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What Exactly is Alternating Current

  1. Jan 3, 2013 #1
    I understand direct current perfectly fine as it is just the continuous flow of electrons, but what is alternating current. I have heard it explained as the continuous switching of a currents direction 50 or 60 times a second, but that is so vague and confusing. Do the electrons move back and forth? Why would this type of current help?
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  3. Jan 3, 2013 #2


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    yup! :biggrin:

    (a very short distance, not all the way to the mains and back)
    suppose you're trying to heat an ordinary heating element …

    all that matters is how fast the electrons are moving through it. not which direction they're going in

    obviously, the power is sometimes zero, but it's never negative, so it heats fine! :wink:

    (i forget the advantages of AC … something to do with lower transmission losses, i think … but you can easily look that up)
  4. Jan 3, 2013 #3


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    Yes, in alternating current the electrons flow first one way then the other, back and forth.

    This is useful because one can use a transformer to convert from low voltage alternating current to high voltage alternating current.

    A transformer is essentially a pair of coils. A changing input electrical current through the one coil generates a changing magnetic field. The changes in the magnetic field experienced by the other coil induces an output electrical current. If the number of windings in the output coil is greater than the number of windings in the input coil then the output voltage can be greater than the input voltage. If the number of windings is lower then the output current can be greater than the input current.

    Note that energy is conserved. The output voltage may be higher. But the output current will then neccessarily be lower and conversely, the output current may be higher but the output voltage will then neccessarily be lower.

    By contrast, a constant direct current flow produces a constant magnetic field. A constant magnetic field does not induce an electrical current. So a transformer will not work to convert low voltage DC to high voltage DC.
  5. Jan 3, 2013 #4


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    The transmission loss in a wire goes as the square of the current, while the amount of power transmitted is equal to the voltage times the current. So the efficiency goes up as the voltage increases; you can move the same amount of power with less current and hence less transmission loss.

    But at the point where the power is produced, you want to work with whatever voltage gives you the best mechanical efficiency out of your generator; and at the point where it it used you want whatever voltage matches the need. So it's very desirable to be able to step the voltage up and down, and as jbriggs said, that's much easier with alternating current.
  6. Jan 3, 2013 #5


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    this may be true for the "ordinary" heating element. but it is not necessarily true for anything with a "reactive" component. if you hook your AC up to a capacitor, you will have voltage and you will have current (both AC) and half of the time the power going into the capacitor is negative.

    it's much much harder to make a transformer (to step up or step down voltages) that works for DC.

    for transmission, the losses that are due to the flow through the cables are [itex]I^2R[/itex] which means that by lowering the magnitude of current [itex]|I|[/itex], you lower the loss. to do that and still transmit the same power, you need high (really high) voltages. to convert to/from high voltage from/to low voltage, and to do that efficiently, you need a transformer. and basic transformers don't work for DC.

    that said, there are now modern high-voltage DC transmission lines for dedicated transmission of power (they're not connected directly to "the grid"). so evidently the DC lines have less loss than the AC lines (i think due to what is called "skin effect" that decreases the effective cross-section for the current flow and increases the cable resistance). so, evidently, the difficulty of converting from AC to DC and back to AC again, those losses must be less than the losses avoided in the transmission line by going with DC.
  7. Jan 3, 2013 #6
    The DC transmission of power is much more appropriate for use with undersea cables such as the quite small system between England and France or the much larger schemes contemplated to bring geothermal power from Iceland to Scotland or Germany.
    When AC is used with undersea cables there is considerable loss do to the current flowing in and out due the capacitance of the cable.
  8. Jan 3, 2013 #7
  9. Jan 3, 2013 #8


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    I can't understand this preoccupation with discussing electrical phenomena in terms of electrons - as if it will really help with understanding. Tiny Tim mentioned that the electrons in AC don't get very far. The actual distance that they do travel is worth mentioning. In simple terms the average drift speed of the electrons in a metal is in the order of 1mm per second. Now you have to realise that, with 50Hz AC, they are only drifting one way for 1/50s so that implies a maximum movement of around 1/50mm in either direction. At the same time as this really slow drift speed, you also need to realise that the (conduction) electrons in a metal are thrashing about randomly in different directions at very high speeds due to thermal agitation - much much higher speeds than this snail-like drift speed.
    Does that really help in the understanding of what is going on in the miles of cables of the National Grid or in a Transformer, motor or light bulb? Electrical Engineers (who are the people who tend to know and understand these things) pretty much never refer to electron movement because it is Irrelevant. If you just treat electricity as a net flow of charge in one direction or another, which causes a very rapid transfer of Energy from one place to another you will start to get somewhere. The word to use is Current, which is all you need to consider. Expecting electrons to help ones understanding is a delusion, based on notions of water flow - possibly worse than the water flow model.
  10. Jan 4, 2013 #9
    The only aspect as to which the model of waterflow would be accurate to an extent is the fact that the time it takes for electricity to reach far distances in a conductor is very very small as to the EM wave that it makes and which travels near the speed of light in non vacuum conditions.
    Just like when a pipe is already full of water all it takes to make the water come out in one end is to supply more water at the other end.

    I am sorry if this is not a desired or unscientific explanation
  11. Jan 4, 2013 #10


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    Absolutely. It's very limited but vaguely useful when waving arms about. As with the electron-based discussion of electricity, it is deceptive and can give a very false sense of understanding.
  12. Jan 4, 2013 #11
    But to a person who seeks deeper answers and knows some basic things very well the electron part of electricity may only come as a further deepening of knowledge I guess it's just about how seriously we take things that we want to learn and understand.
  13. Jan 5, 2013 #12


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    Too many people think they fall into that category but, if they need electrons to describe what happens in a circuit, then they clearly do not belong there. If you are involved with the details of semiconductors, superconductors, Hall Effect, etc, then electrons may come into it. Using electrons to 'explain' how a circuit works is a fruitless exercise. 'Electricity' is not a concrete phenomenon and needs to be treated with due respect before you can expect any understanding of it. It's the equivalent to trying to describe Differential Calculus with a pile of beans. Why do people have a different attitude to pure Maths from their attitude to Physics theory, I wonder? They both require formal thought processes.

    There is another real risk when your hypothetical 'informed' person tries to explain using electrons - they give the erroneous message to the 'uninformed' that 'electricity' is trivial.
  14. Jan 5, 2013 #13
    Well maybe because in pure math as you said there are specific numbers and equations going on and there is no room for blind guesses or even close speculation you have to know what your doing to solve those equations. While in theory you can make analogies and try to make imaginations of those processes and it is easier then.

    Also there are more people which can't understand a BH from a mathematical formulation but they have a clue about a BH when they are being told about the physical processes going on around it and close to it.I believe even many high school students learning physics get a clue about some proceses only after they are introduced the story of it not the numbers and equations. And then when they have understood the story behind it they put in the numbers and see how it works out.Well I do that kind of way too.Maybe not the best way I agree but what else a chance do I have if I can't handle math that good.?

    Speaking of myself and electrons I love physics and I think I understand physics quite well for a guy my age(23) and educational background (not that big) but I have to admit mathematics is not my strong side and that causes some problems and slows down my ability to construct different electrical equipment but I still love the process.Even though everytime when I come up with an idea and start to build it i realize that the world has built or tested that already.
    I mean there is so much knowledge already know to human that to catch that is almoust impossible.
    Basically the previous post about the person who seeks deeper was about myself... :D
    Last edited: Jan 5, 2013
  15. Jan 5, 2013 #14


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    I understand what you are saying but I still hold that, if people don't feel comfortable with applying the simple formulae that are used in basic electrical circuit work then a model based on pushing metaphorical peas through a tube is not really going to help them much in improving their understanding or ability to predict what will happen in a given circuit.
    Going from V = IR to 'electrons going along a wire' is a backward step.

    That's absolutely fine and very few of us can ever help to make significant original contributions. The pleasure is always there when something works that we have devised. We all get that glow of pride when the lamp actually does light up when we first press the switch or the display lights up and shows us the right characters.
    Personal pictures of what happens in electrical circuits are fine as far as they go but my worry is when those pictures are often presented in forums like PF as being 'gospel'. They tend not to be presented with all the necessary caveats as to their validity and can serve to muddy the waters for people who really want to find out more.
    If you want to get deeper into the subject then my feeling is that you really need to get away from the 'concrete' and into the stricter, formal ideas that have been established. For instance, it is far more fruitful to talk in terms of 'charge' than 'electrons' because talking about charge encompasses all ways in which electric current is passed and avoids all the random aspects of electron movement and QM. It's a bit of a generational thing (I may even have a tiny chip on my shoulder) but, when I was taught 'electricity' initially at school, we just talked about charge and were strongly discouraged from touching on electrons on the grounds that they would confuse us. My teachers were proved right because later generations of students were all told at their Mother's knee about particles. Consequently, these pages are full of confusion generated by the perception that 'Scientists got it wrong about the direction of electric current'. At an appropriate time in my Science education, I easily came to terms with the negative sign of the charge carriers in metals and there was no confusion.
  16. Jan 6, 2013 #15
    I understand what you are saying but I still hold that, if people don't feel comfortable with applying the simple formulae that are used in basic electrical circuit work then a model based on pushing metaphorical peas through a tube is not really going to help them much in improving their understanding or ability to predict what will happen in a given circuit.
    Going from V = IR to 'electrons going along a wire' is a backward step."""

    Basically I agree to what you say.It's like a grandma doesn't need to know the molecular composition of milk to know that her cat certainly likes it.Also I first hear and learned electricity from the observations about what she does starting from few nasty shocks as a little boy up until all the parasitics and EMF from large transformers and other things when built my first amplifier in school.And now when I know the basics I can try to step further and visualize electricity in terms of electrons but even though I may fail at some point of that deeper visualization that will not erase all the fundamental laws that I know about electricity as a phenomenon not as a force carried by certain elementary particles .

    So fundamentally I agree with you as I think people should start to learn things in the order in which they were invented and after that they can build up some more advanced bricks of today's science.As it would be funny and stupid to know the flow of electrons in the opposite direction to that of the current and not know Ohm's law.
    Last edited: Jan 6, 2013
  17. Jan 6, 2013 #16


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    This says it all, I think. What anyone 'visualises' is not necessarily near what is the accepted model so how is it helping at all (apart to make them feel better)? It can be a positive disadvantage if they then want to take things further. Of course, everything we 'know' is based on analogy and models - unless it is really basic - like Grandma and the milk.

    Piling onto a Science and assuming, on no firm basis, one has an understanding, just using a personal model, is a bit like making up your own version of French and expecting it to have any meaning to a French person. My particular bugbear is even worse than the electron thing - it's when people talk of photons as if they have the remotest resemblance to little particles streaming through Space from one point to another. Nobody who has a deep knowledge of these things would give such a model houseroom (or brainroom) yet the notion turns up on a daily / hourly basis when EM waves are discussed.

    My only real worry is that people's personal pictures can be passed on and received by the uninitiated as if they constitute 'real Science. This does no one any favours.
  18. Jan 6, 2013 #17


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    AC is neccessary to transform the voltage down or up in a transformer. It is also neccessary to run electro motors. Either the AC is made inside the motors windings via commetators (Brushed DC motor), or the AC is fed directly into the windings (Brushless AC motor).

    The frequency is a drawback for transporting energy over long distances. The frequency will cause loss due to the inuctance of the long wires that transport electricity. DC would be the best option for the transport part, but DC cannot be transformed down to a voltage that can be handled safely. Transportation voltages often exceeds 50 000 Volts to keep the current as low as possible. As current will cause loss over such a long distance with an increased resistance in that cable the longer it is. Keeping the current low, will cause less energy loss.

    50 or 60Hz is standard frequencies. An AC motor depends on this frequency to perform a given RPM. Except for the number of poles in the stator and the rotor, the RPM is set based on the input frequency.

  19. Jan 7, 2013 #18


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    It's interesting to note the contrast between the quote and the response in the above post. The use of 'electrons', which is in the quote (noname2020x) is totally ignored in the response (Low-Q). Clearly, the response is from someone who has more knowledge of electricity and realises that bringing electrons into the topic really doesn't help.

    I wonder if the message has sunk in yet. It may be possible to give some sort of limping explanation for DC using electrons but trying to take them with you to into AC explanations (and the way that transformers and synchronous motors work, for instance) is fruitless.
  20. Jan 7, 2013 #19
    @sophiecentaur : By that visualizing I meant as close as possible to the model physics knows the best.It's not like one just hears a word and then makes a totally self sustained story out if it atleast not in my case. It's rather you read the most accurate theory to date and try to grasp it , ofcourse there will be some misconceptions from person to person I guess that's just human nature.It's like 10 persons traveling down the same road with the same map and still after the trip everyone will have a slightly different account of how the road looked like and what was there.Same in physics and even worse as the things are much more complicated and in the case of electricity- hidden from the naked eye.
    Ah the photons , let's be honest understanding all these things can be compared to the world's economic situation , there will be rich people there will be middle class and there will be poverty and homeless people not to go into detail it's just the way things are because of how we live how much we understand and want to do.Same in physics if someone understand something good then that isn't a sign that some other will understand those things even close.
    The only responsibility to those who know is to spread the knowledge as unedited and close to the truth as possible.

    @Low-Q:"adding to what you just said is that nowdays DC lines are starting to evolve as we have high power thyristors and all kinds of "smps" equipment to step up or down even high voltages of DC and to then invert those to usable AC mains.
    A German company "ABB" and "Siemens" are currently building such high power DC 800kv lines in China.


    @By the way "sophiecentaur" is right because in electrical equipment there are very rare times when something from the quantum level of physics needs to be considered like electrons for example of photons in EM vawe, so when it's not needed we can survive with the classical laws of electricity that we all know better of like Ohm's, ampere's and others.Each level of competence has certain things you need to know but not more, like a guy who repairs tv sets probably doesn't know quantum mechanics good or even at all but that doesn't make him a bad repairmen I guess as long as he is in his field of expertise.
    Last edited: Jan 7, 2013
  21. Jan 7, 2013 #20


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    My point is that the model / visualisation of electrons circuits that most people carry in their heads (due to what they're bombarded with at School and on TV) is damagingly flawed and leads them to seriously wrong conclusions if and when they start to think about it independently. It's not merely inadequate; it's bad, imo and it really has no place in PF discussions - or even the discussions in secondary schools. It's nowhere near the 'closest model'.

    Back to photons: The picture that people (yer average Jo public) have of photons is a throwback to the old Corpuscular Theory of light that went out about 400 years ago. So much for Science education.
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