More to life then just Picaxe fun

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leftyretro

New Member
Ran across the below video yesterday and had a lot of fun researching about them. Not real useful for anything practical as it drains the battery pretty quickly being a almost dead short across the battery. However it should be fun to try and impress my grand daughter with them. I found a ton of Neodymium Magnets available on E-bay and sent an order off for some last night.
Be sure to watch the whole clip as my favorite version is the last one shown. That last one might be an interest to you cat owners ;)

http://www.metacafe.com/watch/1294270/marks_homopolar_motors_02/

Lefty
 
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BeanieBots

Moderator
Magnet + wire + current = force.

How to make an inductor!
Inductance of single length of wire?
(ans. enough to be a problem in FM receivers and switchmode power supplies)

Hope I've got this right from 40+ years ago!
Left hand rule...
Hold your left hand so that your thumb points up.
Index finger pointing forwards.
Forefinger pointing to right.

Thu(M)b = Magnetic field.
(I)ndex finger = current.
(F)orefinger = Force
 
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alband

Senior Member
Ah yes like in bubble chambers for measureing sub-AP's.
Magnetic force is straight up in this case (thumb)
How is the current and force moving.
The current is going in a big loop from + to - yet the force is always at a tangent to the circle it is splinning in?
 
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BeanieBots

Moderator
Draw it out.
Show the magnetic lines, then the current lines and where they cross.
From the "left hand rule" you'll get the force.
From then on it's down to Mr. Newton. F=ma. s=ut+1/2at^2, u=at.
v^2=u^2+2as.
 
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alband

Senior Member
I cannot get this to work.
I've used a normal 1.5V like in the video and it diddn't work. I'm using quite a small magnet so I swapped to a half used mini 12V battery (described as "A23" size) and it still doesn't work.
I've got it set up very similar to my diagram. I've checked the resistance through the magnet and it is vertually nil. I've tried the magnet both ways round.
Has anyone got any ideas?
 
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BeanieBots

Moderator
alband said:
I'm using quite a small magnet so I swapped to a half used mini 12V battery
Click to expand...
Are you familiar with ohms law?
V=IR or more to the point, I=V/R

What's the internal resistance of your 12v battery?
Can it deliver the several AMPS required to make this work.
Even a good condition AA 1.5v cell will struggle and be flat in no time.

The "armature" resistance is a short piece of wire. VERY low resistance. Almost a dead short. It is a single "turn". Therefore requires HUGE current to develop any force. If your magnet is weak, then the current needs to be even bigger to get the same force. Unless it's a 'super' magnet, probably won't work at all before the wire melts.
 
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Rickharris

Senior Member
You can do this by putting a small neodymium magnet on the head of a screw - Put the point to the bottom of an alkaline battery and the screw will stick (by magnetism)

Hold one end of a length of wire to the positive terminal and touch the magnet with the other to complete the circuit - the screw will turn like crazy.

Draws much current though and the wire gets hot so take care.

Just to make this a picaxe thread ! - you can easily build motors where the current through a simple coil is controlled by a picaxe triggered by a reed switch to swap the current at the right time - Hey that's a stepper motor!
 
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BeanieBots

Moderator
alband said:
So basically, bigger magnet.

How do I measure the internal resistance of the battery?
Click to expand...
No, a STRONGER magnet.
As for battery internal resistance, that's a six million dollar question. However, you can get a good idea by measuring the OC voltage, then load with a known current and measure again.
You know the current and the drop in voltage, so from V=IR
R=V/I.

As Rick says, the wire will get hot. Several watts needed to make it do that.
Power=I^2.R R is VERY small, so I must be very large. Several Amps!
 
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alband

Senior Member
I tried that one show in the video using a screw and it still doesn't work?
The magnet is bout 4mm dia and 3mm thick, neodymium.
 
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leftyretro

New Member
alband said:
I cannot get this to work.
I've used a normal 1.5V like in the video and it diddn't work. I'm using quite a small magnet so I swapped to a half used mini 12V battery (described as "A23" size) and it still doesn't work.
I've got it set up very similar to my diagram. I've checked the resistance through the magnet and it is vertually nil. I've tried the magnet both ways round.
Has anyone got any ideas?
Click to expand...
Yes, you must use Neodymium Magnets, they are very strong and this application requires a strong magnetic field supplied only by the magnet.

From some research I did:

"Neodymium magnets are necessary because 1) they are much stronger and 2) they have a nickel surface that both conducts electricity and allows the wire to slide easily.
Update: It has been suggested that you can wrap a standard ceramic magnet with aluminum foil to make it conductive. However, if you do this your motor probably won’t spin as fast as it would with a neodymium."

And as far as how it works, I found this statement:

" The force that causes the wire to rotate is called the Lorentz force. The magnetic field is in the axial direction of the magnet, and the charge flows radially. From F = qv x B, the direction of the force is perpendicular to both, in this case tangential to the surface of the magnet. "

Lefty
 
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alband

Senior Member
I've been trying this for a while now. The crucial difference being I know the equation is F(N)=IxLxB not voltage.
So, using an ordinary AA. I've had some success but not much. I'm using an "uberorb". It isn't round though and doesn't conduct so I've had to wrap tin foil round it and put a smaller, circular magnet on the bottom. This thing is, I cannot get the calculations.
Force in Newtons = Current in amps x length of the conductor in meters x flux desity of the magnet in gauss (B).
An uberorb is made from neodymium and so has 12000 gauss.

I = V/R. The voltage off the AA battery is 1.2V (NiMH AA shaped) and the resistance of the wire is 0.1ohms.
I = 1.2/0.1 = 12amps (this is the first bit I'm not sure about).

The length of each wire is 11cm = 0.11m (my wire is shaped roughly like the big butterfly one in the video and each side is 0.11m). So:

F(N) = 12amps x 0.11 x 12000 = 15840N
Now that just can't be right. I suspect my current calculation is wrong...
 
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ccdubs

Member
Hi Alband,

The problem is that you have got your units mixed up. Flux density should be measured in Tesla not gauss, where 1T = 10,000G. So your peak resulting force would be 15.84 N. This still sounds high (about the same as the force on a 1.5kg weight due to gravity) and in reality would be much lower because the flux density of the magnet will decrease rapidly in air.

You would have to look up the formulas for flux density attenuation in air.

The current from the battery will also be limited to a lower level than 12A by the batteries internal resistance. If you have a multimeter you can measure the current (use the highest current setting)!
 
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eclectic

Moderator
@ Alband.
I am NOT a physicist, and I genuinely do not want to criticise,
but some of the logical steps are errrm ...

“An uberorb is made from neodymium and so has 12000 gauss.”
Is it? Has it?

“It isn't round though and doesn't conduct so I've had to wrap tin foil round it and put a smaller, circular magnet on the bottom. “
Please explain.
How can wrapping a magnet in Aluminium foil, with another magnet,
maintain the Flux Density?

e

Added. ccdubs had a more erudite reply.
 
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alband

Senior Member
I've tried measuring current from batteries before and I always get zero, every which way I try it.
The exact formula for the force is:
F (newtons) = I.l.B

This explains it, and this explains the magnetism/B/G/T side of it.

eclectic: "“An uberorb is made from neodymium and so has 12000 gauss.”
Is it? Has it?"
Click on the 12000 gauss in that post and use find (normally Ctrl+F (you'll know this I'm sure)) to look for "12,000" with the ",". However, I no nothing of magnets (comparatively) and so am not sure just what gauss, Tesla et-al are. Is gauss a constant that depends of substance like mass per gram or does it depend on material, size shape etc. like mass.

What on earth? I've just edited this ^ last bit in and instead of "save" I've got "Vote Now", I'm gonna click it an see...
2nd Edit: Yep is saved fine, there must be some other voting feature of Vbulletin that does voting and it got the software equivalent of "mixed up"; never mind...
 
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eclectic

Moderator
alband said:
eclectic: "“An uberorb is made from neodymium and so has 12000 gauss.”
Is it? Has it?"
Click on the 12000 gauss in that post and use find (normally Ctrl+F (you'll know this I'm sure)) to look for "12,000" with the ",". However, I no nothing of magnets (comparatively) and so am not sure just what gauss, Tesla et-al are. Is gauss a constant that depends of substance like mass per gram or does it depend on material, size shape etc. like mass.
Click to expand...
I DID read the reference,
which appears to be from a vendor of magnets.

Do you have a valid, reliable, scientific, source of information?

e
 
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alband

Senior Member
My apologies, I didn't mean to insult. :eek:
This is another vendor, this is well something, this is another vendor and this appears to be a little more scientifice but still a vendor.
So, no; I can't find any specifically science sites that give the figure, but the general consensus is 12000.
 
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ccdubs

Member
If you can't measure the current, then either your meter is maxing out or you have blown the input fuse. Most likely the latter.

Also, remember when measuring current that the meter must be in series, i.e., one lead to +battery, the other to one end of wire.

Ideally you would put a known resistance across the battery, say 10 ohms, and then measure the current. Without internal resistance you would get 1.2/10 = 0.12A. To determine internal resistance take the measured result and plug into:

Rint = V/I - R (derived from V=IR=I*(Rint+R)

Where V=1.2V
I = Measured value (say 0.1 A)
R = Known load = 10 ohms
Therefore: Rint = 2 ohms

Alternatively see if you can find a datasheet for your battery on the interweb!
 
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slurp

Senior Member
This is FAB!

I've got a small pile of neodymium magnets and had a play this evening, it works a treat but there are a couple of thing I think make a read difference.

1. Make a circuit
2. Don't let the contact press to hard
3. Balance the wire (or wires if it drops both sides) to aid #2
4. Avoid other points of contact (drag).

I used 3 12x5mm magnets (haven't tried variations yet!) one AA battery that had been chucked into the recycling bag (so relatively flat!) and a bit of solid copper wire that had been recovered from an old electrical cable (typical of 240v house hold wiring).

A spiral down the battery works well as it can be balanced easily, symmetrical can be more problematic in terms of balance vs. contact pressure. A little tuning can work wonders, something I made that was slow was much faster when the kids tweaked the shape a little.

Excellent toy :)

regards,
colin
 
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alband

Senior Member
Right, I've just measured the battery. The problem was actually that I am dumb and forgot to switch one of the leads :rolleyes:.
I got about 8 amps max.
So, let's try this again (also using 12 tesla not 12000 gauss):
F (newtons) = I.l.B
F = 8 x 0.11 x 12 = 10.56N
Thats about 1kg yes? That still can't be right...

slurp: Have you seen the video at the top of this thread? The little car one at the end looks good.
 
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slurp

Senior Member
alband said:
slurp: Have you seen the video at the top of this thread? The little car one at the end looks good.
Click to expand...
Yes, saw the video... that's why I tried! I need some fresh batteries for the last one, I don't think there's enough juice left in these old ones.

Looking at these I think 1Kg is a bit optimistic! Very low grams, if the balance is anything to go by.

regards,
colin
 
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InvaderZim

Senior Member
The Left Hand Rule for electric motors. I always learned it with the Right Hand Rule, which I guess is more universal in math and physics. But I think for engineering, the left hander is more practical; I wish I'd learned that a long time ago!

For a ton of fun with magnets, check out this collection of do-it-yourself experiments: Sci Toys. I love all the magnet ones, but this boat (you have to scroll down a bit) is the best! There's a movie too.
 
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alband

Senior Member
That makes more sense then. My wires are about 1-2cm away from the magnet at the point where they would cross the field lines.
I think I'll need to buy some better neodymium, circular magnets at some point.

Really cool site, will definitely try the ion motor.
 
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geoff07

Senior Member
fridge magnet

For me the screw version worked first time with just a single 1cm disc magnet from a fridge thingy. I found touching the unthreaded section of the screw just by the screw head worked better than touching the magnet, probably due to less friction. Strong smell of ozone!

The link with picaxe is that it Just Worked!

How nice to see such an absolutely elemental demonstration of the principles of electro-magnetism!
 
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alband

Senior Member
Well, I got some magnets off eBay and it works a treat, thanks retro! :D

Could current flow be replaced by an ionic substance i.e. one that has extra electrons e.g. O-2?
So that a substance can move. Could this be used to make a wheel that would keep spinning when magnets are either side of it?
Also, if the wire was replaced by a resistor, would this stop the battery running down?
 
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BeanieBots

Moderator
Anything with excess electrons will move in a magnetic field. (until ballanced again).
That's how things like spectrum analysers work. Oxygen (O-2) however, does NOT have any excess electrons. They are both bonded.
If it was as simple as strapping magnets next to a wheel of the right material, we would all be driving around in perpetual motion machines by now:cool:

The closest you can get is with a shorted superconductor that already has a current flowing in it.

Replacing the wire with a resistor would reduce the rate at which the battery runs down. It would also reduce the force and hence the speed. The resistance of the wire is about as high as you can get away with. As above, you don't get something for nothing. (with the exception of this forum).
 
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alband

Senior Member
I meant an oxygen ion. It would for a negative charge, yes?
But anyway, solid oxygen is rather chilly.
are there any materials that are solids and form negative ions?
 
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