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Induction, magnetism and conductivity 
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#1
Dec1412, 01:14 AM

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Relative motion between a permanent magnet and a conductor wire produces electric current in the wire. Would induced electric current be greater in a wire made of "magnetic" material like iron, "nonmagnetic" material like copper, or it doesn't matter? In other words, is there relation between material magnetic properties and its inductivity? And similar but I think different question, is there relation between material magnetic properties and its conductivity?
How come magnetic field of a permanent magnet can interact with magnetic fields inside a wire that is overall magnetically neutral? Does the same thing happen with electric fields, so if instead of permanent magnet we had some electrically charged object, could we also induce electric current in a conductor wire by their relative motion? 


#2
Dec1412, 08:57 PM

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The induced emf in a conducting loop depends on the time rate of change of the magnetic field through the area enclosed by the conducting loop (Faraday's law). So induced emf has nothing to do with the type of conductor. The current that flows in the conducting loop in response to that induce emf does depend on the type of conductor. In order to maximize the induced current you would want to minimize the factors that impede current flow in the conductor. How would you do that? AM 


#3
Dec1512, 03:59 AM

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#4
Dec1512, 06:09 AM

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Induction, magnetism and conductivity
Yes. That means the induced emf around the wire loop is determined not by the type of wire but by the rate of change of the magnetic field and the area enclosed by the wire. If you are asking if the induced emf depends on the wire, the answer, as I have said, is no. The current that flows in the wire due to that emf does. The current can be affected by the magnetic properties of the conductor. In order to maximize current flow, the magnetic field inside the conductor must be constant (that is just from Farraday's law). There are quantum effects that actually eliminate the magnetic field inside the conductor in order to achieve superconductivity. AM 


#5
Dec1512, 09:07 AM

P: 56

It seems to me there are more factors that would come into play than just conductivity, like the number and strength of magnetic fields per volume of substance that would be available (unpaired) to interact with this external magnetic field. But also not all of those magnetic fields would interact in the same way. Some would be due to electron spin and some would be due to electron orbits, some would repel while others would attract, so depending on how many of them there are, how "free" they are to move, and depending in what direction they prefer to move, on average and relative to that external magnetic field, it seems it would result in greater or less induced electric current. But then what I just described could be the same thing what defines conductivity as well, and if so it would all boil down to be the same after all. In any case the situation is very complex, especially considering the temperature and heating of the substance would be a factor too, so I'm afraid the answer I'm looking for is specific and experimental rather than general and theoretical. 


#6
Dec1512, 01:19 PM

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Are you suggesting that since the iron wire has a higher permeability, the magnitude of the magnetic field changes inside the iron wire will be somewhat greater than in the copper wire? Just applying Faraday's law, one can see that this would tend to increase the induced emf in the iron wire, and, therefore the current. But the increase would be in proportion to the area of the wire loop itself (the diameter x length of the wire) compared to the area enclosed by the wire loop. If you are making the diameter of the loop much larger than the diameter of the wire, it should not be significant. AM 


#7
Dec1512, 07:30 PM

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Claude 


#8
Dec1512, 08:32 PM

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#9
Dec1512, 10:54 PM

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Faraday's law has some very subtle aspects that cause all sorts of confusion. Professor Lewin at M.I.T. has a very good lecture on this that can be found here. . It can be found on Youtube here (it is easier to navigate in the Youtube version). AM 


#10
Dec1612, 05:20 AM

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#11
Dec1612, 01:50 PM

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AM 


#12
Dec1612, 07:39 PM

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Whether we measure voltage or amperes we would take the same measurement with both iron and copper wire, and the point in that lecture is about two DIFFERENT measurements. It's about two different measurements of the potential difference related to the direction of the current and two different resistors, which is indeed peculiar, so perhaps it's best to just measure amperes instead of voltage, although since we have no uneven distribution of resistance, like they did, we would not need to worry about anything like that. 


#13
Dec1612, 10:24 PM

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This is a side issue to the issue you raise about the effect of the permeability of the conductor on the induced current, but it is important to appreciate it. Measuring voltages where there is a time dependent magnetic field present is very different than measuring voltages in a circuit containing a battery and resistance. But iron does have an effect on the magnetic field, which is the basis of your original question. To determine how it would affect current is very complicated and involves quantum effects as well as Faraday's law. Physics is not intended to make you happy. AM 


#14
Dec1712, 03:24 AM

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#15
Dec1712, 07:38 AM

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"The circuit would be closed by connecting voltmeter at the ends, but loop is fine too." I simply observed that measuring the induced voltage in a section of conductor like that will not give you induced voltage between the two ends of the conductor. It measures the emf around the whole loop comprised of the conductor and the voltmeter leads. That is why I suggested you use loops. In the above example, the magnetic field enclosed by the voltmeter leads is not significant so a voltmeter will measure the induced emf across the ends of the coil, which is a function of the rate of change of the flux through the coil loops. If you were replace the coil with a single straight conducting wire and pass a magnet in a direction perpendicular to the direction of the wire (replace the coil with a straight wire and have the magnet moving perpendicular to the page) as you were suggesting, a galvanometer connected as shown in your diagram would not measure the induced voltage in the conductor. That was my point. AM 


#16
Dec1712, 10:05 AM

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#17
Dec1712, 10:53 AM

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If you think it does not matter then apply Faraday's law and tell us what the induced voltage is in a straight conductor of length L as a function of dB/dt. AM 


#18
Dec1712, 03:56 PM

P: 56

 Faraday's Law for an uncoiled conductor states that the amount of induced voltage is proportional to the rate of change of flux lines cutting the conductor. Faraday's Law for a straight wire is shown below. http://www.sweethaven.com/sweethaven....asp?iNum=0201  Faraday's Law for a Straight Wire: The amount of induced voltage is proportional to the rate of change of flux lines cutting the conductor. 


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