Solenoid

[stockgameecon2005]

I'm a high school senior trying to figure out this for an engineering project.

How do you make a solenoid with wires wrapped around a 12 inch by 9 inch (and 1/4 inch thick) piece of metal? We wrapped it around and ran a current through it (using a 12 volt voltage from a transformer pluggged into a wall outlet) (we measured the resistance in our wire to be about 30 ohms), but the big piece of metal wasn't getting magnetized.

What I need to know is why it isn't working, and how might I get it to work.

I understand that normally, solenoids are made with wires wrapped around a metal rod or that type of thing, but right now we're wrapping it around a big slab of metal because we want a giant magnet. We want it to be used as a magnet behind a "white board" that we are making, using pixels filled with iron filings.

Please help me, thank you!

[Ceal Craig]

[quote="stockgameecon2005"]I'm a high school senior trying to figure out this for an engineering project.

What I need to know is why it isn't working, and how might I get it to work.

I understand that normally, solenoids are made with wires wrapped around a metal rod or that type of thing, but right now we're wrapping it around a big slab of metal because we want a giant magnet. We want it to be used as a magnet behind a "white board" that we are making, using pixels filled with iron filings.

Hi! Interesting question!

First thing to recognize is that the physical size of the magnet is not the key driver for magnetism strength. Next, not all metals can be magnetized. The metal needs to be ferromagnetic to start: for example, not all steels can be magnetized. Next, the shape of the metal can, I believe (though I'm less certain of that), influence the result as well.

Here are some sites to check out:
http://www.wondermagnet.com
http://science.howstuffworks.com/electromagnet.htm

Let me know if this helps and if it works out!

Ceal Craig
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[EDS]

Hi folks,

Hope you don't mind if I add a few thoughts.

As Ceal Craig said, the type of metal is important. Your best bet is to use something that will stick to a refrigerator magnet as the core of your solenoid.

I'm curious what sort of geometry you're using for this.

For an ordinary solenoid, the magnetic field runs along the axis (long side) of the solenoid in the region inside the wires. Along the sides of the solenoid, the field is pretty small. Here's a crude picture:

Code: Select all

     ^
     |
     | B field
     |
+--------------->  
+-------+
+-------+
+-------+
+-------+
+-------+
+-------+
+-------+	
+-------+
+-------+	
+--------------<--  current
     ^
     | B field
     |

If I were you, I'd start by wrapping a small iron bar (like a big screw or nail) to get a sense for how strong a magnet you can make with your materials. If you then place a small sheet of ferromagnetic material (like iron, some steels, etc) across the top of the solenoid, you may be able to create a flat surface that will hold iron filings. (I've never tried it myself.) Play around with a couple sizes and see what improves things and what doesn't. You can probably buy sheet metal from a local hardware store for a few bucks a square foot. Bring along a fridge magnet to find a good ferromagnetic metal. Baking pans or tea tins will sometimes work too.

Another option is to try to make a really big solenoid such that the white board area fits inside of it. You can certainly try it, but you may discover that you need much more current than you can produce with your power supply. It's hard to make big magnets!

Also - you may already know this, but you should be careful not to make a coil with such a low resistance that it draws more current than your power supply can handle. From Ohm's Law, the current, I, is given by V/R, where V is the voltage in volts and R is the resistance in ohms. For your setup, I = .4 Amps (or 400 milliamps). Your power supply may have a maximum current printed on it. Chances are that if it's an ordinary household wall-wort supply it's something like .3 to .6 amps. If it doesn't have something printed on it, touch it to see if it's getting hot.

Good luck!

Also, if you want to talk about how you might go about calculating the sort of magnetic field you expect from an electromagnet, I'd be happy to help you get started.

Best,
Erik

[Ceal Craig]

Erik, thanks a bunch!! Great thoughts and suggestions

Good luck Engy!

Ceal Craig
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