How AC current works

  1. Please remember that I'm not a science or physics student, so please explain as simply as you can. Thanks.

    In the case of DC current electrons flow from a negative terminal to a positive. A negative terminal is full of electrons and the positive is electron deficient. I'm trying to understand how AC current flows. It is said AC switches on and off several times a second, let's saying it's 50 times. Now please have a look on the linked diagram:

    Does that 50 times on/off mean that A (or B) changes its polarity 50 times in one second? When A is +ive, B is -ive. When B is +ive, A is -ive.

    Please help me to understand this. Thank you.
  2. jcsd
  3. mathman

    mathman 6,395
    Science Advisor
    Gold Member

    The current oscillates (not on and off, but back and forth). US standard is 60 cycles per sec. If your diagram is about battery terminals, it doesn't apply. Batteries are always DC.
  4. It may be easier for you to think of AC initially as the voltage/current varying from some maximum value to 0 and then from 0 to some maximum value. In the case of an ideal square wave this is frequently the case. You can imagine this as connecting a battery to a light bulb via a switch. If you "flip" the switch Up and then Down 1 time each second you have a square wave AC signal of 1Hz. If you flip the switch Up and then Down 60 times per second, then you have a 60Hz square wave.

    Because power is more readily generated using machines (alternators & generators) the power rises to maximum and then declines back to 0 rather than "jumping" from 0 to max and holding @ max then "jumping" back to 0 and holding at 0. There are some very important reasons for this periodic rise and fall, but I suspect you will need to wrap your head around the concept of "on and off" before we get too far into sine waves.

    As mathman points out, AC behaves very differently than DC, but it is certainly possible to convert AC to DC and DC to AC. There will be times when it is not convenient to think of the minimum as 0V, but it is important to remember that Voltage is simply a measure of the potential difference between two points. If this difference is constant, then we call the Voltage Source DC, if it varies with time, we call it AC.

    Hopefully some of this will help you get a grasp on AC!


  5. They key property of alternating current (AC) is that it periodically reverses direction in a circuit. If the current varies from some max value to zero and back as you describe, it is called a pulsed current. The concept of 'back and forth' should be understandable, even for a non-scientist student :)
  6. rcgldr

    rcgldr 7,398
    Homework Helper

    In the case of a light bulb, it's a simple resistor that both current and voltage oscillate back and forth through, creating enough heat in the element to produce light.

    If the load has a capacitive or inductive nature, then a phase shift occurs, causing current peaks to be offset from voltage peaks.

    AC motors are more complicated, wiki article:
  7. Let's call me a non-scientist type for a moment, and let's explore the difference between a pulsed current and an alternating current with a DC offset.

    In a similar thread ( ) the subject is kicked around for the purpose of removing the DC bias.

    I would contend that "back and forth" and "pulsed" are indistinguishable w/o reference to a DC source, and even then that the distinction is trivial in understanding AC.

    For DC the first derrivative of V wrt Time = 0, and it is a special case of the Voltage function. If the first derrivative of V wrt Time <> 0, then the voltage is changing. If the voltage is changing (alternating) then the alternating component is NOT DC, regardless of the DC bias. Barring the cases of an infinitly rising or falling voltage, the first derrivative of V(t) gives us alternating signs (+/-) indicating the relative direction and magnitude of the voltage compared to its previous value, nothing more. It does not require or imply that the voltage change from positive to negative, only the slope of the voltage is positive or negative (rising or falling).

    As an analogy we might cut a piece of wood with a bandsaw where the blade is always moving one direction with respect to the wood. The speed of the cut is a function of the speed of the blade, and the feed rate. With a hand saw the saw blade moves back and forth with respect to the wood and the speed of the cut is a function of the length of the stroke, the number of strokes per unit time, and the feed rate. While the hand saw moves back and forth, this does not imply that the work being done "reverses direction", rather the work varies wrt time. If our hand saw is six feet long, but our stroke is only 1 foot the same amount of work can be done to the wood by varying the number of strokes per unit time wrt a longer stroke, but where on the blade our 1 foot stroke occurs is trivial. We could choose to draw a line at three feet from either end of the saw and define that line as "0". If we choose our stroke to be bound between +1 and +2 the work imparted is no different than if we chose -2 and -3.

    Extending this analogy to electricity. DC is generally conceptualized as electrons "flowing" from one battery terminal to another. The natural assumption about AC is then that electrons flow from one terminal to the other for some period of time and then flow back from that terminal to the other. Neither case is really accurate. What actually happens in DC is that electrons "bump" into each other and generally "drift" from one terminal to the other. The net result for a very thin cross-sectional area of the conductor is that some number of electrons cross over in any given period of time, but this in no way implies that any of the same electrons will cross another very thin cross-sectional area some distance from the first, only that the same number of electrons move across it as the first. In alternating current the electrons are more accurately described as "vibrating". A thin cross-sectional area will have some number of electrons crossing from one side to the other at a rate that varies wrt time, then for some other period of time some number of electrons will travel back across it. Again, this does not imply that any of the same electrons are involved in any type of "regular flow".

    To return to the saw analogy, if the hand saw were connected to a hydraulic cylinder and the hydraulic cyclinder were connected by a hose to another hydraulic cylinder such that if we push on the remote hydraulic cylinder the water compresses and the energy is translated through the hose and the hydraulic cylinder connected to the saw moves "outward". If we then pull on the remote cylinder the water is drawn back and the saw, in turn, moves "backward". In this case the water moves some distance in one direction and then moves some distance back, but there is no reason to assume that the water in one cylinder recieves any of the actual water from the first, though over time one might expect the disturbance in the water to cause a homogeneous re-distribution of the water; this is only a statistical approximation; it would be impossible to predict where a particular water molecule might be at any given time. This redistribution is also of secondary interest to the primary action of the cylinders/hose/saw/wood.

    I hope this helps clear things up a bit for the OP. As far as arguing about pulsed DC vs AC, if we want to argue that, it should really be in a new thread, I am afraid too much on that subject would confuse the OP.

  8. Thank you very much, everyone. Your replies are really helpful. But I some time to get this stuff digested. So will ask further questions later. Once again, thanks.
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