Experimenting with Magnet Drop: Feasibility of Using PVC vs. Iron Tube

[edwinem]

So for a project what I want to do is experiment with dropping a magnet or a piece of metal down a tube and seeing the effect on its falling speed. My first idea is to wrap copper wire around a piece of PVC pipe. I would then hook it up to a battery to make it into an electromagnet. If the field is to weak then I would try to wrap it around a iron tube to strengthen the effect of the magnetic field instead of the PVC. My question is will it work? I remember once reading somewhere that the field would cancel out in the tube. Thus even if I drop a piece of metal/magnet through it there will be no change.

If it is possible to do would it be ok to use a PVC pipe or should it be a iron tube?

 

[Phrak]

That should be do-able. Make a solenoid using PVC or using some other thin-walled plastic or paper material and magnet wire.

Usually an experiment involves measuring something. How would you measure fall-times with the magnet off, and then on?

For a conductive material the difference in fall-time would be most pronounced using a short piece of aluminum tubing dropped inside the solenoid (electromagnet).

There are some more things you should know about if you want to drop a magnet inside the solenoid.

 

[protonchain]

Usually an experiment involves measuring something. How would you measure fall-times with the magnet off, and then on?

I guess I would also try to measure the intensity of the magnetic field? Just a thought/addition to what you said.

 

[G01]

I guess I would also try to measure the intensity of the magnetic field? Just a thought/addition to what you said.

How will this give you information about the fall time of the permanent magnet?

 

[protonchain]

How will this give you information about the fall time of the permanent magnet?

It doesn't, it's just a space filler/something else to do while you're dropping things on the floor.

 

[edwinem]

Well my changing variable would be the amount of current flowing through the electromagnet. I have a variable resistor and an ammeter. I hook these up in a circuit and then just change the resistance thus changing the current. With different currents the strength of the magnetic field should change and then the falling speeds would be effected.

For timing I have a timing system built that would allow me to time the differences.

The original idea came from the phenomena that if you drop a magnet through a metal tube it will fall slower. My only problem with this is that I need to be able to change a variable to see the if it has an effect. Thus the idea with the electromagnet came to be.

 

[negitron]

The original idea came from the phenomena that if you drop a magnet through a metal tube it will fall slower. My only problem with this is that I need to be able to change a variable to see the if it has an effect.

- Use tubes of different materials, but the same wall thickness and diameter.

- Use tubes of the same material and wall thickness but different diameters.

- Use tubes of the same material and diameter but different wall thickness.

 

[edwinem]

Yeah I thought about that except that buying different tubes costs too much money. I tried using aluminum foil to see if I could get the same effect but it failed.

 

[Phrak]

edwinem. It's going to be difficult to measure fall time. It would be easier in the long run to make two identical tubes. Drop different materials and shapes, and change the currents in the solenoids.

 

[Bob S]

This sounds like an interesting project, but at the same time challenging. Measuring the fall time will be difficult because the transit times are so short. For example, a 1-meter vertical fall is 0.45 seconds, so you probably need to measure the fall time to about 0.001 seconds accuracy. You could measure the simultaneous fall times of two objects, but then you have to release the two objects at exactly the same time. A differential start-stop timer (started by the faster one and stopped by the slower one) could be made out of several Radio Shack IC chips.
You could slow everything down by tilting the tube to an angle θ, so that the acceleration is g sin(θ) rather than g, but then your falling objects would need to be balls (ball bearings), because they have to roll. The acceleration is a = (5/7) g sin(θ). This is because the moment of inertia of a solid ball is I = 2/5 m R².
You should not try to use an iron pipe, because the longitudinal magnetic field inside is very weak (it is about 1/5000 the field in the steel itself). It is because H_tangential = constant across air/steel boundary. Aluminum is OK. If you wrap a coil around a PVC, aluminum, or cardboard tube, and you use a steel ball bearing, eddy currents in a rolling ball bearing will slow it down when you turn the current on. You would not see this effect if the ball bearing was dropped vertically (i.e., not rolling).
The best choice for wire would be enamel-coated copper ("magnet wire"), about 20 Ga (3 amps peak),18 Ga (5 amps peak) or 16 Ga (7 amps peak) depending on how much current you can put in the wire. 20 Ga will take more time to wrap, but 16 Ga requires more current.
Bob S

 

[edwinem]

Thanks a lot guys. Your advice was a lot of help.