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I am doing science with my 6th grade son, and there is a circuit with an electromagnet (as below)

When asked to describe the energy conversion when the switch is turned on, part of the answer provided by the teacher indicates that electric energy is converted into kinetic energy when the electromagnet attracts the hinge.

Ignoring the obvious complications in understanding how magnets work (I am sadly still confused about that), this statement seems wrong for the following reasons:

  1. A magnetic field is created which attracts the hinge. There is no direct conversion from electric energy to kinetic energy.
  2. Any energy conversion seems to have been from the chemical potential energy in the battery to the current flow. The movement of the hinge seems to be an indirect effect rather than a direct conversion.
  3. Is there is additional resistance added by the presence of the electromagnet? If not, there should be no energy loss, if yes, what's the mechanism of that loss?

Clarifications will be very much appreciated, I tried to work through the above questions with my kid but ended up being very confused myself.

Lastly, if someone can share an answer that will be appropriate as an answer in a 6th grade exam, yet technically correct - that will be great. :)

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Any energy conversion seems to have been from the chemical potential energy in the battery to the current flow. The movement of the hinge seems to be an indirect effect rather than a direct conversion.

Indeed, but they are simply ignoring that conversion. The electricity is treated as stored so you're simply "releasing" it. If you replaced the battery with a mains wire and switch it would have the same outcome, so ultimately it's the electricity that's doing the work.

Is there is additional resistance added by the presence of the electromagnet? If not, there should be no energy loss, if yes, what's the mechanism of that loss?

Yes, the magnet definitely acts as a resistor, or more specifically, reluctance.

Magnetic fields are created when a current flows. In the case of a straight wire, the right-hand-rule tells us there is a field around the wire. For instance, if you hold your hand with the thumb pointing up representing the current in a vertical wire, the fingers of your closed fist represents the direction of the field.

When you bend the wire the into a ring, the field created at point A begins to overlap the field from point B. Lens' Law tells us these will interact in a way that resists the flow of electrons. From an electrical standpoint it looks like resistance. And yes, it definitely dissipates heat. The motion of the striker also effects this field.

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  • $\begingroup$ Thanks for the great references! Lenz law cleared up a lot of my misunderstandings. Is it correct to say that some of the electrical energy was indirectly converted to kinetic energy? And a stupid question, does metals moving in the magnetic field cause more or less resistance? I’m guessing more, since electricity should be generated in the metal itself. $\endgroup$ – ming yeow Apr 12 '19 at 6:47
  • $\begingroup$ Oh yes, not even that indirectly... the electricity caused a magnetic field that pulled the striker. As to the second, yes! You might want to check out "magnetic brake". $\endgroup$ – Maury Markowitz Apr 12 '19 at 15:24

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