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Some questions regarding Electromagnetism

  1. Oct 28, 2006 #1
    Hello, this is my first post on these forums, and I hope to get a response.

    I am doing an experiment about building a type of an amusement park ride on a much smaller scale, and I have some questions and need some advice.
    Firstly, when I make an electromagnet by wrapping a piece of metal with a wire, and then run a current through the wire, how strong can the magnetic field be? I know it depends on the current but how man volt would i need to make the field strong enough to pull a very heavy metal. say around 3-5kg.
    What if I wrap the wire around a magnet? would that be stronger?

    Second I need something like a transformer which can alter the flow of the current regularly, for ex. if the current coming in is around 30 V. I need something which can regulate it regularly. What type of a mini transformer can I use? I want that transformer to be such that it should be a sort of a button, the harder I press the more volt it should let pass, and if I don't press it at all then it should pass almost none. Is it possible that I build this device myself, can I get instructions?

    Thirdly I need to know how can I create an AC generator. So let's say it can change the current continuously at a rate of around 1 second, or any other time which would be more sufficient.

    Fourthly if I want to build a sort of a banner that would go up and down. Suppose I want to use a solenoid, nothing else. And it should go up and down withing one second. Suppose the tube is 14 inches long. I wrap it with a wire, place a thick steel rod suppose of 1.5 inches. Inside the rod I place a strong magnet, so the rod would go in and out of the tube with a lot of force.I would attach the rod with the tube in such a way that it isn't shot out. then it moves back in by reversing the current. How do I do this, and is using a solenoid a good idea?

    I'm sure such problems are common here on this forum. But since I don't know a lot about physics, I need to know this information.
    Last edited: Oct 28, 2006
  2. jcsd
  3. Oct 28, 2006 #2


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    Welcome to PF, Apples.
    I'm going to leave all of the the technical stuff to the experts, but I have a suspicion that buying off-the-shelf units would be a lot simpler and possibly cheaper than building them yourself (and definitely safer).
    That having been said, a strong magnet doesn't need a lot of input. As I mentioned in another thread, Edmund Scientific has a unit that runs on 2 'D' cell batteries and lifts 500lbs.
    Wrapping your solenoid coil around a magnet won't work for 2 reasons. The first, obvious, one is that you wouldn't be able to turn it off. Secondly, most good permanent magnets already have their domains pretty much saturated (it's as magnetized as it can get). Adding electricity won't help any.
    I also suspect that there's a problem with your banner-raising device, but I'm not sure enough about it to say anything now.
    Good luck with it.
  4. Oct 28, 2006 #3
    Thanks a lot for the reply!
    What I'm doing is kind of urgent that's why i'll be checking here regularly.
    The problem with buying stuff of the shelf is difficult for me since I've shifted from USA, and where I am buying already made stuff is very difficult, because they either aren't available or are difficult to find.
    If that 2 'D' electromagnet is really that strong, I have no other reason to use some other type of magnet. But I need to know some details on how to make it. Which metal should I wrap the wire around? How big should it be? What type should the wire be? Do the wires get hot? etc.

    I too think the solenoid idea is bad, and for the time being I'll drop it.
    And what about that transformer sort of the thing to control voltage mentioned above.

    What about the AC generator. How do I make that.
    EDIT:what about a commutator? what if I use that instead of an AC generator to switch the current continuously... if yes, how do I make one?
    Last edited: Oct 28, 2006
  5. Oct 29, 2006 #4
    I really need help on this. I'll appreciate if people reply.
  6. Oct 29, 2006 #5


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    Sorry; I was too busy to log on again yesterday. I'm afraid that I've about expended my knowledge of magnets.
    As for the banner, what kind of mechanism will work partially depends upon how big the thing is, how much space you have to work with, etc.. If space and design considerations allow it, I'd go ahead and use a solenoid except attach it to one end of a pivoted swing-arm (like a railway crossing gate). A slightly more complicated version, if it has to rise vertically rather than in an arc, would be to use a normal flag-pole pulley system. Mount the lower pulley on the shaft of a reversible motor. There should be enough friction that the (fish-line?) is pulled up and down consistently, but also enough slippage that the motor won't stall out at the end of travel. That way, your limit switches don't have to be deadly accurate (and you won't need them at all if you manually work the main switch).
  7. Oct 30, 2006 #6
    Thanks a lot for the reply. However, what your recommended won't work for what is in my mind. The banner project isn't exactly a banner project, but it's something similar.
    However, I'l be able to go along with it smoothly if those questions in my 2nd post in this thread would be answered somehow.

  8. Oct 30, 2006 #7


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    You'll have to get that from someone else; I just don't know enough about it. Be really careful when working with alternators, though. They can put out some really deadly jolts. Good luck with it.
  9. Oct 31, 2006 #8
    So do I wait for someone to see this thread or what?
    Or can I somehow call the other smart members of this forum to this thread.
  10. Nov 1, 2006 #9


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    You can post a request for attention in Feedback, but I don't know if it will help. Certainly others have read this; I don't know why they aren't responding.
  11. Nov 2, 2006 #10
    That's probably because they get no incentives.
  12. Nov 2, 2006 #11
    I think what you're looking for is a potentiometer, not a transformer. (imagine a dimmer switch on a light). It's basically an adjustable resistor that you could use to smoothly control the current to the load. Most transformers only come with preset "taps" of certain voltages. Sure you can rig switches to move the load from one tap to the next but it wouldnt be a smooth transition. Although I'm not too clear on what you need it for.
  13. Nov 3, 2006 #12
    I just need it for simply adjusting the voltage very frequently. The voltage would be around 10 or maybe less V.
    I'll be connecting an electromagnet powered by whatever is sufficient (danger says 2 D batteries). And I'll be needing something to adjust the voltage to reduce/increase the power of the electromagnet with whatever you want it to.

    If a potentiometer is the thing, is it possible for me to build it myself? If yes, how? If not, can I use a dimmer switch, but using a dimmer switch would to have problems.

    What about the AC generator thing?
    Last edited: Nov 3, 2006
  14. Nov 6, 2006 #13


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    I think the reason why you don't get any response is that your question is a bit "how do I become an electrical engineer through some forum discussion, and quickly please", and you start off by quite some confusions.

    First of all, it is not a piece of metal, but a piece of ferromagnetic metal (iron, say). If you wrap a wire around an aluminium bar, that won't work very well. Next, "how strong can the field be" is not very clear. It seems to be that you relate strength of the field to "weight lifting capability", which is a rather complicated relationship. If I tell you that a good classical electromagnet can give you induction strengths of the order of, say, 0.5 Tesla, does that help you ? It won't, because it depends on the geometry of your magnetic circuit.

    First of all, a transformer doesn't do what you say, current is not measured in Volts, and the thing you want to construct needs some basic knowledge of electrical engineering: do you mean a dimmer, or do you mean a potentiometer, or do you mean an autotransformer or...
    All that 1) depends on the specific application, and 2) needs some knowledge in order to build one, if you don't want to do silly or dangerous things.

    An AC generator is rather an electromechanical device used to generate electricity from mechanical work (as in power plants). However, it seems that you are more looking for a kind of electronic chopper or oscillator. Again, one needs electrical engineering knowledge to build one - especially if it is a power device.

    Seems you want to build some kind of linear motor. Again, some basic electrical engineering knowledge is needed in order not to do stupid things.

    Don't be offended, I'm just trying to point out why you don't get any response you'd like: the "knowledge gap" seems far too big between what you seem to know right now and what you want to realise quickly as a project. It's bound to fail and it's gonna be dangerous too. You could just as well ask how to replace a kidney on your little sister, and what kind of anesthesia to use, and what kind of knife to buy and how to test if the kidney of the donkey you have in your fridge is going to be ok or not :bugeye:
  15. Nov 6, 2006 #14
    Thanks a lot for posting even though you embarrassed me quite a lot:).
    I know I'm using such terms, because I don't know the thing, so I'm just saying things which might be close or give you people an idea of what I want.
    I'll provide all the details you ask.
    Yes, I do need a potentiometer. If I can't build one, I'll try finding one. But it has to work on small currents. I don't think a dimmer switch will work, because here it operates for fans where the V is ~220.

    About the 'current-flow switcher', I don't know what it is supposed to be. I just need something which automatically switches the flow of the current in very less time spans.

    About building the electromagnet, I don't know what you want me to tell you :P.

    Is there anything else?

    And no I don't want to be an Electrical Engineer overnight. Electrical engineering involves more complicated stuff, while I , an illiterate, am just looking for the 'right' electrical appliances.

    Looking for more responses and help.
  16. Nov 6, 2006 #15


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    As vanesch tried to point out.
    Tell us what you want to end up with.

    I have no idea if your current questions have any relation to the desired result.
  17. Nov 7, 2006 #16
    OK let me put what I want very simply:
    I want to build an electromagnet with the North and South in different directions(This means that It should not be like a horseshoe). This should be strong enough to pick around 3 KG(I am not sure about the weight yet, making the measurements is too early for me yet). I want to be able to adjust the voltage manually. And I want the North and South sides to switch (I don't want the iron piece of the electromagnet move, I just want the sides to switch) frequently around once in 0.5 seconds (again this is too early for the exact measurements).

    That's it. What I did was ask each thing separately. I asked for the potentiometer to adjust the voltage, I asked for the 'current flow switching' thing because once the flow of the current switches the North and South of the electromagnet will automatically switch.

    Now that I have explained myself as much as possible, if there is still something I am missing, do specify it and I'll tell you.

    Now please post solutions, answers, what electrical appliances I need etc.

    Thanks a lot.
  18. Nov 7, 2006 #17


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    Off the top you might look into DC power sources, H-bridge, pulse width controlers, timers and magnets.

    Does your magnet just have to hold 3kg or does it have to pick that up from a 10cm distance?
    Why do you want polarity reversal? Not normally important for picking things up.
    How long does it operate in use? 10 sec or 5 hours.
    How long is it off before the next use?
    How many people get killed if the device fails?
  19. Nov 7, 2006 #18
    It needs to pull that ~3kg, not pick.
    The purpose of the electromagnet is not picking. Also, I'm hesitant in giving details which are not relevant.
    How long does it operate in use, it can be 1 second or around 6 hours.
    What does that mean? As in when I switch of the electromagnet and then switch it on again? If that's the question then it might even be 1 second, or 24 hours.
    ROFL:rofl:, well I hope it doesn't fail and no one gets killed. But on a serious not, I don't know.

  20. Nov 7, 2006 #19


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    Err yea :uhh:
    Any reasonable design would have to take that into account.

    I suggest you hire a design engineer.
    They will need the plans for the complete unit.
    Not just the electrical part.
  21. Nov 8, 2006 #20
    All I need are answers to my questions. The rest I'm capable of doing myself. Thanks for the suggestions anyway.
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