Today I was at my local activity centre and was demonstrating how when a magnet is dropped down a copper pipe, it falls slower than when deopped from the same height without the pipe. This being caused by the induced current in the copper creating an opposing magnetic force.
A bright ten year old then asked me a question I do not know and cannot find the answer too.
The "work" required by the falling magnet is provided by gravity, so his question was " is some gravity consumed as it falls" his argument being that an alternator/generator needs a motive power to work, usually by an engine of some sort that consumes a fuel to provide the motive force, so he wonders (and now so do I) whether or not gravity can be consumed and if so could it in theory be depleted if enough magnets were dropped down copper tubes all at the same time. If not consumed, then it's energy for free. I found links to the Johnson motor, but that has nothing to do with gravity.
Can anyone provide a link to where I can find an explanation?
What was that old saying? never work with children or animals ha ha.

 

Conservation of energy still applies. There is no free lunch.
Gravity is not consumed. The work was done (energy put into the system) when you lifted the magnet to the top of the tube. You exerted energy and gave the magnet potential energy.

As the magnet falls inside the tube, energy is generated as heat (eddy currents inside the tube). The magnet exists the tube with some kinetic energy (it is still moving). Some energy was imparted to the surrounding air by pushing air out of the bottom of the tube and sucking air in the top of the tube (though this is minimal compared with the heat generated in the tube).

 

And there is friction.

 

And there is friction.

I did think of that but thought that the movement of air would consume more energy than friction of a free falling magnet inside a hollow tube.

 

Air movement takes the energy? Are you serious? Take the air away and repeat.

It's magnetic interaction....just like a motor. Magnetic field impedance. Counter EMF.

The KE of magnet is transferred to angular energy of tube free charge. If you remove tube free charge......magnet will fall un impeded.

 

Another way of looking at it:
• With the copper tube or not, the energy available during a fall of the magnet is the potential energy imparted on the magnet by the person raising it up.
• Tube or no tube, all of that potential energy is converted to some other form by the time the magnet hits the bottom of its fall.
• With no tube, the potential energy is converted to kinetic energy and a small amount of air drag. The kinetic energy turns into heat when the magnet hits the bottom.
• With the copper tube, some of the potential energy is converted to heat by the magnet passing a conductor, with the remainder of the available potential energy converted to air drag and kinetic energy of the slowly moving magnet.

The source of energy in this experiment is a human, by exertion of force (equal to gravity) over a distance (the distance the magnet is raised). No energy is created or lost in the subsequent steps, only converted from one form to another.

 

You mean there will be no air friction ?

 

The magnet resting on the bench is consuming as much energy as it would if it were stuck to the bottom of a steel plate. No work is being done. Nothing is being consumed. Already several people have pointed out that gravity is there, same as the magnet at rest on the table or on the side of a refrigerator. The energy is there - it's just not being utilized. When you lift the magnet you impart potential energy. As the magnet falls it consumes all of the energy you put into lifting it to the top of the tube. Some energy is converted into EMF and BEMF. Some into heat and some into air movement. Absolutely all energy is accounted for. There was no potential energy until you lifted it up. Same would be true of a spring. At rest it has no potential energy until you pull or push on it. Once the spring is loaded there is now potential energy.

 

I did think of that but thought that the movement of air would consume more energy than friction of a free falling magnet inside a hollow tube.

If by "hollow tube" you mean a cardboard tube or plastic or some other non-interactive metal, then yes, the bulk of the air movement would consume more energy. Especially if the air flow were restricted somehow. But through a copper tube as the original post states, more energy is consumed as electro motive force.

 

You mean there will be no air friction ?

I said the opposite.

This experiment would look the same in vacuum as in air. Showing the effect of air would require very precise measurements. But if there's air, some of the energy will be lost to friction.

 

whether or not gravity can be consumed and if so could it in theory be depleted if enough magnets were dropped down copper tubes all at the same time.

Not at all.
You can look at gravity as a very long spring.
It stores energy when stretched (lifting the object from the ground) and returns the energy when released (falls back to ground), but is not altered otherwise, no matter how often you repeat the process.

 

Not at all.
You can look at gravity as a very long spring.
It stores energy when stretched (lifting the object from the ground) and returns the energy when released (falls back to ground), but is not altered otherwise, no matter how often you repeat the process.

Yes that does seem to be the most logical explanation. I think my confusion arose from thinking the magnet or anything really, is caused to fall by gravitational attraction being static.
As the air resistance is not really a consideration in this particular scenario, ( it would to all intents be the same in a vacuum) and air friction and compression aside as they are only adding minor amounts to the slowing of the magnet's fall, Do I assume that you mean that the gravity acting on the magnet is already stretched while it is held up even without the tube, and the gravity "spring" tries to pull it towards the centre of the planet until it hits the floor, which now is the only thing opposing the pull of the gravity "spring"? The slowing of the fall only being caused by the magnetic induction
into the wall of the tube as it travels, that then sets up an induced current that in turn creates an opposing magnetic force" repelling" the magnet.

 

The slowing of the fall only being caused by the magnetic induction into the wall of the tube as it travels, that then sets up an induced current that in turn creates an opposing magnetic force" repelling" the magnet.

You got it.

 

Do I assume that you mean that the gravity acting on the magnet is already stretched while it is held up even without the tube, and the gravity "spring" tries to pull it towards the centre of the planet until it hits the floor, which now is the only thing opposing the pull of the gravity "spring"?

Yes, that is the analogy I was trying to make.
The gravity "spring" is attached to the center of the earth.

 

Today I was at my local activity centre and was demonstrating how when a magnet is dropped down a copper pipe, it falls slower than when deopped from the same height without the pipe. This being caused by the induced current in the copper creating an opposing magnetic force.
A bright ten year old then asked me a question I do not know and cannot find the answer too.
The "work" required by the falling magnet is provided by gravity, so his question was " is some gravity consumed as it falls" his argument being that an alternator/generator needs a motive power to work, usually by an engine of some sort that consumes a fuel to provide the motive force, so he wonders (and now so do I) whether or not gravity can be consumed and if so could it in theory be depleted if enough magnets were dropped down copper tubes all at the same time. If not consumed, then it's energy for free. I found links to the Johnson motor, but that has nothing to do with gravity.
Can anyone provide a link to where I can find an explanation?
What was that old saying? never work with children or animals ha ha.

The magnet dropped (from rest) from one height to a lower height (and ending up at rest) converts the exact same amount of gravitational energy into some other form whether it is dropped in a vacuum, in free air, in water, in corn syrup, or down a copper tube.

When dropped in a vacuum, all of the energy is first converted to kinetic energy as it accelerates and then converted to some other form, primarily heat but possibly some other form depending on the specifics, by the impact forces when it is stopped upon hitting whatever stops it. In all of the other cases, some of the kinetic energy is converted to some other form, primarily heat, by whatever is causing the drag that prevents it from accelerating as much as it would if it were dropped in a vacuum. The rest is converted to heat by the eventual impact which, since it is at a lower speed, doesn't dissipate as much -- but it doesn't need to since there is less kinetic energy to convert. In the case of the copper tube, some of the kinetic energy is converted to electrical energy, which is then dissipated as heat in the resistive losses as the currents circulate in the tube. You could also rig up something else to extract energy to ship of somewhere else, leaving less kinetic energy in the falling magnet. This is why the water discharged at the bottom of a hydroelectric damn is a lot tamer than it would if it were just spewing out of a pipe -- as much of the energy has been converted to electrical energy and distributed into the grid as possible.

 

Air movement takes the energy? Are you serious? Take the air away and repeat.

It's magnetic interaction....just like a motor. Magnetic field impedance. Counter EMF.

The KE of magnet is transferred to angular energy of tube free charge. If you remove tube free charge......magnet will fall un impeded.

Yes, air absorbs some of the energy. Take the air away and repeat and you will discover that the magnet falls faster through the tube than it did before. For the difference to be readily noticeable, you need a light magnet that has sufficient air to create decent drag.

 

I said the opposite.

This experiment would look the same in vacuum as in air. Showing the effect of air would require very precise measurements. But if there's air, some of the energy will be lost to friction.

A slug that leaves only a small gap between it and the tube walls can show a real difference in fall rate with air and in vacuum, because in air the slug has to compress and push the air below it as it falls.

If you take this to extremes -- close the bottom of the tube and increase the gravity (simulated by applying mechanical force to the slug) it is a very simple matter to convert enough energy into heat in the air to ignite paper placed in the tube.

 

I thought the question was on the magnetic affect. Take a plastic pipe and a copper pipe with same clearance.

Now you see the magnetic difference. Air has been nullified.

 

I thought the question was on the magnetic affect. Take a plastic pipe and a copper pipe with same clearance.

Now you see the magnetic difference. Air has been nullified.

Yes, the effect of the air has been nulled on one level as far as the experiment goes because it's effect is the same in both measurements, and so if the results are different then it points to something OTHER than air causing a difference. But that is very different from your suggestion of taking the air away and repeating. You will see different results than you did before.

 

I thought the question was on the magnetic affect. Take a plastic pipe and a copper pipe with same clearance.

Now you see the magnetic difference. Air has been nullified.

Yes, the effect of the air has been nulled on one level as far as the experiment goes because it's effect is the same in both measurements, and so if the results are different then it points to something OTHER than air causing a difference. But that is very different from your suggestion of taking the air away and repeating. You will see different results than you did before.

Have you guys ever even done the experiment (magnet in a copper pipe)? If you have, you'll realize how stupid the conversation about air resistance is. It takes about 5 to 10 seconds for a good magnet to fall through a 1-foot pipe.

Your concern and prolonged hijacking of the thread to discuss air resistance on the magnet's fall through the pipe is like discussing how insects hitting your windshield lower fuel economy. If you guys don't have anything to add for the OP, sit quietly instead of hijacking.