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When a magnet travels linearly through a solenoid at a fast enough velocity if the ends of solenoid are connected to each other, the induced EMF generated and Lenz Law the solenoid opposes the motion of the magnet. But, when a bulb is connected to the solenoid the bulb/led lights up, the circuit is still complete but the force with which the solenoid opposes the passing magnet decreases dramatically and the magnet emerges with much more velocity at the other end. I am confused why is that so?

The circuit is still complete, the magnet still falls at the same rate, the number of turns in the solenoid are still the same. Then why this behavior.

Can someone please explain?

And what if we connect a resistor instead of a bulb.

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  • $\begingroup$ yes. the ends of solenoid connected: (1) directly and (2) to a bulb to complete the circuit in two cases. $\endgroup$
    – xyz
    Commented Jan 2, 2023 at 18:40
  • $\begingroup$ Do you really mean a solenoid, that is a long tube-shaped coil? $\endgroup$
    – Philip Wood
    Commented Jan 2, 2023 at 18:44
  • $\begingroup$ yes. A solenoid. I am confused if there is a problem? $\endgroup$
    – xyz
    Commented Jan 2, 2023 at 18:45
  • $\begingroup$ Yes. It's been dealt with on this exchange this very afternoon; see "If a magnet is completely inside a solenoid, but is moving, Does this induce an emf?" Sorry that I don't know how to do links. $\endgroup$
    – Philip Wood
    Commented Jan 2, 2023 at 18:48
  • $\begingroup$ I just saw. coincidence. :) but its a bit different. $\endgroup$
    – xyz
    Commented Jan 2, 2023 at 18:54

1 Answer 1

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The size of the opposing force experienced by the solenoid depends on the magnitude of current flowing through it as it traverses the magnet, since the strength of the magnetic field produced by the solenoid depends on the current.

IN the case of the short-circuited solenoid, its own internal resistance is the thing that limits how much current can flow. this is typically low and so a lot of current can flow, a strong field is produced, yielding a strong force which slows down the solenoid (or the magnet, whichever is free to move).

But when we connect a resistive load to the solenoid, there is hence more opposition to the flow of induced current and the resulting field is weaker, and all the forces become weaker too.

In the limit of an open circuit, no current flows, no field is produced, and no forces result.

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  • $\begingroup$ thank you. I understood the resistive load case. And for a highly efficient bulb would behave like an open circuit since it converts current to photons? Is that correct? $\endgroup$
    – xyz
    Commented Jan 2, 2023 at 18:53
  • $\begingroup$ nope, the bulb behaves like a resistor. each different wattage and voltage rating for a bulb represents a bulb with a different resistance. $\endgroup$
    – niels nielsen
    Commented Jan 2, 2023 at 19:05
  • $\begingroup$ I am a bit confused. Shouldn't bulb be by definition have less resistance so that the current losses are minimum and it can convert maximum input power to photons. Or is the analogy that since its converting maximum power to photons therefore there is no emergent power on the other end so it seems like a resistor? $\endgroup$
    – xyz
    Commented Jan 2, 2023 at 19:10
  • $\begingroup$ A bulb consumes power by turning it into heat which is radiated away. a resistor consumes power by turning it into heat which is radiated away too. the difference is that the bulb runs at a temperature so high that the radiation is visible. The resistor runs at a temperature too low to produce visible radiation. $\endgroup$
    – niels nielsen
    Commented Jan 2, 2023 at 22:08
  • $\begingroup$ @xyz An interesting thing is that most electronics is done with voltage sources, but a lot of the equations flip over when you talk about designing them for use with current sources. With a voltage source a small load is a high resistance but if you want to put a small load on a current source then you want a low resistance. In fact both infinite and zero resistance both use zero power, but the difference is what happens to the source, and that changes depending on whether it's a voltage source or current source. $\endgroup$
    – Stack Exchange Supports Israel
    Commented Jan 3, 2023 at 0:03

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