School Magnetic Project: How to (Best) Wrap Cu Wire Around A 16d Nail 3

[racookpe1978]

I need to wrap thin copper wire around sixteen 16d nails for my wife's up-coming middle school "science" experiments. Power will be either 1x C cell batteries, or 2x C cell batteries in series. She wants the kids to compare the number of paper clips picked up under four conditions: +3 volt dc or +6 volt dc, 10x wraps of copper or 40x wraps of copper.

So, I plan to wrap 8x nails with 10 loops, and 8x nails with 40 loops.

Realistically, will it really make any difference if I:

1) Wrap the copper tightly around the 16 nails, keeping each loop right up next to each other? Or wrap the nails loosely, with some space between subsequent copper loops?

2) Will it make any difference if I wrap the nails near the head, in the middle, or near the point?

3) Will it make any difference using a galvanized nail, or smooth nail?

 

[MagMike]

A1: Magnetically it won't make too much of a difference, but the visual impact on the students will be better if the wraps are tightly wound. Try a layer of 2-sided tape on the nail to help hold the windings in place.
A2: Location of the windings won't matter too much.
A3: Galvanized vs. smooth: Absolutely no difference.

 

[ScottyUK]

Use a drop of cyanoacrylate glue at the first and last turn to help you start and finish the coil. Would probably work better on a plain steel nail than galvanised.

 

[waross]

It depends on how much the loosely wound coils increase the average diameter and circumference over the tightly wound coils.
With a small core diameter in relation to the coil thickness there may be quite a bit of difference, and with the same voltage applied the fewer turns may be able to pick up more paper clips
Factors:
Amp turns. If the length and resistance per turn of the additional turns is equal or close to equal there may be little difference.
Twice the turns equals twice the resistance and half the current. The Amp turns remain the same. strength remains the same.
With a small core diameter, twice the turns may be more than twice the resistance and the Amp turns and the strength will be reduced.
Battery internal voltage drop. If the lower resistance coil causes a much greater internal voltage drop in the battery then the Amp turns of the coil with the lesser turns will be reduced.
Coils closer to the core have less leakage and are more effective. If the loosely wound coil has a much greater overall diameter, it may have more leakage and so less strength for the same Amp turns.
With a coil geometry such that added turns have little added length and resistance per turn and the battery has little drop under the load of the fewer coils then the strength of the electro-magnet has more to do with wire gage than with the number of turns.
Saturation is not considered.
You may have more interesting results with a two to one turns ratio between the coils and another set of coils with a different wire gauge.
I would suggest 15 wraps and 30 wraps if allowed.
I would check the battery voltages under load to see if battery terminal voltage drop is a factor.
Good luck.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

Use a round nail. I supose there might still be a few square nails about.

 

[stevenal]

Why are adults doing the wrapping? The kids should benefit from the entire experience, and not have it all laid out so it is easily forgettable.

 

[waross]

wrap thin copper wire around sixteen 16d nails

Why are adults doing the wrapping? The kids should benefit from the entire experience, and not have it all laid out so it is easily forgettable.

I agree with you completely stevenal, but from the memory of a similar project, time constraints probably prohibit having the students wind the coils.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Time constraints? My memory of middle school (known as Jr. High at the time) involved very little time constraints. Whatever couldn't be finished in the class period became homework due the next day.

 

[ScottyUK]

Ah, the old days... :-D

 

[cranky108]

Why the magnitic stuff? Why not show how pressure in a bottle will make the lid fly? Then you can enjoy the beer as the kids figgure this out.

 

[racookpe1978]

My thanks to all for your time and the comments.

So, to keep the currents the same, I'll cut every Cu wire the same length to get the same resistance, have longer straight leads in the 8x short-wrapped nails, and shorter leads on the 8x long-wrapped coils.

The wires are insulated, so I'll slip shrink-wrap over the coil around the nail to minimize the unraveling. Probably won't stop it, but it may help reduce it.

You're right, it would be nice to have the kids wrap - and I'd do it that way if I were still helping Boy Scouts and Cub Scouts, but only the first period class would get the experience, and - as poorly as these kids attempt to "print" their names - I have no expectations that they'd finish wrapping nails in time to measure anything. Blah. (Lost four more days this week with that snow/ice storm, so she's already short some class time.)

 

[waross]

Leaving the leads short illustrates the importance of Amp-turns over the number of turns without regards to current.
Popular wisdom aside, increasing the number of turns does not always increase the strength of an electro magnet. It does increase the induction however.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

Hmmmn.

So, as the C cell batteries discharge over time (from her 1st period to 7th period class), the amp-turns flowing will decrease and the number of paper clips would be expected to decrease as well. Maybe from 7-8 in 1st period, to 5-6 by the end of the day.

Sounds like I'd need to split for the $1.95 per switch to get 8x push-and-release (NO) Radio Shack switches to reduce battery loss also. Push the button, get current, pick up paper clips. Release the button, current stops.

 

[waross]

Good idea. Some spare batteries may be an idea also.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

Did you say shrink-wrap? Unless I am incorrect, I think most shrink-wrap is colored, which may make obersavation of the wire impossible. It won't change the outcome of the demonstration, but it may confuse the event in the eyes of the viewer. Having wires wound neetly may not be worth the misunderstanding of the students. So be careful.

I have designed some experments for my kids, and I was careful to use knife blade switches, and the like, because I wanted to be sure there was illusion of magic. This really is simple physics, no magic.

 

[ScottyUK]

Translucent heatshrink is fairly commonplace.

 

[racookpe1978]

The nails and battery cases with insulated wire test lead clips were delivered today, and much awe and gratitude was emitted from attending science teachers. 8<)

Again, my thanks to all.

2. Photo's coming.
3. I used black shrink wrap around the coil ends to "lock" the loops from pulling apart. Couple of other attempts didn't work too well: Ended up passing the final wire loop back under the n-1 loop, but not reversing the loop direction. Passed a shrink wrap band around the wire loop end and over the wire lead, but pushed the 18 gage Cu wire through the middle of the shrink wrap to make a "tee" shape. Once shrunk down, the "tee" of the wire was still poking through the middle of the band.
4. Was unpleasantly surprised by how long the wire needed to be cut to get all 40 loops wrapped. My fault, I had a 0.188 (3/16) OD nail, but didn't add the 18 gage Cu + inuslation thickness wire OD before running L = lead in + 40 x 3.14 x .188 + lead away. Needed 48 inches in each wire!