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Help with basic electromagnets

  1. Mar 22, 2016 #1
    I am currently working on and electromagnet experiment with my 8th grade students. I have been given an experiment where we build a basic electromagnet with an iron bolt, enameled wire and a 6V Alkaline Battery, (lantern battery). The goal is to test the strength by picking up BB's, Paperclips, or small washers. My problem is the battery drains extremely fast. What can I change to prevent this from happening. I've can't continue to spend the money out of pocket for all these batteries, but the students love this experiment. Please help.
     
  2. jcsd
  3. Mar 22, 2016 #2
    As you likely know, the strength of the magnet depends on how much current flows. You can go with a weaker magnet. To limit the current flow place a small value resistor (Perhaps 20Ω?) in series.

    Another option is to use more turns of the wire. More turns will multiply the current's effect (turns2) while increasing the parasitic resistance.

    You might also consider a rechargeable battery. These are somewhat pricey, but cheaper than lots of disposables. Also consider a battery holder from Radio Shack and perhaps 4 D cells. (Make sure you use the resistor; otherwise the lower resistance of the D cell could start a fire.) D cells carry more energy for the buck than 9V batteries, but it takes about 6 in series to get 9V. (4 will give 6V nominal and should work for a magnet.)

    Whatever you use, test your setup first in a safe place.
     
  4. Mar 23, 2016 #3

    davenn

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    lots of great suggestions from Jeff :smile:

    The other thing you should really consider is a decent variably voltage and current PSU for the classroom
    and do away with the batteries completely

    here's some suggestions from eBay, probably fin them on Amazon as well

    http://www.ebay.com.au/sch/i.html?_nkw=variable+power+supply


    cheers
    Dave
     
  5. Mar 23, 2016 #4
    I find it works with a small diameter nail, a metre of wire and a 1.5 volt cell. To make a good magnet, wind tightly at one end of the nail. I also temper the nails so the iron is "soft". To reduce expense, tell them to only connect for a moment, or make the circuit with a push switch in it. A power supply is not really the answer because the pupils will over heat the wire.
     
  6. Mar 25, 2016 #5

    NascentOxygen

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    It's a shame that bicycles no longer come equipped with a dynamo, or you could be powering each solenoid from a stationary bike peddled furiously. :cool:

    Need a diode, or four, to convert the dynamo's AC to direct current, but thereafter it's free energy!
     
  7. Mar 25, 2016 #6
    Why do you want DC? AC operates just the same.
     
  8. Mar 25, 2016 #7

    NascentOxygen

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    Not exactly the same. AC won't give as good a deflection of a compass needle.

    While AC may suffice, for an introductory treatment it may be desirable that one end of the electromagnet be identifiable as north so direct similarities can be drawn to an ordinary magnet.
     
  9. Mar 26, 2016 #8
    I've run into similar problems with my capstone project, look into lithium batteries. I've found some that can handle high currents for a decent amount of time. Like stated previously your only other option is to get more turns because the magnetic field is proportional to either the current or the number of turns. Also keep in mind that the more turns you have the higher your resistance will be which may decrease current. In my case the increased resistance was negligible.
     
  10. Mar 27, 2016 #9
    ImageUploadedByPhysics Forums1459057983.625165.jpg
    I used to use one if these for electronics projects, it was really helpful. No need for batteries. The downside would be the limit current in the device.
     
  11. Mar 27, 2016 #10

    sophiecentaur

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    on the face of it, they are attractive but they need to be operated correctly, with a regulator circuit, or they can explode due to over-current. If you are in education then your power supplies have to be totally kosher (as your risk analysis advice should have warned you when you were training). You just do not need any form of accident with kids. It really spoils your day.
    Lead acid batteries behave better and (as long as they are appropriately fused) will not let you down.
    Lab power supplies, with variable output voltage, are available and are more 'acceptable' for use in Schools. Cost a few tens of GBP or USD each. More than cheapo mains adaptors but better suited to your requirement, being safe and robust.
     
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