I was watching a Youtube video about magnetic induction and a guy explained to me that when a magnet moves by a piece of copper it causes a magnetic field in the copper which slows the magnet down but in some part of the video and somewhere in the video it showed the magnet slowing down a clear PVC pipe with a copper coil wrapped around it and on this copper coil it had an LED attached. He said "The LED is very efficient so powering it doesn't slow down the magnet quite as much" So my question is what causes the diminishing of Lenz's law to allow what he said to occur.

Link to the video --> https://youtu.be/sENgdSF8ppA?t=110


2 Answers 2


When there is a changing magnetic field inside the coil, Faradays law states that there is an induced emf

This Emf causes electrons to accelerate, producing a current

Amperes law( along with the negative in faradays law, )states that this current produces a magnentic field that wants oppose the change that causes it. when there is an increase in flux, the induced Emf will produce a current to decrease the flux, This manifests it self as a magnetic field that repells the magnet falling ( in an attempt to reduce the change in flux)

Notice there is a gap in the logic from Faradays law to amperes law, For there to be a magnetic field a CURRENT must be induced , not just an emf.

When there is an LED in the circuit, the resistance of the circuit increases, this means a smaller current being induced for the same Emf. Because there is a smaller current, there will be a WEAKER magnetic field that opposes the motion of the magnet. making it fall faster.

  • $\begingroup$ Hey thx for answering my question. I have another question for you relating to the same youtube video "youtu.be/sENgdSF8ppA?t=248" He says that he is able to move the other magnet from afar using the copper plate so how does that work? $\endgroup$ Dec 5, 2021 at 20:19
  • $\begingroup$ Are you talking about 3:33? if so this has nothing to do with copper, he was just showing magnetic field lines pass through copper this is just standard magnetism $\endgroup$ Dec 5, 2021 at 20:53
  • $\begingroup$ "4:21" look at "4:21" $\endgroup$ Dec 5, 2021 at 22:06
  • $\begingroup$ This is just regular magnetic force between 2 magnets. But with the copper there, the force is somewhat dampened due to the coppers ability to fight the change in magnetic flux from the magnet on top, allowing him to have "more control" of the magnet, without it flying off $\endgroup$ Dec 5, 2021 at 22:18

The opposition to the movement of the magnet is due to the induced current in the coil circuit.
The larger the induced current the larger the opposition to the movement of the magnet.
A factor which controls the magnitude of the induced current is the resistance of the coil circuit.
The statement that the LED is very efficient should be taken to mean that the LED adds a small resistance to the coil circuit.
This in turn that means that with the LED added to the coil circuit the resistance is increased by only a small amount which in turn means that the induced current is decreased by only a small amount.
Thus the motion of the magnet is slowed but not by quite as much as when the coil was short circuited.

  • $\begingroup$ Wait that just doesn't make sense. You said that resistance is what slows down the magnet"A factor which controls the magnitude of the induced current is the resistance of the coil circuit." And you also said that the LED adds a small resistance to the circuit"LED adds a small resistance to the coil circuit." But you say that the LED decrease the effect. Can you pls explain? $\endgroup$ Dec 3, 2021 at 21:00
  • $\begingroup$ Increased resistance jn the circuit means it falls faster $\endgroup$ Dec 3, 2021 at 21:36
  • $\begingroup$ Increasing resistance decreases induced current and so lessens the slowing down. $\endgroup$
    – Farcher
    Dec 3, 2021 at 21:53

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