Some time ago I asked you all to confirm my thoughts about why a magnet dropped through a copper pipe has its descent slowed. Of course my thoughts were confirmed by Sibeen who kindly reminded me of Lenz’s Law.

Now what would be the result if you were to first lower the temperature of the copper pipe by emersing it in liquid nitrogen?

I think it would slow the descent of the magnet more than if the pipe was at ambient temp.

All your thoughts appreciated & I do intend to test it when I get to visit my mate in Ambergate that has a dewar flask full of the stuff.

:-)

Less resistance in copper more current flow , more magnetic field

Will be slower fall

Hmmm, I was thinking along those lines myself. Is there an equation that may prove it?

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scond.html

copper doesn’t become a superconductor. and most others do at below what liquid nitrogen can get them. soooo i reckon there will be no difference.

So you think the idea may be counter intuitive, Boris?

http://www.teachengineering.org/view_lesson.php?url=collection/van_/lessons/van_mri_lesson_8/van_mri_lesson_8.xml

i haven’t a clue but i don’t think the resistance of copper will change much with a reduction in temp. and i don’t think -77 is really cold enough. but i’m a printer sooo.

Hey ol’ mate. I ain’t havein’ a go. I don’t know myself. That’s why I’m asking. :-)

Mean time I’m working through Roughies link…

Bulgarian Umbrella said:

Hey ol’ mate. I ain’t havein’ a go. I don’t know myself. That’s why I’m asking. :-)

Mean time I’m working through Roughies link…

didn’t actually say that you’d find the answer there.. ;)

i know bu, i was just saying i don’t have a clue but just post links that i semi understand.

:-)

from morries gif it don’t look like the resistance changes much at the lower end.

>>didn’t actually say that you’d find the answer there.. ;)<<

Doesn’t matter…. There’s some interesting stuff in there. ;-)

lower the temp of a neobium magnet and you might see a difference. plus it would be easier to cool. being small and all.

JudgeMental said:

from morries gif it don’t look like the resistance changes much at the lower end.

It looks like the resistance drops to almost nothing at about the temperature of liquid nitrogen, 77K

http://amasci.com/miscon/eleca.html

Bulgarian Umbrella said:

All your thoughts appreciated & I do intend to test it when I get to visit my mate in Ambergate that has a dewar flask full of the stuff.

:-)

make sure to take a video and post it online :)

>>lower the temp of a neobium magnet and you might see a difference. plus it would be easier to cool. being small and all.<<

I reckon I want to see it. So I need to go see the Mad Russians son & get some liquid nitrogen. At least then we could test both methods.

Duty calls. I’ll pop back shortly.

Bulgarian Umbrella said:

>>lower the temp of a neobium magnet and you might see a difference. plus it would be easier to cool. being small and all.<<

I reckon I want to see it. So I need to go see the Mad Russians son & get some liquid nitrogen. At least then we could test both methods.

If you know anyone in the physics dept at UWA, they do the copper pipe experiment all the time there, as well as liquid nitrogen tricks.

Bulgarian Umbrella said:

>>lower the temp of a neobium magnet and you might see a difference. plus it would be easier to cool. being small and all.<<

I reckon I want to see it. So I need to go see the Mad Russians son & get some liquid nitrogen. At least then we could test both methods.

indeed

>lower the temp of a neobium magnet and you might see a difference

Do you mean neodymium?

“NdFeB can be used at low temperature but at around 135 Kelvin (-138 degrees C), the direction of magnetisation is said to change from a single axis (easy-axis) to an easy-cone which could cause a fall in output of up to 15% due to this spin reorientation”

How about a uranium pipe?

Slower?

yeah, that one.

and from the hyperphysics link

The superconductivity in Type I superconductors is modeled well by the BCS theory which relies upon electron pairs coupled by lattice vibration interactions. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behavior correlates well with the BCS Theory.

A magnet falls much more slowly through a roll of aluminium foil that has been cooled with liquid nitrogen.

BU, perhaps an easier experiment would be to use two different copper pipes having the same inner diameter but one of the pipes having a thicker wall, and therefore less resistance.

In a way the proposition is analogous to the damping you notice of a speaker cone if you short the speaker terminals, or have an amplifier connected and try to move the cone, in which case the amplifier produces the damping.

So a good way to understand it might be that way, you’ll notice system damping, sound pressure and distortion are very much influenced by voice coil temperature.

transition said:

In a way the proposition is analogous to the damping you notice of a speaker cone if you short the speaker terminals, or have an amplifier connected and try to move the cone, in which case the amplifier produces the damping.

So a good way to understand it might be that way, you’ll notice system damping, sound pressure and distortion are very much influenced by voice coil temperature.

you can determine the matching connections on a 4 wire stepper motor by shorting them out and trying to turn the motor. If the motor doesn’t turn easily, then the two wires are a pair.

sibeen said:

BU, perhaps an easier experiment would be to use two different copper pipes having the same inner diameter but one of the pipes having a thicker wall, and therefore less resistance.

good idea.

>you can determine the matching connections on a 4 wire stepper motor by shorting them out and trying to turn the motor. If the motor doesn’t turn easily, then the two wires are a pair.

hints of our hobbies hey what

Another thought is that liquid nitrogen is going to give you a maximum delta T of about 200 degrees. The same delta Tcould be accomplished by using an oxy torch.

sibeen said:

Another thought is that liquid nitrogen is going to give you a maximum delta T of about 200 degrees. The same delta Tcould be accomplished by using an oxy torch.

I expect to see this turned into a working thermometer

BU, I’ve just worked through some figures on my idea of using different wall thicknesses rather than changing the temperature, and I must say the result made me laugh.

I knew sweet FA about copper pipe so had to look it up. I found a plumbing site that had measurements and realised that the outside dimension will be set and the inner dimension will change with the wall thickness.

I chose two different 1 inch pipes.

The thick walled one has an outer diameter of 25.4mm and an inner diameter of 23.8mm.

The thinner walled one has an outer diameter of 25.4mm and an inner diameter of 24.5mm.

Using a one metre length of pipe I calculate the resistances at 20 degrees.

Thick pipe = 68 micro ohms.

Thin pipe = 119 micro ohms.

Now I did a calculation on changing the temperature of the thick pipe by 200 degrees.

Thick pipe (200) = 120 micro ohms :)

Actually, I should have mentioned that I used an oxy in the last example and raised the temperature by 200 degrees.

>I knew sweet FA about copper pipe so had to look it up. I found a plumbing site that had measurements and realised that the outside dimension will be set and the inner dimension will change with the wall thickness

Perhaps one could use one of those circular magnets that can be recovered from old microwaves, with a suitable spacer to make it fit on the outside of the tube?

morrie said:

>I knew sweet FA about copper pipe so had to look it up. I found a plumbing site that had measurements and realised that the outside dimension will be set and the inner dimension will change with the wall thickness

Perhaps one could use one of those circular magnets that can be recovered from old microwaves, with a suitable spacer to make it fit on the outside of the tube?

I doubt it will make that much difference, morrie, as you need a fair bit of room between the magnet and the wall anyway, to make sure it doesn’t drag along the wall as it goes down.

I just did the calculation for dropping the temperature of the thick tube by 200 degrees and get 15 micro ohms.

So there is a substantial difference either way you go.

Amazingly if you drop by 200 you reduce the resistance to a third. If you increase the temp by 200 the resistance will be 3 times higher. Ain’t maths grand :)

copper = .39-.43%/degC increase maybe?

So, for example, a voice coil temp rise 20C -150C with initial DC R of say 6 ohms would be 9.081 ohms (given coefficient .395 ohms/degC), Z change though be more dramatic.

sibeen said:

morrie said:

>I knew sweet FA about copper pipe so had to look it up. I found a plumbing site that had measurements and realised that the outside dimension will be set and the inner dimension will change with the wall thickness

Perhaps one could use one of those circular magnets that can be recovered from old microwaves, with a suitable spacer to make it fit on the outside of the tube?

I doubt it will make that much difference, morrie, as you need a fair bit of room between the magnet and the wall anyway, to make sure it doesn’t drag along the wall as it goes down.

I just did the calculation for dropping the temperature of the thick tube by 200 degrees and get 15 micro ohms.

So there is a substantial difference either way you go.

Amazingly if you drop by 200 you reduce the resistance to a third. If you increase the temp by 200 the resistance will be 3 times higher. Ain’t maths grand :)

I am surprised at the result actually, since that graph I posted doesn’t pass through the origin, so I would not expect the results to maintain a constant proportion. ie no geometric similarity. Unless I am misinterpreting the term resistivity.

morrie said:

sibeen said:

morrie said:

>I knew sweet FA about copper pipe so had to look it up. I found a plumbing site that had measurements and realised that the outside dimension will be set and the inner dimension will change with the wall thickness

Perhaps one could use one of those circular magnets that can be recovered from old microwaves, with a suitable spacer to make it fit on the outside of the tube?

I doubt it will make that much difference, morrie, as you need a fair bit of room between the magnet and the wall anyway, to make sure it doesn’t drag along the wall as it goes down.

I just did the calculation for dropping the temperature of the thick tube by 200 degrees and get 15 micro ohms.

So there is a substantial difference either way you go.

Amazingly if you drop by 200 you reduce the resistance to a third. If you increase the temp by 200 the resistance will be 3 times higher. Ain’t maths grand :)

I am surprised at the result actually, since that graph I posted doesn’t pass through the origin, so I would not expect the results to maintain a constant proportion. ie no geometric similarity. Unless I am misinterpreting the term resistivity.

…and so you should be, morrie. Discount the last sentence in that post, it makes no sense at all and is not even close to being correct. A few too many beers at that point, I fear :)