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I performed an experiment where I connected a magnet to the end of a spring with the north side on the bottom. The magnet at the tip of the coil was aligned to pass through a column of coils of about 900 copper coils. The column of coils has the top coil connected to an extension wire which is connected to a variable resistor. The bottom coil is connected to an extension wire which is connected to the other port of the variable resistor, creating a series circuit. enter image description here Now, what I found intersting is that when I increase the resistance of within the circuit, the damping of the oscillations became greater. Now, this is contrary to what is most logical, since an increase in resistance causes a decrease in the current induced within the coils as a result of the change in the magnetic field relative to the coils (Lenz's law). These I think are known as eddy currents. But the questions still remains, how can an increase in resistance within the coils possibly cause more damping if it is decreasing the current opposing its motion as it enters the coil column.

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    $\begingroup$ In extreme case of infinite resistance, there will be no induced current, thus no damping. It is contrary to the logic. Do you have the experiment data, damping as a function of resistance? $\endgroup$
    – Jian
    Aug 25, 2018 at 11:12
  • $\begingroup$ @wwwjjj I didn't calculate the values for damping as such, but I do have the experimental data for the change in force and induced emf over time. I will work on calculating the damping coefficients. How do I upload screenshots in the comments section of the page though? $\endgroup$
    – Oussama Safi
    Aug 25, 2018 at 11:22
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    $\begingroup$ You can edit the question, then add graphs. It would be nice if you can show the position/force/emf-time curves (same intial condition), for a wide range of different resistances. $\endgroup$
    – Jian
    Aug 25, 2018 at 11:26
  • $\begingroup$ I apologise if it took me too long to repsond with the graphs $\endgroup$
    – Oussama Safi
    Aug 30, 2018 at 10:47
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    $\begingroup$ (a) Terminology: You have one coil with 500 turns, not 500 coils! (b) How, I wonder, did you measure the force? (c) Have you considered starting with a simpler experiment, for example measuring the decrease in amplitude (e.g of up and down oscillations of the magnet or of voltage)over a given time, and repeating for a few different values of resistance? $\endgroup$
    – Philip Wood
    Nov 24, 2021 at 11:54

2 Answers 2

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Damping implies losses.

If the coil is an open circuit, the current is zero. No losses.

If the coil is shorted, the current could be high, but the resistance could be very low, so the losses could be low as well - close to zero, if a coil was a superconductor.

From the above, it follows that, for a given setup, there must be some coil load resistance, which will cause maximum losses and, therefore, maximum damping.

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Larger resistance doesn't mean less current in this case. When magnetic field on a conductor changes, a "current" is induced in the coil. Notice that it is current not voltage. The voltage comes from the current running through some impedance. When you are increasing the resistance, the current is not changing much but the total joule loss is increasing. Hence, more damping.

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  • $\begingroup$ "When magnetic field on a conductor changes, a "current" is induced in the coil. Notice that it is current not voltage." I suggest that you read up about Faraday's law of electromagnetic induction! $\endgroup$
    – Philip Wood
    Nov 24, 2021 at 12:24
  • $\begingroup$ I did. Just for reference emf and voltage are not exactly always same. Faraday's law states that there will be some induction. But how much and which direction come from Lenz's law. Simply think like this, you are bringing a magnet close to a coil. So, there will be some power induced in the coil so that it oppose the magnet getting closer. $\endgroup$
    – Abdur Rakib
    Nov 24, 2021 at 12:40
  • $\begingroup$ i.e., the coil have to behave like a magnet. And the magnetic field (and flux) is related to current through the coil not voltage. Changing magnetic field induces emf is only correct when the coil is an open circuit. If it's closed circuit, it's more appropriate to say that current is induced. $\endgroup$
    – Abdur Rakib
    Nov 24, 2021 at 12:40
  • $\begingroup$ "Changing magnetic field induces emf is only correct when the coil is an open circuit." What, I wonder, is your source for this (incorrect) statement? $\endgroup$
    – Philip Wood
    Nov 24, 2021 at 12:49
  • $\begingroup$ Doing that for active vibration control. The simulations are showing good results. Hoping that experimental results will be as good. $\endgroup$
    – Abdur Rakib
    Nov 25, 2021 at 5:43

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