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

Confused about AC current

  1. Mar 23, 2013 #1
    I know that in AC the current switches between +ve and -ve 50 times a second in the case of mains electricity for the UK. However I was confused about how it alternates, as in when it swtiches how far do the electrons travel before it switches again. I was also told that in a circuit the electrons gain energy from the cell and then this energy is given to the components and it is replenished by the cell when it completes its cycle, is this the same for AC as if it alternates do all the electrons go back through the power supply?

    Im only GCSE so dont expect me to know this already as my teacher isnt the greatest.
  2. jcsd
  3. Mar 23, 2013 #2
    You might find it suprising that the electrons in a conductor move along the length of the conductor at very low speeds when subjected to an electric field, on the order of millimeters per second, be it AC or DC. One cycle of 50 Hz AC will hardly make them twitch!

    What's important to realize is that this isn't what matters with regards to the power delivered to the conductor. What matters is the amount of current flowing in the conductor.

    The following isn't going to be an accurate description of the movement of charge carriers in a conductor, but bear with me:

    Imagine a very long tube (the conductor) filled with marbles (electrons). The tube radius only fits one marble, so if one marble goes in at one end, another comes out at the other. Let's say this happens during one half cycle of AC. If you follow any one marble, it moves very little, but it's important to note that _all_ of the marbles in the tube move this same distance.

    Now, a conductor isn't a hollow tube and electrons aren't marbles, but I hope you can imagine how, even though any one electron will move only slightly along the length of the conductor, you'll have a whole bunch of them that will have moved to a position of lower electric potential energy. The energy that was "lost" during this movement is equal to the energy transferred to the conductor in the form of heat.
    Last edited: Mar 23, 2013
  4. Mar 23, 2013 #3
    So when the current alternates do the electrons change direction? If they don't then I understand and thanks for your help
  5. Mar 23, 2013 #4
    The net flow of electrons will reverse in direction. This doesn't change anything with regards to the power delivered to the conductor though.
  6. Mar 23, 2013 #5
    So was my teacher being technically correct when he said electrons carry energy in joules? As I think this is causing my confusion
  7. Mar 23, 2013 #6
    I'd rather say the electrons have electric potential energy due to their interaction with the electric field, i.e the electric potential energy is a property of the charge-field system, but that might be a bit pedantic with regards to what you're asking.
  8. Mar 23, 2013 #7
    Ok thank you
  9. Mar 23, 2013 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    That's a VERY significant point Miles gave you

    whatever "Charge" is, electrons just carry it for you.
    In fact they move charge by nudging one another along like marbles in that tube.

    That "nudging" propagates at nearly speed of light.
    But the electrons themselves move neither fast nor far.

    SophieCentaur often chides us for the notion of "electrons whizzing around" in wires.
    It is a common misconception.
    Only in evacuated chambers, like CRT's or electron microscopes are they free to move so fast and far.

    Most textbooks teach the subject as if positive particles were your charge carriers,
    so you'll see "current" described as flowing from positive toward negative.
    They call that "conventional current" as opposed to "electron current".
    It is worthwhile to train your brain to think both ways,,

    my guys called flow of negative charge "Real Current",
    and conventional or textbook current, in good spirited teasing, "Engineer's Current".

    So when reviewing Kirchoff's laws for them I drew my arrows and my polarity signs green for negative current and brown for engineer's current, and demonstrated the equations come out exactly the same.

    Your career (not to mention your schooling) will go a lot easier if you are fluent in both.
  10. Mar 24, 2013 #9
    So can current be said to be flowing even if it is the same electron passing the point in one second? So electrons just carry charge and nudge each other, so power is delivered through the electric field and just needs electrons moving to create this?
  11. Mar 24, 2013 #10


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Imagine you are riding your bike and, instead of pedalling round and round, you keep reversing the direction of your legs over an angle of about 90 degrees. The chain doesn't acctually go anywhere. You use the same section of the chain again and again and supply power to the wheel in short jerks. You could design a complicated system of gears and ratchets which could supply power to the wheel for both directions of your reciprocated pedalling. The same short bit of chain would go back and forth but never travel from pedal sprocket to wheel sprocket. With the electrons, in AC, it's the same. They oscillate at 50 (or 60)Hz and never move (on average) more than a few 1/100mm but the power still gets transmitted. Imagine electrons in a radio antenna, radiating a 100MHz signal. How far would they move? Sub-atomic distances! (Hence my problem with people wanting to discuss electric current in terms of electron moving along wires - cheers Jim).
  12. Mar 24, 2013 #11
    Consider cutting the conductor in two, like you would with a loaf of bread. Make a mental note of the flat surface of one of the ends of the conductor (a cross section of the conductor) and put the conductor back together. The current is, for a steady flow, the net amount of charge that passes through this surface per some period of time. In general, the current is the rate at which charge passes through the surface and it's approximately the same value regardless of where, along the conductor, you imagine the surface to be.

    To determine the current, you don't care what electron goes where. You just care about the rate at which charge passes through the surface. During a full cycle of AC, you'll just get a whole lot of electrons moving first one way through the surface, then the other way. Following any one electron won't make much sense.

    I was going to do a bit here about the potential energy of masses moving up and down in a reversing gravitational field, but that would probably be one analogy too many :redface:
  13. Mar 24, 2013 #12
    Ahhhhhh thank you, the last two analogies made a lot of sense. So I was getting confused because I was think of just the one electron.
    Thanks everyone
  14. Mar 24, 2013 #13
    So to supply the power you don't think about one single electron but all of them?
  15. Mar 24, 2013 #14
    How did Jim know sophiecentaur?
  16. Mar 24, 2013 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    I know him from reading his posts here. I've never met him. He's in England I think and i'm in central USA.
    Closest I ever got to crossing the Atlantic was a trip to Bimini in my little sailboat... that storm set meteorological records but that's another story.

    Pay attention to his posts and digest them. You'll learn rigorous thinking.

    Miles Young's post #11 is another good one.

    well, yes because it takes so many of them to carry measurable current.
    From Miles's post:
    Note he said charge passing through the surface, not electrons.
    Whatever charge is, each electron carries a small amount of it.

    Write a 6 followed by 18 zeroes. (or 602 followed by sixteen zeroes, to be more accurate)
    That many electrons carry an amount of charge that's named a "Coulomb"

    A coulomb per second,
    which would be 6E18 individual electron charges,
    passing through Miles's surface every second, is one ampere.

    It is important to get that concept:
    Charge moving past a point is the measure of current.
    Place your fingertip against a piece of wire.
    Now imagine yourself counting charges going by your fingertip so fast you could reach 6E18 in a second.
    That's an amp.
    I use that mental exaggeration when explaining to newbies.

    So get the concept down pat : charge is coulombs, and current is coulombs per second going past a point.
    6E18 is a handy number to remember. My memory aid is "it rhymes with six times three is eighteen"

    It'll help you immensely in your study of electricity.

    Miles was doing great - I hope I haven't just muddied the water.

    old jim
  17. Mar 24, 2013 #16
    No I understand all of that, I seem like an idiot now cause I knew that...
  18. Mar 24, 2013 #17


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    It's a pleasure conversing with people like you Rinn62. You are after answers and not arguments!
  19. Mar 24, 2013 #18

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    well then, i'm sorry I beat the dead horse...

    ....learning is largely discovering what you already know.

    Explain it to some of your classmates and it'll really sink in for you.

    and I second Sophie's comment.

    old jim
  20. Mar 24, 2013 #19
    Oh I'm the 'nerd' so people get bored when I discuss this with them but thanks for the help and nice comments
    Last edited: Mar 24, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook