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Electric and magnetic fields VS electromagnetic waves

  1. Feb 26, 2014 #1
    Capacitors are popular candidates for explaining electric fields. The phenomenon becomes really apparent with the aid of a leiden jar ...which is really just a capacitor. Anyways, as long as you're cranking the handle on the electrostatic machine, the charge is going to build and build until I complete the circuit (just assume I'm holding a leiden jar that's connected to an electrostatic machine, in which you are operating).


    Charge is flowing, correct? And if we define electricity as a flow of charge, then this^ is electricity, correct? And if so, you do NOT need a completed circuit for electricity to flow.

    How exactly does electricity flow? Probably not how you think. It's easiest to imagine a group of people (atoms) that have gathered themselves into a circle (circuit). Every person has a ball (electron). Now tell everyone to pass their ball to the left. Of course, I'm forgetting things like voltage and current, but I think you get the idea.

    Does anyone see where I'm going with this? What happens when you have an incomplete circuit like the leiden jar? How is that last atom (last atom at the end of my hand holding the jar) going to feed the first atom (wherever that is) an electron? Does the whole basketball paradigm break down? Kind of. What actually happens is electrons are sucked up through my feet. Okay. So exactly how far away in/on the ground are atoms passing electrons towards you? I don't know?! 20 yards, 2 feet, 7 inches from the last atom in my right foot. Lol, I'm not sure you can definitely answer that question. You're probably laughing, but I've seen some pretty crazy maths on this forum, so I'm asking whether it's theoretically possible to do such calculation.

    I want to understand fields better. So, a static electron has no field, correct? But once set in motion, this electron takes on a new identity called a charge, and surrounding this charge is a field.

    Sounds right, but I'm forgetting the part where once set in motion, you actually get electromagnetic interaction which radiates outwards. So I guess once set in motion, you get electromagnetic interaction, and then when the charge is in motion, you just get a field surrounding the charge ...that's flowing in the conductor.

    I'm going to take a big leap here and ask why? Why the electromagnetic radiation the instant the charge is set in motion? I'm pretty sure it has to do with relativity. Once the charge is in motion, there will be no electromagnetic radiation radiating outwards. Why? The whole idea of what it means to be at rest. I assume once those charges are moving, then one can argue that they are at rest, and it is everything else that is moving.

    I haven't introduced magnets yet, but maybe it's best I stop here.
    Last edited: Feb 26, 2014
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  3. Feb 26, 2014 #2


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    No. An electron always has electric charge (-1.60 x 10-19 coulombs of it), regardless of whether it's at rest (stationary) or moving. Because of that charge, it always has an electric field surrounding it. When it moves, there is also a magnetic field. Magnetic fields are produced (in classical electromagnetism anyway) by charges in motion.
  4. Feb 26, 2014 #3
    I always thought that electric charge could change. For instance, an electron with a high charge was high in voltage. That statement alone tells you nothing about the rate at which electrons flowed. That was current.

    This^ is how I understood voltage and current.

    I got the current part correct, but I guess voltage is joules/coulomb. Coulomb of course having a fixed value.
  5. Feb 26, 2014 #4
    current is just how MANY charges are flowing through the conductor , charges are made from charge particles like electrons , voltage just simply allows a higher number of charges to flow through a conductor by giving each individual charge higher potential energy
  6. Feb 26, 2014 #5
    So how can you make putting your tongue on a 9V a more painful experience? Raise the I or V?

    You raise the current, you raise the number of charges that flow past a point in a second. You raise the voltage, you raise the number of joules per coulomb, which raises the potential energy, which ultimately raises the number of charges that flow past a point in a second.

    So ultimately, it is charge per second that matters.

    So both.

    If this is all true, then why do certain components such as LED's require BOTH a certain voltage and a certain amperage?
    Last edited: Feb 26, 2014
  7. Feb 27, 2014 #6


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    Raising the volts raises the current (as long as the resistance stays the same). Lowering the resistance of your tongue would also raise the current, but it's probably easier to use a higher voltage source. :tongue:

    Imagine you have an LED with two leads to attach your voltage source to. Applying the appropriate voltage causes the appropriate amount of current to flow through the LED. But, let's say you accidentally got some grease or dirt or something on the leads before connecting your voltage source. This would increase the resistance of the leads and result in a lower amount of current even though you are applying the same amount of voltage.

    Also, it's possible for a voltage source to be able to apply the correct voltage, but be unable to put out enough current. For example, a voltage source may be able to apply 20 volts, but only put out 10 mA of current. If your electronic device required 20 volts and 15 mA, then it wouldn't work properly, if at all.
  8. Feb 27, 2014 #7
    Jim Al-khalili did a series of episodes with BBC on electricity. I remember him talking about the Transcontinental Cable that stretched across the Atlantic. I don't remember the name, but some guy either raised the voltage or current and destroyed the cable altogether. This wads the 1st cable, btw. I believe they got it right the second time.

    From my understanding, you want high voltage, low current for transmission over long distances. Something with the square and loss of power?


    If Ohms law always has to hold true, then how can someone sit there and say I want this voltage and this current?

    Forgetting about heat, resistance is fixed. So if you provide X voltage, then according to ohms law, you're going to get Y current. You don't have a choice.

    Does Ohms Law somehow break down? Again, you want high voltage, low current for transmission over long distances. Looking at ohms law, I'm not sure how you can do that.
  9. Feb 28, 2014 #8


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  10. Mar 1, 2014 #9
    Suppose I have a cell. Now suppose I want to increase the voltage. How is voltage increased?

    I spent some time last night thinking about this, and couldn't figure it out. Voltage is joules per coulomb. If you want to increase the voltage, all you need to do is increase the number of joules per charge.

    So what's the problem?

    Well, from my understanding, it only takes a certain amount of joules to free an electron from it's shell. So if it takes 5 joules of energy to free an electron from it's shell, then that means that electron can only do 5 joules of work from it's higher potential down to the lower potential or from the cathode of the battery to the anode (assuming actual electron flow).

    Yeah and?

    And?! If you feed 10 joules of energy to free that electron, it's still only going to be able to do 5 joules of work.

    So, how is voltage increased?
  11. Mar 1, 2014 #10


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    When you are talking about conduction in a metal, you can forget about 'shells'; the conduction electrons are shared between the surrounding positive metal ion cores.

    To increase the volts from a battery, you need to produce AC - then use a transformer (of some sort) to increase the voltage and then rectify it. Modern switch mode circuits may not use a conventional transformer to do the job but the circuit uses inductance and rapid switching to get a higher voltage.
  12. Mar 1, 2014 #11
    If I'm understanding this correctly, when you say the electrons are shared, what you're really saying is that there is no exact figure holding every electron in place? Because if there was, if one electron were freed, then all electrons were freed. So what's that mean? That would mean that a cell could only have a certain voltage -nothing more, nothing less.

    Theoretically, if every electron has been freed in a closed system like a cell, then the cell has reached maximum voltage, correct?

    Lastly, if resistance was non existent, then we wouldn't have voltage or current. We would only have electricity, correct?
  13. Mar 2, 2014 #12


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    That is incorrect. Some of the electrons are shared in a metal, which means that those shared electrons are in the conduction band and are constantly moving around inside the metal in random directions. Applying a voltage causes the electrons to develop a net movement in a certain direction. They still move around randomly, but they move slightly more in one direction than the other. Add up all the motions and you get an electric current.
  14. Mar 2, 2014 #13


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    This is why metallic bonding is so strong and tough. Unlike in ionic and simple covalent bonding, the forces holding the metal are due to each of the valence electrons hanging on to many of its near positive ion neighbours. If the metal is stretched or bent, the forces never 'let go', so you have ductility and malleability.
    These electrons are highly mobile and transfer heat very easily - just as they transfer electrical energy easily.
  15. Mar 2, 2014 #14
    I don't think we are quite on the same page.


    A cell can be created using two dissimilar metals and an electrolyte.

    The electrolyte (acid) breaks down (I'm not sure what it breaks down), and causes one side of the cell to have a surplus of charge, and the other side to have a deficit.

    What is the source of voltage?

    Let's stop and think about this for a second. Work in equals work out (in a perfect world). What is doing the work here? The acid! The acid is breaking apart (whatever it's breaking apart). It's taking the acid so many joules of energy to do this. This should be the source of voltage.

    My question was, how is voltage increased.

    Hope that helps.
  16. Mar 2, 2014 #15


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    The 'cell' question is off topic, really - certainly off-title. A cell has two plates and, on the surfaces of the plates, a chemical reaction takes place. That reaction builds up a surplus of electrons on one plate and a dearth of electrons on the other. The process continues until the potential across the cell balances the atomic level potentials and the reaction stops. When you provide a path for current to flow round from one plate to the other (a load) the chemical process continues.
    To 'increase' the voltage, you need to choose a different pair of substances on the plates. You notice that all different cell technologies produce cells with different potentials - that's because of the different energies associated with the different reactions. Altering the temperature of the cell can also change the emf slightly.
  17. Mar 2, 2014 #16


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    In a battery? Per wiki: http://en.wikipedia.org/wiki/Electrochemical_cell#Equilibrium_reaction

    Adding more cells in series increases the voltage. For example, two 1.5 volt cells in series gives 3.0 volts.
  18. Mar 3, 2014 #17
    The process continues until the potential across the cell balances the atomic level potentials and the reaction stops.

    Can you elaborate on this? Atomic level potentials? Btw, when I read that^ sentence, I automatically want to think that the voltage is rising until it balances the atomic level potentials. Wouldn't that be something. That would mean a double A battery (1.5V) could kick out the voltage of a 9V if you waited long enough, ha.

    You notice that all different cell technologies produce cells with different potentials - that's because of the different energies associated with the different reactions.

    Aha! Reactivity is the source of voltage! Not the amount of energy needed to free an electron from it's shell... I'm not sure if you guys understood my confusion. I thought the source of voltage was the energy required for the acid to break apart (whatever it broke apart). Suppose it took 5 joules to do this. Now that would mean a coulomb of charge could do 5 joules of work from one side of the cell to the other. This is incorrect of course.

    And my interpretation of how the electrolyte in a cell functioned was flawed as well. I thought the purpose of the electrolyte was to split up charges -one side having a surplus, and the other having a deficit. When you connected a load to the poles, charge would flow to balance out. See, the electrolyte was irrelevant at this point. You didn't need it, because it had already done it's job of creating a potential difference in charges.

    That is incorrect.

    Bold is mine.
  19. Mar 4, 2014 #18


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    Isn't that more or less the same thing? To react, chemically, exactly the same sort of potential is involved as adding or taking away an electron to an outer atomic shell. That is what happens when an atom becomes ionised.
  20. Mar 4, 2014 #19
    Guess so:redface:

    Anyone ever place a permanent magnet underneath a sheet of paper with iron filings sprinkled over the top? You get a neat effect. Lines from both the N and S pole fade into one another all around the magnet. This effect becomes apparent at the sides of the magnet. They curl into one another. There appears to be a straight line running through the magnet. Don't let this fool you. This too, is just lines from the N and S pole fading into one another. You just can't see it because the paper is flat.

    I couldn't sleep yesterday, so I spent some time staring at the nightlight near the hallway. You squint hard enough and you get the same effect, but with rays of light. You can even move your head side to side, and watch the rays curl into one another -just like if you were to take the magnet underneath the sheet and slide it side to side. You can watch the magnetic lines of force fade into one another with the aid of the filings.


    If light always travels in straight lines, then why do I see rays that are curved near the sides of my eyes?


    Light bends in a gravitational field. However, light still travels as a straight line because space is bent with it.

    So my question is, do our eyes contourt/bend the space in front of us to give us a picture of reality?

    Also, Kaku was on NOVA last night. He was talking about how Einsten saved his theory of relativity using the elevator scenario -how there is no difference between accelerating and falling.

    Am I supposed to think of magnets the same way? Is there any difference between attracting and repulsion?
    Last edited: Mar 4, 2014
  21. Mar 4, 2014 #20


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    No, our eyes use refraction to focus light down to a spot on your retina, not a bending of spacetime. You are not seeing bent light rays when you squint. You are seeing a difference in how the light is focused due to your squinting that makes the light change from a circular spot on your retina to a different shape. Also, I'm all for answering questions, but this one is easily explained by looking at wikipedia or another online article. There's no need to try to tie several concepts together before you understand any of them.
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