There is something on Lenz's law about induced current and the magnetic field that I don't quite understand. "Apply Lenz's law that when the magnet is brought near the coil, the magnet's magnetic field through the coil increases, and therefore the flux increases."
Here, why the magnet's magnetic field would increase?? No matter if it goes near the coil, I think it keeps the same, instead, the magnetic field created by the current should increase or coming to existence?
"The magnetic field of the magnet points upward. To oppose the upward increase, the magnetic field inside the coil produced by the induced current needs to point downward."
Here, why the magnetic field in the coil needs to oppose the upward increase??!!
"When the magnet is brought far away from th coil, the flux decreases, so th induced current in the coil produces an upward magnetic field through the coil that is 'trying' to maintain the status quo."
Again, why it needs to main the status quo? I don't get the purpose of all these. Hope you can explain them to me. Thank you very much.
Hello mia6
The magnetic field of the magnet is greatest, therefore more magnetic lines of force,(flux) nearer the magnet. Consequently, the closer the magnet to the coil, the more magnetic lines of flux penetrating the coil. The magnetic field of the magnet remains the same... no increase in force or flux... the penetration of the coil by the magnetic flux is determined by proximity alone.
The orientation of the N/S poles of the magnetic lines of force cutting the coil determine the polarity (direction of flow) of the current potential induced in the coil. The direction of current flow determines the orientation of the lines of force of the induced magnetic field of the coil... one direction when the magnetic flux from the inductor magnet is increasing, the opposite when it is decreasing.
Consider an analogy... the electrons in the wire comprising the coil have an affinity for the atoms comprising the wire... kinda like they are attached by rubber bands. The electrons are displaced by the magnetic lines of force of the inductor magnet creating a positive potential (voltage) that causes them to flow to areas of lower potential as long as the lines of flux displace them.. creating current flow. Remove the magnetic flux that displaced them and the tendency is for them to return to the area from which they were displaced, again creating current flow.. but in the opposite direction.
Hope this helps
Jack
The magnetic field of the magnet is greatest, therefore more magnetic lines of force,(flux) nearer the magnet. Consequently, the closer the magnet to the coil, the more magnetic lines of flux penetrating the coil. The magnetic field of the magnet remains the same... no increase in force or flux... the penetration of the coil by the magnetic flux is determined by proximity alone.
The orientation of the N/S poles of the magnetic lines of force cutting the coil determine the polarity (direction of flow) of the current potential induced in the coil. The direction of current flow determines the orientation of the lines of force of the induced magnetic field of the coil... one direction when the magnetic flux from the inductor magnet is increasing, the opposite when it is decreasing.
Consider an analogy... the electrons in the wire comprising the coil have an affinity for the atoms comprising the wire... kinda like they are attached by rubber bands. The electrons are displaced by the magnetic lines of force of the inductor magnet creating a positive potential (voltage) that causes them to flow to areas of lower potential as long as the lines of flux displace them.. creating current flow. Remove the magnetic flux that displaced them and the tendency is for them to return to the area from which they were displaced, again creating current flow.. but in the opposite direction.
Hope this helps
Jack
QUOTE (mia6+Apr 1 2008, 01:14 AM)
Here, why the magnet's magnetic field would increase?? No matter if it goes near the coil, I think it keeps the same, instead, the magnetic field created by the current should increase or coming to existence?
"The magnetic field of the magnet points upward. To oppose the upward increase, the magnetic field inside the coil produced by the induced current needs to point downward."
Here, why the magnetic field in the coil needs to oppose the upward increase??!!
"When the magnet is brought far away from th coil, the flux decreases, so th induced current in the coil produces an upward magnetic field through the coil that is 'trying' to maintain the status quo."
Again, why it needs to main the status quo? I don't get the purpose of all these. Hope you can explain them to me. Thank you very much.
The magnetic field strength of the magnet itself does not increase. The local magnetic field strength at the coil of wire increases when the magnet is brought closer to it. So, as you bring the magnet closer to the coil the magnetic flux is changing (increasing in this case), and because the coil is a conductor, a current is induced in the wire that opposes the increasing magnetic field flux at the wire. This current lasts only as long as the magnetic flux is changing, so when you stop moving the magnet towards or away from the coil of wire, no current flows.
As you put it, an excellent way to think of it is as if the coil has a "status quo". This status quo is in fact the conservation of energy.
From wikipedia -
To understand the implications for conservation of energy, suppose that the induced currents' directions were opposite to those just described. Then the north pole of an approaching magnet would induce a south pole in the near face of the loop. The attractive force between these poles would accelerate the magnet's approach. This would make the magnetic field increase more quickly, which in turn would increase the loop's current, strengthening the magnetic field, increasing the attraction and acceleration, and so on. Both the kinetic energy of the magnet and the rate of energy dissipation in the loop (due to Joule heating) would increase. A small energy input would produce a large energy output, violating the law of conservation of energy. This scenario can never happen, and is why the induced magnetic field of coil of wire must OPPOSE the change in flux at the coil.
(my bold)
It's not the most intuitive of things... also, if you've never heard of the "right-hand rule" that provides the orientation for cross products in equations such as qv x B, this would be an excellent time to learn. Just google the hell out of it until you're a guru
"The magnetic field of the magnet points upward. To oppose the upward increase, the magnetic field inside the coil produced by the induced current needs to point downward."
Here, why the magnetic field in the coil needs to oppose the upward increase??!!
"When the magnet is brought far away from th coil, the flux decreases, so th induced current in the coil produces an upward magnetic field through the coil that is 'trying' to maintain the status quo."
Again, why it needs to main the status quo? I don't get the purpose of all these. Hope you can explain them to me. Thank you very much.
The magnetic field strength of the magnet itself does not increase. The local magnetic field strength at the coil of wire increases when the magnet is brought closer to it. So, as you bring the magnet closer to the coil the magnetic flux is changing (increasing in this case), and because the coil is a conductor, a current is induced in the wire that opposes the increasing magnetic field flux at the wire. This current lasts only as long as the magnetic flux is changing, so when you stop moving the magnet towards or away from the coil of wire, no current flows.
As you put it, an excellent way to think of it is as if the coil has a "status quo". This status quo is in fact the conservation of energy.
From wikipedia -
QUOTE
To understand the implications for conservation of energy, suppose that the induced currents' directions were opposite to those just described. Then the north pole of an approaching magnet would induce a south pole in the near face of the loop. The attractive force between these poles would accelerate the magnet's approach. This would make the magnetic field increase more quickly, which in turn would increase the loop's current, strengthening the magnetic field, increasing the attraction and acceleration, and so on. Both the kinetic energy of the magnet and the rate of energy dissipation in the loop (due to Joule heating) would increase. A small energy input would produce a large energy output, violating the law of conservation of energy. This scenario can never happen, and is why the induced magnetic field of coil of wire must OPPOSE the change in flux at the coil.
(my bold)
It's not the most intuitive of things... also, if you've never heard of the "right-hand rule" that provides the orientation for cross products in equations such as qv x B, this would be an excellent time to learn. Just google the hell out of it until you're a guru
Electromagnetism is certainly not intuitive... Examples:
- When you approach a contactless smart card to the card reader, the induction increases... Remember the reader powers the card. First and last time I saw a reaction bigger than its cause. I took me some time to understand.
- People put successfully a loading capacitor on top of electrically short antennas. They explain that it increases the current in the antenna. But pity, this is at the expense of a greater opposing displacement current from the antenna top to the ground, and the displacement current induces a magnetic field just as well as the conduction current does. Still not understood.
And just for fun, a patent I read recently. It's an antenna, already a excellent start for esoteric technology.
So it comprises a flat conductor with two crossed tuned slits to radiate a circular field. The lower side is boxed with conducting material as usual. Distinctively, this being the subject of the patent, only one coaxial cable feeds (connected across one slit) the circular antenna, because the upper plate has a strip cut in it, bent to the bottom and soldered there, so both slits are energized in quadrature.
Uuuuuuuuh?
- When you approach a contactless smart card to the card reader, the induction increases... Remember the reader powers the card. First and last time I saw a reaction bigger than its cause. I took me some time to understand.
- People put successfully a loading capacitor on top of electrically short antennas. They explain that it increases the current in the antenna. But pity, this is at the expense of a greater opposing displacement current from the antenna top to the ground, and the displacement current induces a magnetic field just as well as the conduction current does. Still not understood.
And just for fun, a patent I read recently. It's an antenna, already a excellent start for esoteric technology.
So it comprises a flat conductor with two crossed tuned slits to radiate a circular field. The lower side is boxed with conducting material as usual. Distinctively, this being the subject of the patent, only one coaxial cable feeds (connected across one slit) the circular antenna, because the upper plate has a strip cut in it, bent to the bottom and soldered there, so both slits are energized in quadrature.
Uuuuuuuuh?
QUOTE (am_Unition+Apr 1 2008, 03:29 AM)
It's not the most intuitive of things... also, if you've never heard of the "right-hand rule" that provides the orientation for cross products in equations such as qv x B, this would be an excellent time to learn. Just google the hell out of it until you're a guru
Some physics guru should weigh in on the discussion between Gavilan and I about Lenz's law vs. Faraday's law starting here in the "Nasa Baffled By Force Acting On Space Probes" thread.
Some physics guru should weigh in on the discussion between Gavilan and I about Lenz's law vs. Faraday's law starting here in the "Nasa Baffled By Force Acting On Space Probes" thread.
Said discussion is off-topic. No explanation for the Pioneer Anomaly, as was pointed several times.
Said discussion is moderately interesting as this is pretty elementary electromagnetism. Just a matter of what moves and when a voltage is induced.
So I won't jump in - though I have no ambition of being some kind of guru.
Said discussion is moderately interesting as this is pretty elementary electromagnetism. Just a matter of what moves and when a voltage is induced.
So I won't jump in - though I have no ambition of being some kind of guru.
QUOTE (am_Unition+Apr 1 2008, 03:29 AM)
The magnetic field strength of the magnet itself does not increase. The local magnetic field strength at the coil of wire increases when the magnet is brought closer to it. So, as you bring the magnet closer to the coil the magnetic flux is changing (increasing in this case)
What does it mean by Local magnetic field? So you mean the magnetic field of the magnet as a whole doesn't change? but the local part changes??! I am confused with 'local'?
What does it mean by Local magnetic field? So you mean the magnetic field of the magnet as a whole doesn't change? but the local part changes??! I am confused with 'local'?
Hi Mia,
Read the post carefully and you'll see that Unition is not referring to the field of the magnet itself (which does not change) but how the induction of the coil changes as the magnet is brought closer to the coil, hence the 'local to the coil' part of the magnet's field. Make more sense?
Peace,
Ron
Read the post carefully and you'll see that Unition is not referring to the field of the magnet itself (which does not change) but how the induction of the coil changes as the magnet is brought closer to the coil, hence the 'local to the coil' part of the magnet's field. Make more sense?
Peace,
Ron
QUOTE (Ron+Apr 7 2008, 12:31 PM)
Hi Mia,
Read the post carefully and you'll see that Unition is not referring to the field of the magnet itself (which does not change) but how the induction of the coil changes as the magnet is brought closer to the coil, hence the 'local to the coil' part of the magnet's field. Make more sense?
Peace,
Ron
THanks, Ron. Yep, it makes more sense. Is my thinking right over here: when the flux seems to increase,the magnetic field inside the coil will point in the opposite direction of the magnetic field of the magnet, just because of the conservation of energy; when the flux seems to decrease, B in the coil will point in the same direction of B in the magnet?but I have a question here, flux=BA. here Is B the magnetic field of the coil or the magnet? I mean if it is only either magnetic field of the coil or of the magnet, then it doesn't matter if the other one's magnetic field's direction as I stated before. If you don't understand what I meant, tell me. thank you.
Read the post carefully and you'll see that Unition is not referring to the field of the magnet itself (which does not change) but how the induction of the coil changes as the magnet is brought closer to the coil, hence the 'local to the coil' part of the magnet's field. Make more sense?
Peace,
Ron
THanks, Ron. Yep, it makes more sense. Is my thinking right over here: when the flux seems to increase,the magnetic field inside the coil will point in the opposite direction of the magnetic field of the magnet, just because of the conservation of energy; when the flux seems to decrease, B in the coil will point in the same direction of B in the magnet?but I have a question here, flux=BA. here Is B the magnetic field of the coil or the magnet? I mean if it is only either magnetic field of the coil or of the magnet, then it doesn't matter if the other one's magnetic field's direction as I stated before. If you don't understand what I meant, tell me. thank you.
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