Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electromagnetism !

  1. Oct 30, 2011 #1
    Some concepts need clarification. I am about to complete my 14 years and am in class 10th.

    1. From my textbook : Take a magnetic needle freely pointing in north-south direction. Hold a thick insulated copper wire connected to battery above magnetic needle and close the circuit. It is observed that when current flows from south to north direction , the north pole of the needle points towards west as long as the current flows through wire.
    Similarly if current is flowing from north to south direction , the north pole of the magnetic needle deflects towards east.

    If wire is held below magnetic needle and current flowing from south to north , then magnetic needle deflects towards east. If current flows from north to south then magnetic needle deflects towards west.

    Q. Why does this happen ? How can I determine the direction in which magnetic needle deflects with respect to current flowing in particular direction ? Is there any concept behind it ? Why is magnetic field set up by a conductor acts at right angles to the direction of flow of current ?

    2. From my textbook : Imagine you are holding the conductor with your palm such that fingers encircle the conductor showing direction of magnetic field and thumb shows the direction of current. This is Maxwell's Thumb Rule.

    Same way imagine a screw rotating , the linear distance showing the direction of current and rotation , the direction of magnetic field. This is Maxwell's Cork Screw Rule.

    Q. Suppose you have current flowing in this direction
    N ^
    S | South to north ​

    Then isn't it that the magnetic field will be in anticlockwise direction ? In one book it was shown that south to north flow of current is clockwise direction. Is it ? But by cork screw rule it shows opposite ! I am confused. Am I missing something ?

    Q. In an electromagnet , how will we determine that which end is north pole and which is south pole in a coil ?

    Q. Why is it that a magnetic field in a magnet starts from north pole and ends in south pole ? Why ?
  2. jcsd
  3. Oct 30, 2011 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    The answer is in your text-book. When you have a current in a wire, you have a magnetic field around the wire. Look through your text-book for this. The underlying concept is called "Ampere's Law" and it is related to the relationship between electricity and magnetism.

    if the current flows north, horizontally, then the magnetic field will be circling the wire clockwise when viewed from the south end. The part of the field by the magnet is pointing E-W, above the wire it is W-E. Go get a right-handed cork-screw and look closely.

    The north pole is the one that the magnetic field lines come _out_ of.
    There is also a trick involving the letters S and N but that's the physics.

    It's a convention - you can set up the theory of magnetism the other way and change all the signs around. That end is only called "north" because a compass points to it.

    As it happens, it a convenient convention - the math turns out simpler if you do it that way.
  4. Oct 31, 2011 #3
    @ Simon Bridge
    You told me nothing new though. That was already in my textbook. Can we not apply Maxwell's Thumb rule here ? Can't we ? Right ?

    Thanks anyways.

    We assume south pole to be the pole of the arrival of current ? If we look the same wire from opposite direction then we'll get the magnetic field in opposite direction. That is if we look at the wire
    N ^
    S | South to north

    We can get by Maxwell's rule the magnetic fields in anticlockwise direction. But if we change the face or look from the opposite face of the same place then will not it mean that magnetic fields are in clockwise direction ? One guy pointed out to me that you are confusing yourself by inserting poles in such a rule. Is he correct ?
    Yes but why ? And what is the trick you're pointing out ?
    So that means when we trace magnetic field of bar magnet by a magnetic needle the the bar magnet faces in north south direction right ? And so is the needle (right ?). So the north pole of both repels and hence we say magnetic lines of force flow from north to south ? I am getting confused. Hmm but will not south pole attract north. We could also take south to north pole flow ? But not ! Is it an experiment ? Please clarify. :confused:

    Thanks very much !!
  5. Nov 1, 2011 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Then you already had the answer to your question. I'm sorry, I can only answer the questions you write down.
    Yes - that's what I said.
    You can apply the right-hand-screw rule ... yes. Do you know why it works?
    No worries.
    No. We don't.

    The magnetic field forms loops around the wire so there are no exposed ends to make a pole. When you put a compass in the field it aligns itself in the direction of the field.

    Because thems the rules.

    There is a common confusion about compasses. I'll explain:

    A compass needle is a light-weight magnet mounted to rotate on a pivot. It is often shaped like an arrow - and one end (the pointy end) is often painted to show it is special. The pointy end is the needle's south pole.

    This south pole is pulled in the direction of the local B field and the north pole is pulled against the local B field. This puts a torque on the needle, so the needle turns until it is lined up with the B field. Often, this turns out be close enough to the direction of the Earths north pole to be useful for navigation.

    The little arrows we draw on the B field lines are just little compass needles.

    It's hard to describe in text. The letters N and S have opposite-sense rotational symmetry.
    If the rotation of the current in a loop when you look at it square has the same sense as the rotation of the letter, then you are looking at the pole indicated by that letter. It's a handy accident. (I didn't describe it right away because I didn't have the picture.)

    Here is how the magnetic field around a current turns into the magnetic poles in an electromagnet:
    ... see how the poles result from adding together the loops of magnetic field around the wire?
    The current enters the coils at the end of the wire at the top.

    If you put a compass needle to either side of the electromagnet in the pic, the needle will point upwards. But if you put it inside the electromagnet, the needle will point downwards.
    Last edited: Nov 1, 2011
  6. Nov 2, 2011 #5
    Yes , I know how the Maxwell's rule - right hand thumb/grip rule or right hand cork screw rule work in the case of deflection of magnetic needle due to the flow of current in conductor kept above or below the needle but yet I fail to tell you, why it works in this case. I know that direction of magnetic of an electric conductor is given by this rule.

    In case of How:- imagine a current flowing through wire like this : -------->
    Then thumb points towards the direction of current and fingers encircling show its direction. So I think magnetic field will be umm anticlockwise which deflects the needle. The same thing can also be concluded by analyzing the motion of a cork screw.
    Yes now I guess you are talking about the coil or solenoid. When current flows in a single coil , say then one end will be south pole where current seem to flow clockwise and other will be north pole with current imagined to flow anticlockwise.

    And this compass needle when kept or hanged on a string follows a directive property that it aligns itself in North - South direction. The end which points towards south pole of earth behaving as a magnet is north pole and other special pointy end is south pole. Magnetic needle when kept near a bar magnet aligns itself in magnetic field of bar magnet such that the north pole deflects and so we conclude that magnetic field flows from north pole to south pole , right ?

    I guess I will have to reread 9th text now.

    Got it , thanks !

    You still have to answer these questions which are left pending :

    1. Why is it that a magnetic field in a magnet starts from north pole and ends in south pole ? Why ? (I think you answered but that did not satisfy me.)

    2. Why is magnetic field set up by a conductor acts at right angles to the direction of flow of current ?
    Last edited: Nov 2, 2011
  7. Nov 2, 2011 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    1. The right hand screw rule works because the math of the magnetic force has a cross-product in it. The cross-product is dictated by the Universe.

    But if you mean, why not, say, a left-hand screw rule or something else?
    It is an accident of evolution that our hands work this way - if we had tentacles, for eg, we would just have to memorize the raw math. However - Cartesian coords are "right-handed" because most humans are right handed. Same with screws. A cross-product in right-handed coords therefore follows the right-hand screw rule. So the relationship between the cross product in math and our handedness is not an accident.

    That's the one. The compass needle is a magnet. If you hang a bar magnet on a string, it would try to orient itself wrt to the Earth's field with the south pole pointing north. There would be no special direction if we didn't draw an arrow on it. The arrow got drawn the way it did because people ages ago cared about the direction towards the pole star. If compass-using civilization developed in the southern hemisphere we'd probably have the arrow drawn the other way.

    I'm sorry about that. Unfortunately I can only try to give you true answers. Satisfaction is up to you. The Universe doesn't care either way. The magnetic field lines have that direction because that is how the compass needle points. That's what the direction of the field means.

    It just does. If it were different, then the Universe would look different and whatever Beings were doing physics in that situation would be asking "why is it not right angles?" It's a meaningless question: congratulations - you've just hit the intersection between natural and pure philosophy.

    If you want to have a shot at knowing how things happen - Science is your friend. You want to know why? Go for pure philosophy.
  8. Nov 2, 2011 #7
    My goodness, how complicated!
    A simple experiment is to use a vertical copper wire passing though a hole in a horizontal piece of cardboard. Attach this wire to a power supply so that the electron flow direction is upward.
    Place a small compass on the cardboard and move it around. You will clearly see that there is a circular magnetic field around the wire. If you grasp the wire with your left hand with your thumb pointing in the direction of the electron flow, your fingers will wrap around the wire in the direction of the magnetic field. (This direction shows the pointing direction of the north seeking pole of the compass.)
    This is called the Left Hand Rule.

    Note: some books like to have current flow from positive to negative (based on early historical errors in the understanding of electrical current). If your text is one of these, you have to use your right hand instead. Guess what the law is called.
  9. Nov 3, 2011 #8

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    @daqdaddyo1: yes - OP understands this. The problem is, he wants to know how come this fortunate relationship exists. Note: a negative current flowing down is equivalent to a positive current flowing up. It is possible to have a current made of physical positive charges, you can even have a current made of both. But you are correct that it is important to distinguish between the convention of electric current in a wire and the reality of currents of moving charges.

    Anyway, I stuffed up.
    The direction of the magnetic field is, indeed, the direction the magnet needle points. What I got wrong was that the Earth's geological north pole is a south magnetic pole.
    It's called "north" because that is the direction towards the "North Star" - Polaris.

    Thus the north-seeking pole of a compass is actually a north magnetic pole.
    The north pole of a magnet is so-called because it points north when suspended on a string.

    I got over enthusiastic - my apologies.
    Here's a demo (needs flash):

    I need another whiskey!
  10. Nov 3, 2011 #9
    Yes , my textbook is the one which assumes that the conventional current flows from positive to negative. So I would rather prefer to call this rule - Maxwell's Right Hand Thumb/Grip Rule. Yet I find using Maxwell's Right Hand Cork Screw Rule more simpler than this.

    We should always think in the direction of flow of electrons. Before electrons were discovered , people assumed the direction of flow of current from positive to negative , i.e. opposite to the flow of electrons. This is too silly !!

    My text book says that now we know electrons travel to their deficit zone. So we take the flow of electrons to be electronic current. This makes the matter even worse ! We have two damned terms - electronic current and conventional current to completely confuse us though we know that only one (electronic current) term is correct and other is just a confuser.

    Precisely current is the flow of electrons , and not the flow of charges (positive charges) , which every textbook proclaims , I guess.
    This is because charges don't travel !! Hence we see that current flows from negative to positive and not vice versa. Thank you. :)

    There are no positive currents. Actually the flow of electrons constitute the current and not the flow of charges! The assumption we have is wrong ! There is even no negative current. Current is but a scalar quantity. It is positive in magnitude. I=Q/T , here Q is positive or x coulombs , say and time is positive. Current flows just from negative to positive. We just assume it conventionally to be flowing from positive to negative but that is wrong ! You cannot have current of both. As charges don't move ! They develop due to deficiency or excess of electrons.

    Got it ! *sigh* I see science cannot answer every question. That flash demo is awesome , thanks !


    Yet I cannot get this aspect : We get magnetic lines of force of bar magnet by tracing them with a magnetic needle. We can to it both the ways , either from south pole or from north pole of bar magnet. So why we say magnetic lines from north to south ? Is the direction of magnetic field the direction of the north pole of magnetic needle ?

    Found something very interesting here : http://www.howmagnetswork.com/

    Thank you anyways. :smile:
    Last edited: Nov 3, 2011
  11. Nov 3, 2011 #10

    Your question about why the magnetic field lines are directed one way and not the other has another historical explanation: The early exploroers who used comass needle was all from the northern hemisphere. They were all closer to the North (the one in the Arctic) Pole so it was the custom to make the north seeking pole of their compasses look unique (e.g. painted red, etc).
  12. Nov 3, 2011 #11
    My remarks weren't directed at you particularly. I know from past teaching experience that the topic of electromagnetism is full of mind traps.
    For instance, the helix diagram in your earlier post can be confusing as it is difficult to tell which half loops are above the plane of iron filings and which are below.
    When I drew this diagram on the blackboard, I always showed "breaks" in the lower half loops at the places were they were hidden by the upper loops.
    This is especially important when one delves into Lenz's Law and sticking a magnet into one end of a coil to induce a current.
  13. Nov 3, 2011 #12

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Oh you mean that "My goodness, how complicated!" was not in response to what I'd written? OK - no harm.

    I have had almost 30 years teaching this from pre-school to post-grad college :) OTOH: I don't expect to stand on my experience or any kind of authority this may or may not provide. After all, I could just be an old fogey set in my wrongheaded ways. What I write here should stand or fall on it's own merits.

    You are right about the mind-traps. You saw how I got struck in one by forgetting the Earth MNP is also a magnetic south pole. It does not help that it is still labelled "NMP = North Magnetic Pole" by geologists.

    Unfortunately I was stuck with the diagrams I could find online - I noticed, though, the author carefully drew in the overlaps and added breaks with the current direction. I have seen no evidence that OP was confused by this... though I was concerned about it. I could have used a row of parallel wires instead - but then the link with electromagnets would still need to be made. I chose the diagram because it illustrated the way the circular fields turned into the electromagnet field - recall OP first indicated thought that the poles referred to the direction of the current. The last link needed is between the ideas developed so far and permanent magnets and their magnetic domains. OP has not asked that question... yet.

    A persistent idea is that all currents are made of electrons.

    But what then of a current made up of a stream of alpha particles?
    It is clearly moving from the alpha-emmitter to the cathode, from positive to negative, and yet the conventional current points the same way.

    There are also times when it makes sense to talk about currents of positive charges (holes) in an electric circuit, though we understand them as the "lack of an electron" moving backwards as the electrons move forwards. I've commonly used the hall effect (compared between positive and negative doped germanium) to demonstrate this.
    Last edited: Nov 3, 2011
  14. Nov 4, 2011 #13
    Ok , I get it - you mean that it was an early convention that the magnetic field in a permanent , artificial or electromagnet was assumed to be the direction of the north pole of the magnetic needle (it points towards south pole of earth not true geological north) when it was used to trace magnetic lines of force of a magnet.

    Ok. It is for Sir Simon.:biggrin:

    "The last link needed is between the ideas developed so far and permanent magnets and their magnetic domains. OP has not asked that question... yet."
    Which question have I not asked ? Please read my post #9.

  15. Nov 4, 2011 #14

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    "Electromagnets get their magnetism from the electric current - where do permanent magnets get their magnetism from?"

    If I missed you asking that, I apologize.
    I think the outstanding Q from #9 was
    The answer is "yes". See bottom of post #2.
    daqddyo1 also answered this in post #10 with a historical reference as to how this convention was chosen.
    Last edited: Nov 4, 2011
  16. Nov 4, 2011 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    There are way too many errors in here that I'm not sure where to begin. I'll try to tackle a few:

    1. Whether it is silly or not, the mathematics works by designating current as the opposite direction of the flow of electrons. Claiming something to be "silly" has never been a valid argument against something like this. This is not a matter of opinion, or a matter of taste.

    2. We DO have positive charge flow. In proton particle accelerators, the current is due to the flow of positive charges. In p-type semiconductors, the current is due to positive holes.

    3. Current CAN be a vector! One can define current density j as

    [tex]\mathbf{j} = \sigma \mathbf{E}[/tex]

    where [itex]\sigma[/itex] is the conductivity (which could even be a tensor).

    4. If you are truly "fascinated" by charge transport, especially in solids, then you should look up at Solid State Physics text, or take a class in it.

  17. Nov 4, 2011 #16
    Here's the problem:
    I remember as an enthusiastic first year student at university being astounded to find in my newly written electronics text a circuit with a vacuum tube where the "current" was flowing from the plate to the heater. I knew from my electronics hobbies and earlier correct teaching that this was backward. How could a university text get it so wrong?
    When I asked, I was told of the historical reason for this which did not satisfy me. Were the authors of new texts afraid to change for fear of embarassing earlier authors?

    Certainly one can talk of the fact that postiive current flow in one direction has the same external effects as electron current flowing in the opposite direction, but why bother?

    It is also fine to talk of flow of positive charges as possible but on this earth, the vast number of situations involving electrical currents, electron flow is where it is at, e.g. in transmission lines, household wiring, flashlights, etc. Certainly one can have positive charges flowing in accelerators and gas tubes but not in copper wires. As well, the number of examples of positive charge flow pales in comparison.

    With the number of mind traps in physics that student can experience if not taught carefully, he or she would be better off not being exposed to conventional current until university and then being exposed to it in correct situations (unlike in my experience).
  18. Nov 4, 2011 #17

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    ... look at the definition of en electric current. It's as if we had a kind of river that flows uphill as well as the normal kind and needed a way to make sense of this in mathematics.

    They don't have to satisfy you though, that's not the point.
    More that the change, in the next texts, would have created even more confusion than it saved.

    If we pick a set direction for a conventional current, the math gets easier. It doesn't actually matter which direction this is.

    Because it makes the math easier later on and makes no difference to how an electrician wires your house.

    The choice for current and charge signs were made by hard-headed engineers to cover, then emerging, situations like electric street lighting (Edison et al). They were chosen to be convenient for the men who owned the power company and had to lay the lines and build the light-bulbs. They were not chosen to be convenient for you and me.

    In physics though - we are not about trying to make things convenient for people just starting out. The idea is to uncover general laws of the Universe. These conventions have proved themselves useful for this. At college level, we usually pick the conventional current to suit us: usually it is the direction of the motion of the majority charges in the situation.

    The main job of a HS education is to equip regular people to cope with life in a technological and changing World. One of the things they have to be able to do is understand what some scientist is going on about on TV. They also have to be able to understand the contractors who they hire to fix their technology - which will be electrical. In order to have at least a chance of this, they need to know about conventional currents but also about the ideas behind them. We absolutely do not need to dumb-down HS education.

    BTW: you computer most likely has majority positive charges running it. Certainly parts of it do ... all that p-type silicon.

    Consider: however hard you think this is at High School - College physics is harder than this and contains more subtle traps.

    Interestingly, the people who naturally question everything like you do are the ones who have the most trouble with this. And they are the folks we want in College... but specifically, we want the inquirers who are also able to cut through the confusion.
  19. Nov 4, 2011 #18
    All good points Simon although with house wiring, it does matter how the electrican connects things. Because the current is alternating, one wire (black) is hot. That is, its potential varies between +120 and -120 volts. The white wire is called neutral or the return. It is connected directly to the grounded breaker box which is also attached to the ground (earth). Each outlet has three holes: the smaller slot is the hot side and the longer slot is the return side. The round hole is connected directly the ground at the breaker box. If the white and blck wires are connected in reverse, short circuits and/or shocks can result.
  20. Nov 4, 2011 #19
    Ok , I will try to answer this by recalling my knowledge from class 9th to the present class 10th.

    The answer is but the Earth's magnetic field. It flows from south pole to the north pole. When it links itself (magnetic flux) with a permanent magnet , it induces opposite poles in permanent magnet in which magnetic lines of forces flow from north pole to south pole. This is due to the process mutual magnetic induction.

    Now the question arises that if we keep an iron bar will it behave as a magnet ? No ! We must convert it to a magnet : Single touch method , divided touch method , double touch method or electrical method. If we follow either of methods electrons come in pairs in straight lines having net electrostatic force. This is Ewing's molecular theory of magnetism.

    e- e- e- e- e- e- e-
    e- e- e- e- e- e- e- --> Like this in a permanent bar magnet.

    Like this in normal iron bar :

    e- e-
    e- e- e-

    No worries.

    I think this can be correct. It is bit commencing. Current can be treated as a vector quantity.
    For example : We have misconception that area is scalar quantity but it is a vector quantity.

    ZapperZ , I = Q/t
    If I is vector then it must be division of a vector and scalar. Time is of course not scalar. So , is Q or charge a vector ?:confused:
  21. Nov 4, 2011 #20
    Some current flows from + to - ; for example positive ions in a plasma or in a liquid.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook