here’s a simpler way- electric motors and generators are roughly 70-80% efficient. Meaning, 70-80% of the electrical power you put into one comes out as mechanical shaft power. vice versa for generators. the rest is lost as heat.
So let’s look at it simply in terms of efficiency. let’s say for the sake of argument, it would take a 1 hp electric motor to run the generator. 1 hp is 746 watts. But, that electric motor is only about 70% efficient, so it would need roughly 969 watts electrical power from the generator. So it would have to spin the generator hard enough for it to generate that 969 watts. But the generator is also only about 70% efficient, so the electric motor would have to be putting out 1260 watts mechanical power (1.7 hp) to get that 969 watts from the generator.
see how fast that breaks down?
The three Laws of Thermodynamics, simplified:
You can’t ever do any better than breaking even,
You can’t break even unless you’re at absolute zero
You can’t reach absolute zero.
You can’t win, you can’t even tie, and you’re stuck playing the game.
It is possible to connect the spindles of two motors together, then wire them to each other in such a way that turning one generates current that turns the other n the same direction. It’s not free energy, but the system as a whole behaves a little like a flywheel.
With the circuit completed, this would dissipate energy through ohmic losses in the wiring (along with bearing friction/windage). I would expect this system to spin down more slowly if you snipped the wires.
If you have access to any Lego robotics kits, it’s easy to demonstrate this.
Connect two motors together and turn one–you’ll find that it spins easily, and the other one spins as well (almost in lock-step). Add some mechanical resistance to the other motor, and spin again–you’ll find that it’s much more difficult.
Well the voltage generated is just about fixed, because its a function of the field strength and the number of coils and speed.
Then the load is setting the current, and the current acts to make turning the generator harder… or slowing it down, depending how you look at it.
either way, power = force * velocity… so either you push harder and make more power, since the load is drawing more. or go slower and don’t provide the power to the load.
I can’t recall why exactly, but it makes sense from a symmetry argument (if turning the generator makes electricity, then the electricity must be resisting turning … acting as someting to push against.
so that power is above zero… if force is zero, there’s just no power in it… ) .
If you think of it in terms of a single permanent magnet and a coil: the movement of the magnet (actually movement of its field) generates current in the coil; but the flow of current in the coil also generates a magnetic field - symmetry is what makes these the opposite of one another.
If the coil is open circuit (i.e. has no load), not much current can flow (or none, ignoring capacitance) - no current flow means no opposing field.
If the coil is a closed circuit (i.e. it has a load), the induced current can flow readily, and is able to generate its opposing magnetic field.
I don’t recall the doorknob, but what I remmber was that the flywheel had radial slits in it; I have no idea why. Since I left Philly 54 years ago, I have been back to the Franklin Institute just once. And was rather disappointed since the planetarium show was run by a showman, not an astronomer as in the good old days.
I haven’t liked the show since they replaced the old Zeiss planetarium projector (the one that looked like some giant insect) with a more modern unit. Must have been some time in the 90s. The old sky with the old projector was beautiful - pinpoints of light on a deep black background. With the newer projector it was clearly some sort of raster scan - not nearly as sharp and way less contrast.
I built a home-brew battery charger for the cottage that demonstrates this exactly.
Basically, it was a bicycle attached to a car alternator using a belt running from the back rim instead of a tube and tire. It worked OK but was near impossible to pedal when the battery was completely dead.
Adding some weight to the “flywheel” and a load resistor between the battery and alternator helped a bit but it was still a pretty tough workout.
I still vividly remember the generator-load demonstration from a high school physics class. It would be no understatement to say that the light that turned on in my head was brighter than the light that turned on when the switch was flipped and the generator I was hand cranking suddenly was connected to a load.
The other one was the swinging sheet of copper passing between magnets. The solid sheet just hit the brakes and heated up like crazy. This is equivalent to dripping the magnet down the copper pipe, referenced upthread. The kinetic energy is converted to current to heat.
A similar copper sheet with multiple slits (like a comb) swung back and forth, almost freely. Something to do with smaller current loops available, I think, but I did not fully grasp that one.
Gasoline/electric hybrid cars generate power when braking. The motors are switched over to generators and this helps slow the car. This helps charge the battery/s.
Electric trains are using regenerative braking now, too. Newer models of NYC Subway cars use their brakes to dump some energy back into the third rail when stopping.
I have always been amazed at how so many so called proffessional auto mechanics think you can use an alternator to run an electric automobile engine and have frree energy. No fuel at all! They all think they were the first to think of it.