Dismiss Notice
Join Physics Forums Today!

Electro magnets / DC / metal wire / vibrating?

  1. Apr 15, 2006 #1
    Electro magnets / DC / metal wire / vibrating???

    Hi, just a very basic question:

    I just tried making my own electro magnet (copper coil, steel core, connected to a 12V 2A DC power source) and i noticed that when i hold a metal wire (or just anything that's normally attracted by magnets) in it's field it starts to vibrate.

    Is this because the dc output of an ac adapter is not a perfectly straight current but a rectified version of the ac?
    Last edited: Apr 15, 2006
  2. jcsd
  3. Apr 15, 2006 #2
    You are correct.
  4. Apr 15, 2006 #3
    Ok thanks.

    What is used to convert it to a straight current?
  5. Apr 15, 2006 #4
    usually a capacitor is used to release stored charges slowly, which is exactly what an adaptor does, so that the resulting current is approximately DC. The more slowly the capacitor releases stored charge, the less undulation in the electric current, and the less vibration in the wire. so my answer is to get a capacitor that can store more charge.

    But again, this is an approximation, so there will be a little, probably still be a little bit of vibration left.
  6. Apr 16, 2006 #5
    So you get one easy shot somewhat leveling the dc out by using a better capacitor inside the adapter? One because a capacitor doesn't work with dc right?
    Or did i miss something in high school?

  7. Apr 16, 2006 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What do you mean by a capacitor doesn't work in DC?

    Bobbytkc: As far as I know a capacitor doesn't convert AC to DC, there is simply a 'lag-time' between the voltage and the current.

    You would need a transformer to convert AC to DC.

  8. Apr 16, 2006 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    :eek: :bugeye: Worried look
  9. Apr 16, 2006 #8
    a transformer does not convert AC to DC. The purpose of a transformer is to turn a larger voltage/lower current from an electrical source to a lower voltage/higher current that our appliance can accept without overheating. It works through the induction of two coils, one with a larger number of turns than the other. The coil with the larger number of turns (the primary coil with the electric current from the source) induces an EMF in the secondary coil with fewer number of turns.

    The capacitor is the only known way ( I believe, i have not seen any other method) to convert an AC to DC, but only through approximately leveling the current. As a rule, a capacitor that can hold more charge and release them slowly over a longer time will make AC closer to DC.
  10. Apr 17, 2006 #9


    User Avatar

    When I get a signature, I will put "please don't try this at home, I'm an expert."
  11. Apr 17, 2006 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I apologise bobbytkc, I confused myself :confused: but I still stand by my comment that a capacitor does not convert AC to DC. However, they are used to smooth the DC output once it has been converted from AC.

    To the OP;

    A the most basic form of a rectifier uses a transformer, a dioide and a resistor, this is know as a half-wave recitifier. Basically the diode only allows current to pass in one direction, say the positive direction, thus blocking the negative half of the AC current, resulting in a pulsing DC current with significant gaps between each current. Imagine drawing a sine wave but just not drawing the curve underneath the x-axis Half-wave rectifiers have a theoretical efficencey of around 40% if I remember correctly.

    A more advanced form a recitifier is known as a full-wave rectifier. This is a circuit composed of a system of four or two diodes (depending on the type of circuit), a tansformer and a resistor. This again only allows current in one direction but converts both polarities into a DC current. Imagin drawing the modulus of a sin function ([itex]|\sin x|[/itex]). Again this produces 'pulses' of DC but there are no gaps between pulses. Full-wave rectifiers have a theoretical efficency of around 80% (I think).

    The current output from the rectifier is technically DC, but is not at a constant level, therefore the output must be smoothed. This is done by using a network of capacitors to reduced the amount of 'ripple' in the output.

    I've put down as much as I can remember there, unfortunatly its a while since I've done any electronics. A quick google should reveal loads of information about rectifier circuits.

    Last edited: Apr 17, 2006
  12. Apr 17, 2006 #11
    Thanks Hootenany, that helped!
  13. Apr 17, 2006 #12
    Thanks hootenany, I forgot about the diodes that allow/do not allow currents to pass depending on its direction. I remember learning this rectifier in high school though, and once you described it to me I realized my mistake. lol.

Have something to add?

Similar Discussions: Electro magnets / DC / metal wire / vibrating?
  1. Electro Magnetic Waves (Replies: 4)