I have conducted an experiment with a magnet, 2 distinct coils (800/1600 solenoids) and a galvanometer to measure the electric current produced by the magnetic flux.

I'm having some issues understanding/relating the concepts observed with Faraday's and Lenz's law:

  • I connected a led to the circuit, observing that it only turned on when I inserted the magnet in the coil facing a certain direction (positive pole of the magnet + positive pole of the coil). However, testing this same effect in an online simulator, I see that the led should have turned on whichever way I inserted the magnet. Did I setup my preparation improperly?
  • In the 1600 solenoid coil, I noticed that I needed to move the magnet with much less speed for the led to turn on (and with more intensity), in comparison to the 800 solenoid coil. I measured that for the led to turn on, I needed 40 mV / 15 uA of tension using the 800 solenoid coil, but 60 mV / 20 uA using the 1600 solenoid coil.

Regarding Faraday's law formula: R . I = -N d ψ/ dt

Knowing the Resistance (R) of the 800 solenoid coil is 32.2Ω, and the resistance of the 1600 solenoid coil is 64.0Ω, is the difference in the resistance values the reason for the difference in necessary tension?

  • With the measured "necessary tension" values for both coils, can I theorically calculate the minimum tension value for it to come on without having measured the magnetic flux (d ψ/ dt)? I know that the reference value for the led is 1.68V.

Thank you in advance for your time!


1 Answer 1


The lamp in the Phet simulation is an incandescent bulb, if you used an LED then the experiment you performed is not the one simulated because LEDs behave differently than incandescent lamps.

In particular an LED is a "light emitting diode" and like other diodes it allows the flow of current in one direction (and lights up) and resists the flow in the other direction (and if you force it you let the magic smoke out). Incandescent bulbs, on the other hand light up which ever way the current flows through them.

You could restore the behavior seen in the simulation by using two LEDs connected in parallel running in opposite directions, with the added bonus that which LED lights up tells you the direction of current flow—using different color LEDs would make this easy to see.


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