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Help needed with concept of electricity, magnetism and EM

  1. Apr 23, 2015 #1
    Hi, so I've just started learning about physics and I get the gist of most of it. I understand motion, torque, energy, light, radioactivity. But there is one topic that absolutely stumps me. Electricity.
    Can someone explain to an absolute moron like me what it is? It seems like everyone around me understand it but me. Questions in red. Please read everything and if I get something wrong please correct me.

    This is all I know or think I know:
    Charge is a property of matter. I don't know what causes charge. A proton has a relative positive charge of +1, an electron has a relative negative charge of -1. In circuits electrons are the charge carriers?. Charge is not electrons, the electrons carry charge. Ions also carry charge. In a circuit we assume conventional current flow from positive to negative, i.e from the deficiency of electrons to the surplus when it is actually the opposite.
    Charge is measured in coulombs. The flow of 1 coulomb in 1 second is equal to 1 ampere. 1 coulomb is equal to the charge of 6.02x10^23 electrons. Why do electrons flow in a circuit? Are they attracted to the positive terminal of a battery? Why don't they just go through the battery instead of all around the circuit? How does the force of attraction reach the electrons through the wire? What is voltage (p.d). What does "p.d across its ends" mean? What is resistance? If a component has a high resistance how is the circuit even completed? Shouldn't the electrons be stopped or take a long time to reach the end? How does a.c work? I think with a.c the electrons change direction according to frequency? Doesn't that mean they don't go anywhere? When rubbing a polythene rod with a cloth, are electrons actually stripped off the atoms? If I keep these items seperated forever will the polythene always contain an abundance/lack of electrons? What exactly is earthing/grounding?

    I know some people will say I am being lazy and I should look it up myself, but I did do that. So many different sources give vastly different answers so I have no idea what is right. Any help is greatly appreciated.
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  3. Apr 23, 2015 #2


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  4. Apr 23, 2015 #3


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    It's an intrinsic property of some elementary particles. There is no explanation of what causes it. It just exists.

    Usually, but not always. Some components, such as transistors, use both electrons and 'holes', which are empty spaces where electrons use to be. These act as positive charges an flow opposite of electrons. In batteries, you have both electrons and positively charged ions that are moving around, with the ions moving through the electrolyte solution.

    Indeed. This is due to historical convention and dates back to the 1800's, prior to the discovery of the electron.

    Electrons flow because they are subjected to a force given by the power source, whether it's a battery, generator, capacitor, or whatever. Electrons can't go through the battery because of the way the battery is constructed and the way the chemistry works. I'm not entirely sure of all the details.

    Through the EM field. Because a wire is made up of many mobile charges, the field is able to 'travel' along the wire. A very simplified view is that when you exert a force on the electrons, they end up creating regions with a greater or lesser amount of negative charge, which then propagates down the wire. But this is a very simple view and not one that explains very much when you really get into the details.

    Voltage is defined as the difference in electric potential energy between two places. An electric potential difference means that the electric potential at one point is different than another point. Electric potential is the amount of potential energy a charge has in an electric field. In other words, when you place a particle in an electric field is will have some amount of potential energy (because the field can accelerate it, turning the PE into kinetic energy). If one point in the field has a different amount of PE than another point, then there is a difference in electric PE between these two points. Voltage is defined as this difference. To put it simply, voltage is a measure of how much work a single charge can do. Voltage is not a force itself, but a measure of the work that a source can do.

    As an example, consider two capacitors with an equal amount of displaced charges. One capacitor's plates are closer together than the others. This capacitor will have less voltage to use than the other capacitor, and hence can perform less work, but it will take less work to charge this capacitor compared to the other. So, for a given amount of charges, more voltage means that more work can be performed by those charges.

    Resistance is commonly stated as a resistance to the flow of current, however I think it is more useful given the definition of voltage to think of resistance as a property that causes charges to lose energy as they pass through a material. In other words, the higher the resistance of a material, the more work that must be done to move a unit of charge through that material. So if you double the resistance of an object but keep the voltage the same, only half the current can move through it per unit of time since you've doubled the amount of work required by each charge.

    It does indeed. This points to a very complicated aspect of electricity that isn't obvious at all. The energy transferred from a power source to a load is transferred through the EM field, not through the charges. I can't hope to explain this, as I don't understand it very well myself.

    Yes, the electrons are removed from the atoms of one material and transferred to the other. If you separate the items then they will keep their accumulated charge until it can be balanced out by transferring more charges, usually by touching another material, especially a conductor (like when you shock yourself on a doorknob).

    I can't hope to explain all the differences. If you haven't already, read this: http://en.wikipedia.org/wiki/Ground_(electricity)

    The biggest difference is that Earthing is connecting the circuit to the Earth, as in the literal ground beneath your feet. Grounding usually refers to connections within a circuit or to another circuit for various purposes.
  5. Apr 23, 2015 #4
    Thanks CWatters for that link and Drakkith for the explanations. I understand what voltage and charge are now, though I still struggle with the concept of E-fields, M-fields and EM-fields.
    One thing I need help on with charges is the "point charge" and field produced by a charge.
    bh9uO.png I'll call this pic 1. So this is a negative point charge, I'm assuming this is the electric or electrostatic field (not M-field not EM field) produced by an electron in isolation in a vacuum?

    W04yzMO.jpg Pic 2. This is a positive point charge. So this is the electric or electrostatic field produced by a proton or H+ ion in isolation in a vacuum I think?

    I am confused as to what these arrows mean. The arrows go into the negative point charge. Where do the arrows come from? What the the arrows even mean?

    There's also this picture 300px-VFPt_charges_plus_minus_thumb.svg.png which is supposed to be the electrostatic fields by 2 opposite point charges yes? It looks like the magnetic field here http://www.northeastern.edu/sunlab/mom/img/How%20Magnets%20Work/magneticFieldLinesAttractive.gif [Broken] so is there something in common with electric and magnetic fields? Do the magnets contain charge? Do the magnetic poles N and S contain protons + and electrons - or maybe lack of electrons + surplus of electrons - ?
    Last edited by a moderator: May 7, 2017
  6. Apr 23, 2015 #5


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    Also yep.

    Consider your third picture, where you have lines running between the positive and negative charges. Now, let's say that I want to separate the charges until both are well outside the boundaries of the picture, so that we only see the field lines. If I place a negative charge in this field, what will happen? The arrows simply allow us to show where a more positive electric field is located at. So when I place my negative charge, it will want to accelerate in the direction opposite the arrows, or towards the positive charge. If the charge was positive it would accelerate in the direction of the arrows instead. The direction of the arrow, pointing away from positive charges and towards negative charges, is purely by convention. It wouldn't matter if we swapped the direction of the arrow as long as everyone uses the same convention.

    Like electric charge, the magnetic field has two 'charges', though I hesitate to call them charges, as you can't have a solely positive or negative magnet. Magnetic field lines ALWAYS form loops and can never simply end, unlike electric field lines which start and stop on electric charges (the magnetic field lines don't actually end inside the magnet. They keep going, looping around and exiting through the other pole). In other words, if you break a magnet into pieces, you will not find a piece that is solely a north (+) pole or solely a south (-) pole. You will simply have several smaller magnets, each with both a north and south pole.

    Magnetic fields are not generated by an accumulation of charges, like electric fields are, but by the motion of charges. For example, in an electromagnet the motion of the charges in the wire creates the magnetic field. In a bar magnet it is the motion of the electrons in their atomic orbitals. Non-magnetic substances aren't magnetic because the magnetic field from the electrons is cancelled out by other electrons, while in magnetic substances this cancellation is incomplete.
  7. Apr 23, 2015 #6
    Hello Swaggboy,

    A point charge is really an idealization of a group of charges. We use this idealization for many things. One of those things is to make the concepts easier to grasp and when you're dealing with a symmetric body it turns out to be equivalent to what's really going on. Let's take an example to make this mean something. :D

    Ok, so imagine you had a basketball that you rubbed with a material that left a positive charge on the basketball. Let's say this charge was 70micro-coulombs. All 70micro-coulombs would be spread evenly over the surface of this basketball. But because of a net mathematical tool developed by Gauss we can treat all 70micro-coulombs as if they were at the center of the basketball and at a single point there. So that's your point charge here. I suppose a single electron could be a point charge, but you're not going to be dealing with one of those. Thus we use this concept of point charges for more practical problems.

    Now, once you have your point charge you have an electric field it creates. This field is really a very mathematical concept but let's just take it in a conceptual way, which is what I think you want. Conceptually the electric field (for your purposes at the moment) can be thought of as the "desire" that two charges have for each other. So let's take the typical dating game for example. Boys and girls like each other. There's a "field" of attraction that exists between them. Here let's let boys be protons and girls be electrons. A boy will go to a girl in the direction of the arrow and a girl will go to a boy in the opposite direction of the arrow. If neither one of them is fixed they'll both fly towards each other (wouldn't it be cool if this happened in real life - whooossshh!!!). But those lines you see simply describe the attraction between them, at least in a concept. Notice how in your third picture that the two point charges are bending each other's fields toward the other? Well that has to do with the same rough idea here. So if you shot some of the charges off of one of these point charges (which remember could be a collection of charges that we idealize as being at one point in space) it will have a propensity to follow the field lines as described earlier. If it's an electron it will want to flow against the field lines, if it's a proton it would want to flow with the field lines. It should be realized that in the picture you're seeing there the two point charges are not just opposite, they are equal in their MAGNITUDES.

    I would recommend that you first study the electric field until you're comfortable with it. Forget about the magnetic field for the meantime. They are related for sure, but try to get the E-field down first. When you do you'll be ready to study the magnetic field.

    (2days later...) Now let's move on to the magnetic field. Just to put you ahead on the game the magnetic field is actually referred to as a B-field. The magnetic field is a bit more complicated. First we do not have "mono-poles" notice with the electric field you could have a positive or negative point charge all alone. This lonesome charge has field lines coming out of it (when positive) or into it (when negative). Cool!

    When you're dealing with B-fields you'll notice there are always both poles at once. So we never have a mono-pole when dealing with magnets. Now what is the magnetic field?

    The magnetic field is quite complicated. Magnetic fields come as a result of moving charges. How they work in a magnet is something I'll leave for you to study in your own time, but realize that yes they come from the structure of the material as a whole and specifically dealing with their electrons. Thinking of this for a permanent magnet can get complicated so maybe by-pass that for a while. But an electromagnet should give you enough of a feel for what's going on to let you be comfortable. So when you get comfortable with the electric field look up an electromagnet and then consider it's magnetic field.

    I hope this has been helpful and I'm sure others will develop on it more to give a real technical understanding if you'd like. :D
  8. Apr 23, 2015 #7
    Thanks again Drakkith, I understand simple bar magnets up to a basic level now. And DrPapper your description of the field lines was brilliant!
    So I see that electromagnets are basically just wires of a metal with free electrons, they're coiled up to pack more material in a smaller space resulting in a stronger B-field. And the reason that electromagnets need electricity is because magnetism is produced by the movement of charges.
    For the electrostatic charges, both charges of equal magnitude are attracted to each other with equal force along or against the same field lines right? And since they're attracted to each other it produces motion, which would create a B field right? Is this the phenomenon of electromagnetism?
  9. Apr 23, 2015 #8


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    As far as I understand it, that is indeed correct.
  10. Apr 23, 2015 #9
    Yes! You've got a good basis going. Electromagnetism is called as such because as you're beginning to see the two are inseparable connected. Isn't that amazing!?!

    However, the reason an electromagnet is coiled is not really to pack more material into a space, it's because of the way the magnetic field works. Look at an introductory textbook and examine the formula for turns of an electromagnet as related to it's B-field. You'd get a much better B-field with many turns of a thin wire than the same amount of copper in one thick wire wrapped only a few times. This part gets more into mathematics. But think about how each wrap of the wire produces it's own B-field, then think of how their "combining". You should be able to get a pretty good intuitive feel from that. Other than that, you're quite right. :D Try to have some fun with these ideas. Check out how to make a simple electromagnet, then look at how to make a simple electromagnetic motor at home using a simple battery and some copper wire and a few other items laying around.



    As a word of caution, DO NOT DO THIS WITH LARGER BATTERIES OR AC Voltage from your wall! Use only what is provided in the instructions and preferably with the guidance of an instructor. Either way wear safety glasses! :D Other than that have a great night. Remember PHYSICS CAN BE FUN!!!
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