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Unsolved Mysteries? When like charges attract!

March 18, 2009 on 2:29 pm | In Physics |

Attraction and repulsion are two of the simplest concepts in electricity and magnetism: like charges repel and opposite charges attract. (Two steps forward, two steps back.) For electricity, this is how positive and negative charges work:

Two positive charges will repel each other, two negative charges will repel each other, but one positive and one negative charge will attract one another. Simple enough, right? Except, everything is made up of atoms, which have positive and negative charges all throughout them. Certain materials are excellent conductors, which means that positive and negative charges are relatively free to move throughout a material. So if I bring a negatively charged rod close to a neutral conductor, the following happens:

The positive charges line up on the side closest to the rod (because they’re attracted to it) and the negative charges line up on the side farthest from the rod (because they’re repelled). Since the “opposite” charges are closer and the “like” charges are farther, this means that the force from the opposite charges is slightly stronger, and so overall, the negatively charged rod attracts the neutral conductor.

Now, here’s the weird thing: what if you charged up the metal with some negative charges? Would it repel the rod, since negative charges repel? Or would it still attract the rod, since the “opposite” charges are closer than the “like” charges? Well, the answer is both! Check it out!

What I’ve graphed here is Force vs. distance for a negatively charged rod brought close to a negatively charged conducting piece of metal. When the distance is large, they repel each other. But when you bring them close enough, the fact that the opposite charges are closer becomes more important than the fact that there are more like charges, and they attract! There’s even one perfectly balanced point where the force is exactly zero!

And of course, that’s just for electricity. Do magnets do the same thing?

Absolutely! North repels north, south repels south, but north and south attract each other. Magnets make powerful magnetic fields; the stronger the magnet, the stronger the field. What’s really neat, though, is that if you apply a strong enough magnetic field to some materials, like iron, they magnetize, too!

Well, if you bring a weak iron magnet’s North pole close to a strong magnet’s North pole, what do you think will be more important? The fact that the weak magnet is made of iron, and can be magnetized, or the fact that the North pole repels the other North pole? Let’s go down to the molecular level to find out…

Weak magnets have little tiny molecular magnetic moments pointing in many directions. Overall, though, there will be more pointing in one direction than any other, and that’s how your material is “magnetized.” But if you bring a strong magnet close enough, it applies a strong enough magnetic field, and will re-magnetize the weak magnet material to attract the strong magnet! So, is it the same deal as the electric charges? Let’s have a look:

It’s exactly the same! Far away, they repel, close in, it magnetizes and attracts, and yes, there’s a point right in the middle where it balances perfectly, and the force is exactly zero!

So the next time someone tells you that like charges repel each other, you’ll know the exception to the rule!


26 Comments »

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  1. This article has been added to the Astronomy Link List.

    Comment by Astronomy Link List — March 19, 2009 #

  2. I took the introductory course on EM and despite knowing that dipoles wear off like r^-3, this never ‘clicked’.

    Now I feel dumb. Again.

    Comment by Sili — March 19, 2009 #

  3. […] Unsolved Mysteries? When like charges attract! Starts With A Bang! Yes positive charges attract negative charges and positive charge repel positive charges. But there is an exception to the rule as Ethan explains here. […]

    Pingback by Link list – 19th March 2009 | Astronomy Link List — April 7, 2009 #

  4. hello Sir!
    I need the answer of the scientific problem i.e, “Why same charges attract each other and different charges repel each other?”
    plz sir,
    I am kindly requesting you to give me the guidence about the above problem.
    I’ll be very greatful to you.
    Shehzad Ahmed student of FSc pre-medical in Northern Areas Gilgit Pakistan.

    Comment by Shehzad Ahmed — July 7, 2009 #

  5. DCEWXn

    Comment by Velrlmvv — July 13, 2009 #

  6. WHY? why do the charges attract and/repel??? WHY.

    Comment by carrie — February 19, 2010 #

  7. The basic question remains there…
    Why do like charges repel and unlike charges attract each other…WHY???
    Why electrons like protons???
    .
    .
    .
    I am a high school student in India and i know about behavior of charges from my childhood, but still i dont know the reason.
    .
    .
    There’s not much about it on the Internet too..
    .
    Please Reply

    Comment by Avijeet — April 8, 2010 #

  8. this concept is based on quarks which are the fundamental particle of which proton and electrons are further made.
    There are six kinds of quarks. There names are: up, down, strange, charm, bottom, and top. They are in order from lightest to heaviest in weight. All quarks have antiquarks. They are opposites of quarks. Quarks have positive charges, while antiquarks have negative charges. There is also another type of matter that makes up atoms. They are called leptons. Leptons are similar to quarks. Although some leptons are larger than quarks, and some are smaller. The most well known lepton is the electron. The other types of leptons are tau, muon, and neutrinos. Tau and muon leptons have some electrical charge, and are heavier than electrons. Neutrinos have very little charge, and very little mass. They are very difficult to find. Neutrinos are so stable, that when they pass through the atompsphere, they don’t react with anything.

    further 1electron is made up of three down quarks carrying -1/3 charge and a proton is made up of two up quarks and one down quarks.the charge on up quark is -2/3 .
    due to same composition of a two proton ,the same quarks exert repulsive force on each other.thats why same charge repel each other.
    same is the case with two electron
    on the other hand a electron and a proton are made up of different quarks which exerts attractive force on each other .thats why unlike charge attract each other.
    the attractive nature of diiferent quarks and the repulsive nature of same quarks is due to fact that the quarks doesnot exist independently

    Comment by ashray — April 16, 2010 #

  9. I absolutely LOVE this website, when I was doing this in physics i almost jizzed there and then. Infront of all my ” football friends ”

    Comment by Coner Cutland — June 17, 2010 #

  10. the quetionas to how the like poles repel each other and unlike poles attract poles remain unsolved?
    The answers given here is irrelevant

    Comment by mona — July 19, 2010 #

  11. mind bloing / &simaple explanation

    Comment by karan arora — August 13, 2010 #

  12. I am interested to find out out what they call gay electrons? when 2 negaitive charge electrons attract each other. Can you please send me some articals or information about this?

    Thank you.

    Sean

    Comment by Sean Shab — October 18, 2010 #

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    Comment by mbt — May 24, 2011 #

  14. actually why do like charges repel while unlike attract depends in energy needs of particle.any particle that has mass is basically consist of energy in combined state.two similar charged particle will have same energy needs so when they will come closer it leads energy crisis.however it is alwaysnot true that they will repel. under special cases they may attract.any particle consist of various energy interaction like radiation,electrostatic,magnetic,gravity e.t.c.si basically all interaction whether repulsion attraction, gravitation e.t.c are mean to create balance for stable existence of these particles……friends i am still on my way to combine all such undolved puzzle in one grand unified theory…..just working hard lets hope we may get our queries solved

    Comment by davi — June 15, 2011 #

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    Comment by Wally Folmer — September 6, 2011 #

  17. We know that foce of equal magnitude but opposite direction cencel eathother , according to electic field created by positive charge has a outword force , and if the same positve charge is brough close to the charge then both would have same electric field outword and exert same foce on eachother , then how can they repel , as we know force of equal magnitude and opposite direction repel eachother. then positive charges shouldn’t repel eachother , and this phenominan must end in such a way that like charge nither repel eachother nor attrect but remains in equelibrium state.

    Comment by usman — September 13, 2011 #

  18. You really exceeded my desires. Thank you for presenting these invaluable, safe, informative and as well as unique guidance on the topic to Julie.

    Comment by Replicas cmisetas futbol — October 12, 2011 #

  19. Hey I had a question,

    When u charge the plastic rod with negative charges, why don’t negative charges on the rod itself repel each other ? Or do they?

    Comment by Abhimanyu — May 1, 2012 #

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    Comment by birthday quotes — May 31, 2012 #

  21. I think:According to QED(Quantum electrodynamics)charges interact through virtual (invisible) photons.Virtual Photons (bosons)are basically electromagnetic force carriers.Every time charges emit the virtual photons.Suppose,there are two charges in free space & they are continuously emitting the virtual photon which collides the the real charges and transfer the momentum and this way the charges repels and attract each other…….

    Comment by vedant — June 24, 2012 #

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  24. electric flux=EdScos90¬į=0 why dS perpendicular to E

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  25. how to differ the magnetic power in north magnetic pole, how to decide the desire position of pole

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