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Physics/electrical generation


QUESTION: So, I'm teaching the basics of electrical generation to my 4th graders:  basically, that a generator changes mechanical energy into electrical energy by using a magnet to "push" free electrons into a wire which, ultimately, ends up in my toaster which then changes the potential energy of the moving electrons into heat energy which then moves into the bread, the surround air, etc.  
I'm sure that I must be missing a very fundamental idea here!  There is a very large by still finite number of free electrons in the coil of the generator which are being "pushed" by the magnet.  Once that energy is changed into heat and used to make the toast (or whatever) they are, for all intents and purposes, used up and unavailable to return to the coil.  If this is the case, why is it that the generator can continue to transform the mechanical energy into moving electrons?  What piece of the puzzle am I missing?

ANSWER: No, the electrons are not used up.  That's important, good for you, for spotting where it didn't make sense!  They're like water in a hose, they move around in a complete circuit.  They don't go into the toaster, they get pushed to the toaster and electrons in the toaster are pushed out the other side.  On their way through, they scatter around and transfer their energy to the atoms in the heating coils.  It's always a circuit, making a loop.  In fact, if you had to start your car with electrons that came from the battery, it would take weeks to start your car through the coils!  At the bottom, their example gives about 1m/h (yes, just 1 meter per hour for a 3A current in a 1mm diameter wire).  So you knew you were approaching it one way, you described that way and why it didn't make sense.  That's real progress in understanding science.  The changing magnetic field acts like a pump connected to a closed loop of hose, pushing the water inside in a circle.  The hose is the wire (atoms that the electrons are kept with from the electrostatic fields) and the water inside are electrons in the conduction band responsible for the flow of what is not surprisingly called current.

Best question in weeks.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your quick and excellent response!
To extend your metaphor:  I have a closed loop of pipe filled with water (wire w/electrons);  the water is being circulated by a pump (generator).  In the "circuit" of the pipe is a wheel being turned by the circlating water (which represents the toaster).  Some of the water's energy is expended in moving the wheel.  BUT, as it returns to the pump, additional energy is added by the pump and the potential energy of the water once again increases, allowing it to return and re-spin the wheel.

So, the electrons, which are matter, are not "used up" they are simply a means of transferring mechanical energy from the generator to the toaster, and then return for another push.  

Am I getting closer?  Again, thanks a lot.

Not quite, since the pump is the generator supplying the current and the toaster is kind of a thin spot in the pipe where the water rushes through furiously and releases energy.

But yes, closer.  Electrons are not used up and are indeed a means of transferring energy.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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