I believe this is true, but I’d like a demonstration of it or some simple explanation. I’m not getting much on my (admittedly limited) google search.
Is it true that if I hand crank an electric generator with no load on it (i.e. wires are not connected to anything), then it will be relatively easier to turn than if I had the wires connected to something like a light bulb?
And the greater the load, the more ‘resistance’ I feel when turning the generator?
I think this is true, but I have never experienced it myself and would like a demonstration of it to see and show.
Yep, it’s true. If you have any kind of science center nearby, they will often have one set up just to demonstrate this.
ETA: Think of it this way. A generator converts mechanical energy into electrical energy. If it didn’t get harder to turn, then where would the energy come from to light the light bulb? That would defy physics. You’d be getting energy out of nothing. Power in has to equal power out. Your power in is you turning the crank. The power out is the energy to light the bulb plus heat lost in the wires and the bulb and the generator windings.
Yes, I remember the Franklin Institute in Philadelphia had just such an exhibit. You turned a wheel that powered a generator and then closed a switch that completed a circuit to light a bulb and it felt like putting on the brakes.
Generating current requires work. Turning a generator with no load still needs work, but if you want to induce some current to flow, you have to input more work - you feel this as more resistance.
It’s no different to riding a bike - easy on the flat, but harder on an uphill gradient. You have to do more work to overcome gravity in that case. Of course, if you put a dynamo on the bike to run a light, that will need work too.
This confirms what I’ve thought to be true, I just haven’t been blessed with first hand experience with it or second hand video demonstration experience.
I want to be equipped with information before I talk to somebody about this concept.
Ah, the old “Hall of Physics” - my third favorite part of the museum (behind the trains and the planetarium). Tons of exhibits with buttons to push lever to pull, each demonstrating a principle of physics. Back before they felt the need to glitz things up as attention spans shortened.
If I remember correctly that exhibit had a massive flywheel with kind of a doorknob in the center - pretty hard to get spinning.
Last time I was there (at least ten years ago) you could still find some of the old-school exhibits in various hallways or nooks around the place…
You might be able to show this with a coil of wire and a magnet. Pass one pole of a magnet back and forth over the coil of wire with the ends connected and disconnected and you should be able to feel the difference.
Well, electric (and diesel-electric) trains have been using this concept for many decades - regenerative braking. Basically many types of electric motors function as generators if you remove the voltage and drive them mechanically. Imagine that you have a motor driving the train wheels. Remove the voltage driving the motor, now the train is coasting and the motor is being spun by the wheels and is acting as a generator. Now add a load to the generator and it will make it harder to turn, causing the train to slow.
Same concept is used in hybrid cars - except the load on the generator is the battery charger. This allows some of the energy lost in braking to be converted into electricity to be used for driving.
if you apply current through a conductor (wire,) it will form a magnetic field around it.
if you pass a conductor through a changing magnetic field, a current will be induced in it.
this is the principle on how all motors, generators, and transformers work. So in the case of a simple permanent-magnet generator, this is what’s happening:
the generator is a permanent magnet (or set of magnets) on a rotor shaft, which spins inside a housing.
the inside of the housing is lined with coils of wire, which connected to the outside of the housing by wires (for powering a load)
as you spin the magnets inside the housing, the changing magnetic fields are passing over the stator coils. the changing magnetic field is inducing a voltage across the stator wires, now:
3a) if there’s no load (the stator output wires aren’t connected to a load) then no current can flow. a voltage will still be present, but with an open circuit there’s nowhere for current to go, so nothing happens and there’s no effort to turn the rotor shaft.
3b) if you now connect a load (e.g. a light bulb) to the stator output, as the magnets pass over the stator coils it will induce current into the stator coils and current will flow into the load (e.g. the light bulb will glow.) BUT! now that current is flowing through the stator, it will generate “back EMF (electro-motive force,)” meaning the voltage inducted on the stator coils tries to “push back” on the magnetic force of the rotor. So the more power you try to draw from the generator, the harder the back EMF resists, and the more mechanical power you need to apply.
I’ve had amusing discussions on a number of car forums about why cars don’t use an electric motor to drive the alternator instead of the engine.
They still have it, or at least an updated version. It has an incandescent and LED bulbs so you can see how much less power the LED requires for the same brightness.
Every science museum should have one. Heck, you shouldn’t be able to graduate high school or vote unless you’ve tried this once in your life. If nothing else it would cut down on the “why don’t they hook up a generator to the wheels on your car? Free electricity!” threads that pop up here occasionally.
hijack - The Franklin Institute still has the giant heart you can walk through too. Though it seems to have shrunk since I was in 2nd grade.
This is very similar to the situation I’m talking about.
I just realized I have witnessed and done this myself, but it isn’t as easily translatable to someone who doesn’t have the needed education. I’ve dropped magnets down copper tubes before just to watch the induced currents push back on the magnet and slow it down. Unfortunately, doing this with the people i’m thinking of will not immediately translate into resistance in an electric generator.
A similar effect can be observed when jump starting an automobile due to a dead battery, typically moaning and squeeling of belts and other indications that the motor is definitely not in its happy place, because of the heavy load the alternator presents.
If you can find a stepper motor (like, from an old, old disk drive), you can do in interesting demonstration of this effect. Spin the shaft with the leads of the motor free, and then once again with the leads shorted to each other. I have a motor that is almost impossible to turn by hand when the leads are shorted (is a big-ass NEMA-34 motor).
Stepper motors don’t tend to spin freely in any state - it’s actually easier to do this with a small DC motor - spin it with the leads open and observe that the shaft spins for a second or two until it slows down. Short the leads and the shaft will hardly spin freely at all.
When I was a kid I had a hand-cranked generator that was originally from some piece of machinery and I observed exactly this effect quite dramatically. I have no recollection of what the nominal output of the thing was supposed to be but I connected it to various things and could even get it to light up an ordinary 60-watt household bulb. If not connected it turned very easily and slightly less easily with small loads, but with the light bulb connected it was so hard to turn that I couldn’t keep it up very long!
Say… I think you’re onto something there! Use an electric motor to drive the alternator and charge the battery, and use the battery to run the electric motor! I could make billions!
Uhm…I have to be polite here, but this is very close to what I’m dealing with; I was just never 100% sure how I could politely say this wouldn’t work. Now that I’m confident in my original guess that there is resistance in cranking the generator dependent on the load, I can go back and make my case.
Yes, a small DC motor should demonstrate this easily. You can attach a small light bulb to the power leads and feel the resistance as you turn the shaft. Or just tie them together and you’ll see it’s very difficult to turn.
