charges?

 

[sophiecentaur]

I don't think this is worth discussing any more. It's just some basic bookwork and not really a very fruitful topic to have 'ideas' about.
I have no idea what your last sentence is supposed to mean - "cancel out any field from a single plate"??
I think you should do some reading and learning about induction with a bit of antenna theory. It would all become clear then.

 

[I_am_learning]

Then why does an AC circuit with a capacitor have AC current in it? The charges accumulate on the plates of the capacitor and create their own electric fields which oppose further charge being pushed. Is it the fact that capacitors have 2 oppositely charged plates which effictevly cancel out any E field from a single plate which would oppose the movement of the charges?

Yeah quite true. Using just a single plate would quickly saturate (accumulate enough charge to create E field to oppose the applied E filed), So, AC won't flow. But using two plates you got to input a lot more of charge to get that opposing E field or the opposing voltage. (remember v = q / c), so the current flows during the charge flow into the plate. In dc it stops after some time. In ac it flows continuously since the applied voltage changes polarity quickly. There are calculus involved in quantitative analysis of how much time it takes for how much charge to accumulate; but this can give you rough ideas I hope.

 

[sophiecentaur]

The capacity between the your single plate and the rest if the wire (??) will just be very small wrt the capacity between two plates. This presents a very high reactance - not surprisingly. The current will be correspondingly small. For a wire with two bare ends the capacity will be even less and an even smaller current will flow.
Where are the surprises in all this?