# The Naked Scientists Forum

### Author Topic: I'm fairly certain this a physics dilemma  (Read 2618 times)

#### Titanium Egg

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##### I'm fairly certain this a physics dilemma
« on: 02/09/2013 07:53:39 »
I'm trying to build a high powered, lightweight, (long lasting?) electromagnet.  I know it requires a nail, wire, a battery, etc. but I have some questions about that.
1.  What wire should I use?  (What gauge)
2.  How many times should it be wrapped?  (Layers?)
3.  What material should it be wrapped around?
4.  What voltage/battery size will give the most power?
5.  Can it be done to be fitted into a tube roughly 1.5-2 inches in diameter?

#### alancalverd

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##### Re: I'm fairly certain this a physics dilemma
« Reply #1 on: 02/09/2013 08:13:14 »
Field strength B = uNI/L where u = permeability of the core, N = number of turns, I = current, and L = length of the solenoid.

So if you use thin wire with lots of turns (large value of N) you will get the highest field strength. But I = V/R where R is the resistance of the wire and V = battery voltage. Thin wire = high resistance so you need more voltage. The limit is usually set by saturation of the core (u is not constant) at around 1 Tesla for most steels and power dissipation (V^2/R) melting the wire in the middle of the device.

You can find all the numbers you need in any engineering handbook or on line. My preference for a 2 inch magnet would be to go for 40 gauge wire wound on transformer steel, and run the unit at about 50V, but that's only a first guess. For a 0.5T magnet large enough to stand inside, I use 130 tons of steel,  and water-cooled 10 mm copper pipes carrying 140 amps at 200 volts. See www.uprightmri.co.uk

#### allgoaway

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##### Re: I'm fairly certain this a physics dilemma
« Reply #2 on: 02/09/2013 08:25:42 »
I can't tell  you how to billed on but I can give you a few pointers. Your core should be of the hardest iron you can find and make it your core; but if you got the cash get a gold or silver bar. The larger the gage wire you can get the  lower the voltage drop will be. The amount of times you wrap it all depends on the voltage to amps you wont. the more wraps the lower the voltage but higher the amps, but it's always the same watts with AC power

But if your web minded do alancalverd math and you'll get there faster
« Last Edit: 02/09/2013 08:28:31 by allgoaway »

#### alancalverd

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##### Re: I'm fairly certain this a physics dilemma
« Reply #3 on: 02/09/2013 12:35:36 »
Intriguing. What planet do you get your ferromagnetic gold and silver from? The one where Ohm's Law doesn't apply, perhaps?

#### Titanium Egg

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##### Re: I'm fairly certain this a physics dilemma
« Reply #4 on: 02/09/2013 15:39:21 »
I know you're supposed to use some sort of a metal for the core, so i was thinking what metal would work best.  Maybe iron, or aluminum, or titanium, or maybe some alloy.  I don't know, I'm just spitballing ideas.  I have no background in any sort of physics other than last year in junior year high school physics.  I came on here to see if it was even possible and to see if someone could build it.

But from what I gathered, the more wraps around the core I have, the stronger it will be?  With a larger wire, say 40 gauge, at a higher number of wraps around the core, with a higher voltage battery, the stronger it will be?  So if I used a lithium ion battery, because they're smaller, I would want how many volts?

#### alancalverd

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##### Re: I'm fairly certain this a physics dilemma
« Reply #5 on: 02/09/2013 19:24:36 »
Since you don't read the replies, why not just buy an electromagnet?

#### Titanium Egg

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##### Re: I'm fairly certain this a physics dilemma
« Reply #6 on: 02/09/2013 20:38:31 »
Alright, since I read and didn't understand because of my lack of knowledge on the subject, would you be willing to give me a list of materials, and a process on how to build one?  If not, I understand, and thank you for your input and time.

#### chiralSPO

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##### Re: I'm fairly certain this a physics dilemma
« Reply #7 on: 02/09/2013 21:41:17 »
I would suggest using iron for the core. Even as simple as a soft iron nail. You can wind it with any type of insulated wire--make sure that you keep the same direction of winding if you do more than one layer. The strength of the magnet increases with the number of turns, so a thinner wire will allow more turns and therefore a stronger magnet (but will require a higher voltage to get the same current). You can buy very thin, enamel-coated "magnet wire" from a RadioShack, or online. But be careful with the very high guage (thin) wire as is heats up significantly when passing current. (a 9V battery attached to a coil of 80 G wire can burn you pretty badly if you're not careful).

#### alancalverd

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##### Re: I'm fairly certain this a physics dilemma
« Reply #8 on: 02/09/2013 22:21:32 »
Insulated copper wire, iron core (nothing else works nearly as well). Wind the wire round the core.

Look up the resistance of the wire you have chosen. It will be in ohms per meter, R. And check the fusing current (amps) A. Calculate the length of wire you have wound onto the core, L meters - or if you have used it all, look at the packet, which will tell you how much you bought (and probably the maximum current it will carry).

I suggest you limit the current to a tenth of the fusing current or less. So you need a battery whose voltage is not more than 0.1 x A x R x L

#### Titanium Egg

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##### Re: I'm fairly certain this a physics dilemma
« Reply #9 on: 03/09/2013 00:31:17 »
thank you alancalvard for that, it was much simpler.  I can do that.  Thank you for the help

#### Pmb

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##### Re: I'm fairly certain this a physics dilemma
« Reply #10 on: 04/09/2013 00:03:37 »
I'm trying to build a high powered, lightweight, (long lasting?) electromagnet.  I know it requires a nail, wire, a battery, etc. but I have some questions about that.
1.  What wire should I use?  (What gauge)
2.  How many times should it be wrapped?  (Layers?)
3.  What material should it be wrapped around?
4.  What voltage/battery size will give the most power?
5.  Can it be done to be fitted into a tube roughly 1.5-2 inches in diameter?

All this depends on what you desire, i.e. what field strength you're looking for. For the strongest magnetic field with a given current you should choose the metal which best fits your goals. See http://en.wikipedia.org/wiki/Magnet

You should learn about the properties of magnetic materials such as the hysteresis curve of a material. This relates the magnetic field to the magnetization of the material and depends on the history of the magnet.

See http://en.wikipedia.org/wiki/Hysteresis

#### alancalverd

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##### Re: I'm fairly certain this a physics dilemma
« Reply #11 on: 05/09/2013 23:13:27 »

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##### Re: I'm fairly certain this a physics dilemma
« Reply #11 on: 05/09/2013 23:13:27 »