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Electromagnet to attract small iron pieces

  1. Apr 8, 2015 #1
    i want to build an electromagnet which can attract small iron pieces(2-3 mm long) at a distance of 10 cm from the magnet.I am using 24 awg. magnet wire.Please help .answer these questions -
    1) How long magnet wire (24 awg) do i need?
    2)How much current
    3) what shud be least radius of solenoid
    4) can i use iron nails as soft iron core
    5) How can copper loss be reduced
    6) what is effective length of electromagnet

    [mentor note: image URL fixed]


    Last edited by a moderator: Apr 8, 2015
  2. jcsd
  3. Apr 8, 2015 #2


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    hi hackhard
    welcome to PF :smile:

    well you have lots of room for some basic experimentation there
    some of your questions can be answered specifically, some cannot

    how about trying things yourself and make notes of the results of different setups as you go
    till you find a combination that achieves your goal

    your text says lifting items from 10cm but you pic shows 5cm ....

    1) How long magnet wire (24 awg) do i need? ..... lots, probably 30+ metres
    2)How much current ? .... unknown at this stage
    3) what should be least radius of solenoid? .... experiment with 1cm and greater
    4) can i use iron nails as soft iron core ? ...... probably or maybe an iron bolt .... again experiment
    5) How can copper loss be reduced ? ...... what copper loss ?
    6) what is effective length of electromagnet ? ....... dunno what you mean by this ??

  4. Apr 9, 2015 #3
    In magnet terms 10 cm is a vast distance. You're going to need a very strong magnet to lift even tiny particles at that distance.

    I did a little test. I have a stack of 6 rare earth magnets that are the most powerful ones I have around. Their diameter is 18mm and the whole stack measures 28mm. I cut up some fine steel wool into small bits with scissors. At 10 cm I could only get some of the steel wool bits to stand on end and follow the motion of the magnet when I moved it horizontally. It wouldn't lift any of the particles until I lowered it down to about 6 cm. My guestimate is that I'd have to increase the length of this stack of magnets by 3 to lift some particles at 10 cm.

    Based on that, I would get a steel rod an inch in diameter (2.54 cm) and a foot long (30cm) for the core. Then I'd wrap at least 10 layers of wire on it. Then I'd try it with two D cells in series, and just keep adding more D cells.

    The way it works is that, for a given current, doubling the number of turns of wire doubles the strength, and for a given number of turns, doubling the current doubles the strength. However, the strength of a magnet drops off as the inverse cube of the distance, so doubling the strength of the electromagnet will not double the distance over which it can lift a given weight.

    Your main problem is that 10 cm. If you could work with a much smaller distance, like 5mm, it would be very much easier.
  5. Apr 9, 2015 #4
    i wanted 5 cm

    any ideas how to reduce the heat produced
    does heating affect the resistance of solenoid and thus its magnetic strength(because current reduces)

    i tried sticking a copper plate on the windings(along the length). i got +ve results
    any more ideas
  6. Apr 9, 2015 #5


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    That tells me nothing about your electromagnet construction

    If you want us to help you, you need to be forthcoming with information ... OK ?

    what did you use as your core ?
    what diameter is the core?
    how long is the core?
    how many turns of wire did you use ?
    what voltage from the power supply ?

    please show us a photo of your electromagnet

  7. Apr 9, 2015 #6
    Don't leave the magnet energized for more than a few seconds.
    I don't understand the function of a "copper plate on the windings(along the length)." What's this about?
  8. Apr 9, 2015 #7
    just answer this question..
    1)how do i increase the size the magnetic field.Will using a longer core help?
    2) will, keeping 2 electromagnets side by side(as in pic) increase its magnetic field size
    3)wich is better- a shorter core with twice as many turns or a longer core with half the turns(for a distance of 5cm between electromagnet and iron pieces)
  9. Apr 9, 2015 #8
  10. Apr 9, 2015 #9
  11. Apr 9, 2015 #10


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    I got a magnetic 5mm sphere to lift up at a distance of 6cm, using ~200 other magnetic 5mm spheres. I didn't find small unmagnetized iron pieces.
    A large size of the magnet is really important. A larger core diameter, for example.

    I think what zoobyshoe was asking was: "do you think those requests are the best way to ask others to help you with your project in their free time?"
  12. Apr 9, 2015 #11


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    If you REALLY want more help
    Then answer all my questions I put in post #5 !!
  13. Apr 10, 2015 #12
    hey daven , i havent yet completed the electromagnet. Please Help me with 1 more problem .then ill be able to give the no of turns
    I dont have a single 40 ft magnet wire. Ive got it in 5 pieces which are 8-10 ft long .can i solder them together to form a single 40 ft wire
    ps- sorry for my rudeness, zobbyshoe .the hot weather is getting on my nerves!

    iron core - iron nail
    voltage -24v (through my television ac-dc adapter)
    radius of core- 3mm
    (by copper loss i mean the heat produced in solenoid)
  14. Apr 10, 2015 #13


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    yes you could solder them together, that's not a problem but 40 ft of wire isn't likely to be enough ... note what I said my the earlier post on wire length
    and you really need a larger diameter core find a bit of iron that's at least 1 cm in diameter. a offcut bit of rebar ( reinforcing rod) from a building site etc

    Your 24V plugpack, what is its current rating ?

  15. Apr 11, 2015 #14


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    A horse-shoe shaped core will concentrate the field into a narrow region, if that were of any use to you. There do exist U-shaped bolts, and you could wind a separate solenoid on each of its arms.

    How many strands of your magnet wire would you need to lay side by side to make ½ cm?
  16. Apr 11, 2015 #15
    3.4 Amps

    it is 24 awg magnet wire.(0.5 mm diameter). so 10 strands side by side make 0.5 cm

    can i use 5-6 iron nails together to have a core of ~1cm
    Last edited by a moderator: Apr 12, 2015
  17. Apr 11, 2015 #16


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    That sounds way too thin to me. You could parallel 3 or 4 strands to take more current, but then you wouldn't have many turns.

    Do you have access to junked consumer electronics devices or junked cars? These can be good sources of heavier gauge enameled wire, in their transformers, electric motors, or ignition coils.

    Do you have a multimeter to measure 3A of current?
  18. Apr 12, 2015 #17
    yes i have multimeter and the magnet wire if from a transformer inside a mobile charger

    i have access to toy car motors, transformers from mobile charger and electret speaker
    Last edited: Apr 12, 2015
  19. Apr 12, 2015 #18


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    Your multimeter can measure the resistance of that 40' of wire? Scrape off a bit of the enamel so the probes make good contact.
  20. Apr 12, 2015 #19
    1.024 ohm
    but when i connect it in circuit and check ammeter reading to find resistance , it works out to ~4 ohm
    i think its due to heat produced in solenoid( i burnt my finger when i touched it)
    any ideas how to radiate heat produce quickly
    Last edited: Apr 12, 2015
  21. Apr 12, 2015 #20
    how does increasing core diameter increase the strength? i think that it is inversely proportional to magnetic flux density
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