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

Voltage/Frequency/Current Question

  1. Oct 24, 2013 #1
    Hey there, newbie here. =)

    ive been reading up/pondering on electrical phenomenon recently and im still working out the conceptual constructs in my head - which brings me here!

    the issue im wrestling with today is how, as i understand it, different amounts of energy can be moved with a static frequency and voltage (and presumably static speed of wave propagation?).

    so i got my 110v 60hz(where i live) AC current coming out of the wall(well jiggling out / in i suppose).
    but different devices draw different amounts of current from this source.

    it seems contrary to my sense of reason that, based on a static amplitude(voltage) and a static frequency (60hz in this case) at a static rate of wave propagation, to be able to move different amounts of energy.

    so how can i conceptualize the mechanism by which the refrigerator "uses" more power than the clock on my desk?

    i realize there is a factor which i neglected to mention that is resistance. and i have read that less resistance = more current. (I=v/r)
    however i am not able to reconcile, for example, how the utility company can charge more to run a refrigerator than a clock if the signal they send is the same for both.

    -two ducks
  2. jcsd
  3. Oct 25, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF;
    The term "static" does not really work with the concept of things moving with a speed - d you mean "constant" or something?

    In an AC circuit, the charges in the wires move back and forth and the electric field propagates along the wire.
    A handy mental image is of a toy train moving back and forth on a track.

    The Utility company does not "send" you the electricity as such - you pull electricity from the wires with the things you plug in to them.

    It may be a good idea for you to start by understanding DC circuits before you try AC.
    In a DC circuit - the electric field does not change (unlike the AC case) and yet you have no trouble wth the idea that a battery can run a light bulb or that 2 bulbs drain the battery faster than one bulb.
  4. Oct 25, 2013 #3
    indeed! constant is a better word for it =)
    im comfortable with the concept of "AC jiggle" - although, is the electric field a factor of consequence in this? i know of the electric and magnetic fields associated with electron movement. i realize the electric field lines propagate, and this is how wireless transmissions work. but with the case of wired power transmission, are the EM waves important?
    the way you say that it sounds more like the things we plug in are whats doing the work - "pulling" power from the socket =) so what am i paying those guys for? =)
    im relatively familiar with DC circuits...
    DC doesnt cycle, it has a constant voltage. but, with a battery the voltage will cycle, but very slowly. when its fully charged the voltage is high, and as it drains the voltage drops.

    so its kinda like a really slow wave... ? i dunno i was making that up as i went along..

    i would have to read more about DC power supplies(other than batteries)

    but that might be a little beside the point for my question?
    i know most household devices utilize DC current, and they get it by converting AC (going to read about that right now)
  5. Oct 25, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    AC "jiggle" that I'm used to is the deviations from a sine wave that the voltage graph shows.

    Yes. It's what is supplying the energy.

    There is an EM field associated with the electron acceleration but that's not what I'm referring to - there is also an electric field pointing along the wire.

    "drain it away" may be a better metaphore.
    It's like the utility company provides a reservoir and you drain electricity from it.
    The more you drain the harder the utility company has to work to keep the reservoir full.
    If you ran your house of pedal-power - the more stuff you have running, the harder you'd have to pedal.

    No. If you kept running the circuit the battery voltage would drop to zero and stay there.
    In AC it would raise again to maximum without needing a recharge.

    In DC the electric field is a constant. The current and voltage do not cycle.
  6. Oct 25, 2013 #5
    i have only conceived of the field as a 3dimensional radiation wave. that being so, i imagine it moves through the wire along with everywhere else.. but i thought the electrons themselves were the "wave" that transmitted the energy.
    but this reservoir analogy is strange to me since no actual "water" comes to me, it just jiggles at the socket - thereby inducing jiggle in my electronic devices when they are connected.

    i could imagine though, that it takes a quantity of energy to jiggle the "water"/electrons in the "pipes" /wires which run from the power plant to my house. then when i connect a device, which has "water" /electrons in its pipes already, it requires more energy to induce the added "water" to jiggle.

    the more "pipes" connected the more "water" to jiggle and the greater the "resistance."
    yea i was considering the voltage drop from max to 0 as a kind of slow 1/4 wave.

    and as im reading about DC right now, from what i understand it isnt exactly constant... it is essentially phase modified AC current, and there are varying degrees of smoothness.

    getting to sleepy to remember what i was initially "getting at" with this post.
    will review tomorrow =)
    thanks for the input!
  7. Oct 25, 2013 #6
    You really do not need to consider wave propagation in 50-60 hz power. It exists to facilitate generation and distribution ( multiple points of transformation). So you are making this wayyyyy too complicated - per your original post.

    As for AC POWER - that is what we want, Power as V x I - BOTH the V and I have a direction (instantaneously), mathematically we use vectors. As the AC oscillates ( ref SINE WAVE) - ideally the V and I are "in phase" so +v * +I is "+Power" .....then as the V is - the I is - .... now -V * -I = + P

    The refrigerator takes about 1000x the current then the clock, they both run on the same voltage. So the refrigerator uses 1000x ( just a SWAG) more POWER then the clock.

    As for DC - RECTIFIED AC to make DC can have a ripple, but DC does not necessarily have any ripple, it can with the right system be constant - considering the discharge of a battery is again over complicating the issue.
  8. Oct 25, 2013 #7
    i know i dont need to consider wave propagation, i just enjoy building conceptual frameworks - im just doing this for intellectual enrichment... the more aspects of a system i can consider the better i'll understand it. =)
    im familiar(ish) with ohms law and how to calculate power consumption. im trying to build a conceptual framework to understand the dynamics of that relationship.

    so with the analogy used elsewhere in this thread - about water pipes.
    the AC is "jiggling" or "waving" back and forth in the pipe that connects from the utility company to my house.
    when i plug something else into that pipe, im effectively "dampening" that jiggle.
    or! to modify the analogy - the utility company is making waves in a pool. i suddenly connect another pool of calm water to the first. the calm water dampens the "waving" water. the power company "pushes" harder to induce waving in the now larger pool.

    now my analogy gets confusing when trying to switch to electrical terminology.
    because how hard the utility company "pushes" is the voltage, correct?
    and the amount of water in the system would be the resistance - because adding water to the system is what "dampens" the "wave"? or - i see that the resistance could also be the size of the pipes - because the size of the pipe could limit the "size" (amplitude?)of the wave.

    current, however, doesnt seem to fit in this analogy, since the "water" isnt really "flowing" anywhere except back and forth at a consistent rate.
    yea my post about DC was only a few minutes into my reading on ac/dc conversion =)
    though as far as ive gotten it still appears to be only "relatively" constant voltage. but i dunno if thats pertinent for right here anyway =)
  9. Oct 25, 2013 #8

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    But you said that energy can be transmitted wirelessly through EM waves in space... think of the electrons being to the EM field as corks are to the surface of a pond. They bob up and down and drift about in response to the waves on the surface.

    The electric field that is important to your understanding is the one applied to the length of the wire - the part of the field is not generated by the acceleration of the charges in the wire.

    All analogies have their limits - the water-flow analogy for electricity is the same.

    Imagine the old house water supply that had a tank on the roof, and you needed a servant to carry the water up in buckets. Imagine you are the servant. Your boss points out that there is already water in the pipes.

    Thing is - the water is modelling the electric energy, not the electrons.
    It sounds like you don't know how a back-and-forth flow can mean that something is being delivered to you.

    Sound involves a back-and-forth flow - yet sound travels, and can be used to do work.

    The greater the inertia ... the "jiggle" involves accelerating the water - which requires energy no matter which direction the juggle goes in.

    That's right - you don't need t do anything unless you have decided on some purpose - like building a conceptual framework. If you want to be successful then you do need to follow the advise you are given - you do need to use decent tools and solid raw material if the framework is going to be of any use.

    But you are acting as though you are not. You should be using that familiarity as part of your framework.
    What you are doing right now is messing with limited analogies which will give you a shaky foundation for understanding electricity.

    See - any analogy can be pushed too far.

    You want to understand why your house power costs you the way it does - try generating some yourself. Someone in the power company has to turn a crank handle. The more power that is demanded, the faster they have to crank it. That's hard work - in practise they get something else to turn the crank - say, water or air flow: but that can only crank the handle so fast - which means there is a limit to the rate you can supply energy. So you supply the energy to the people who pay you for it.

    You want to understand how a back-and-forth motion can be sending something to you - then look at travelling waves. You do need to understand wave motion to get this.

    Sound waves make the particles in the media move back and forth - do you have any trouble with the idea that energy is travelling even though the particles themselves just go back and forth?

    AC electricity is a travelling wave.
  10. Oct 25, 2013 #9
    I never use a "pool " as my thinking for the infamous water analogy. I think pressure in a pipe - if a valve is closed, the pressure can be positive or negative - but no current flows. The utility pushes on the water - then it pulls on the water - energy (power) can be delivered from the utility to you on both cycles. ( consider a piston type motion) - sorry jiggling and dampening words/language are to vague. The analogy is only a stepping stone.

    For voltage -- also consider a resistor, it does not matter which way the current flows through it, it gets hot, delivering heat energy from the electrical circuit to the user in the form of heat.

    Can I perhaps be more clear -- as for AC POWER distribution systems DO NOT consider 60 Hz wave propagation for this discussion....way off base - this thinking is not used, not necessary, not part of the thinking or math that ANYONE uses....to describe. ( Energizing a system, fault currents are the unit step functions I mentioned - but in these the freq considered is not 60hz).

    Lastly - I have respectfully disagree with SB's statement "AC electricity is a travelling wave" - in POWER the wave is not considered to be Traveling - technically AC only means alternating current/ polarity.
  11. Oct 25, 2013 #10


    User Avatar
    Science Advisor

    I think that goes too far, electromagnetism is electromagnetism.
    The power is in the form of near-field reactive EM fields that travel from the generator to the load in AC. Alternating Current is the effect of the fields on the electrons in the wiring as it moves, as the 'Power' is not in the electrons that restrain the fields near them. Yes, you don't have to deal with far-field wave propagation usually but you still have to deal with wave movement effects similar to umatched coax for RF transmission in long AC power lines that store large amounts of electrical energy in those fields.
  12. Oct 26, 2013 #11


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    There is another thread which discusses this, running at the moment. As soon as the length of a mains transmission line reaches a few hundred km, the travelling wave nature of the mains becomes relevant. Power transmission engineers are only too aware of this. A wavelength at 60Hz may be very long in terms of desktop circuitry but supply areas are a lot bigger than a lab bench. However, engineers are pragmatic people and, when the propagation time is short, they 'ignore' it.
  13. Oct 26, 2013 #12
    Engineers work their whole careers including PEs and never get involved with wave propagation - my point being at 25 hz ( the Amtrak North East line) to 400 HZ power..... wave / traveling phenomena is never considered. Wave propagation in 60 Hz transmission BECAME an issue as the line distances grew- the fact that the AC exists has nothing to do with this. We might as well start saying turning on a flashlight is traveling wave phenomina - techincaly you can work this with Maxwell's. To some one referring to Jiggiling - and make the jump to traveling wave? Sorry I object.
  14. Oct 26, 2013 #13

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You can have lots of fun with analogies can't you.
    We should bear in mind that everything depends on a context - it is possible to have small scale AC that is well modeled without the travelling wave. But if someone is having trouble figuring how something that is essetially a back-and-forth movement could be considered to be delivering something, pointing out that longitudinal travelling waves exist is a valid approach. Here you have a back-and-forth motion that does deliver things.

    As sophiecentaur points out - in power transmission over long distances - the travelling EM wave is important. A lot of industrial standards have been adopted so that the working power co engineer does not have to think about it too much though. Engineers often go their whole careers not considering all kinds of things that are present and important to the physics. It one of the differences between engineering and physics: they are doing a different job.

    It is true that engineers seldom need to understand travelling waves to do their job - but OP would benefit from knowing about them to achieve his stated aims. Feel free to disagree - I'm sure the parallel discussion will be useful.

    I do like the pressure-wave analogy though.
    I normally use a train-set because I happen to have one to show people.
  15. Oct 26, 2013 #14


    User Avatar
    Science Advisor

    Using wave and field theory to design a standard flashlight is crazy but an understanding at even the most simple level of the physics of electical power flow in fields from a battery to a bulb is something that seems to be getting lost in some basic electrical training courses.
    Understanding the concept of field energy flow brings 3D geometry into circuit design and helps you visualize why systems interact in sometimes strange ways like how electrons (charge) travel slowly in a circle in the flashlight wiring but power only travels one way (from the battery to the lamp) on both sides of the circuit at almost light speed.
    When they start AC circuits understanding the relationship between "Jiggiling" electrons (charge) and "Jiggiling" fields/waves is a smaller step that continues to build a electrical visual of 3D space around circuit wiring. Knowing all the details of the math involved is not needed but know the concept is IMO important.
  16. Oct 27, 2013 #15


    User Avatar
    Gold Member

    I sort of got lost reading through this thread, but feel there were some misconceptions in the inital post.

    First, the electrical current does not "jiggle" at 60 Hz. It changes direction and varies in amplitude at 60 Hz. (you sort of stated that). Note that to appear as "jiggle" you would have to look at it in terms of many wavelengths. 60 Hz is ~3100 miles.

    When you attach a load, the current caused by this varying potential is also varying.

    If you attach something with low resistance, there will be higher current, so more power/energy delivered.

    There is no need to think of it as traveling waves since the wire lengths are so small compared to the wavelength of 60Hz. At 1/4 wavelength things might get interesting.
  17. Oct 27, 2013 #16

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    @twoducks: any of this any help for you.
    Things suddenly got a bit specific for a "conceptualization". Let's see if we cannot get back on track ;)

    Hopefully you can see how something that goes back and forth can also deliver something - a travelling wave is an example of that - and you can see why the travelling wave, while strictly present, is not normally used in calculations in real-life circuits.

    Once you see that a travelling wave delivers energy - you should be able to see how an instantaneous back and forth can deliver useable energy - and how the more of that energy you use the more the power company has to work to make the back and forth.

    Or you may be able to skip the travelling wave thing completely.
    Let us know how you are getting on.
    Last edited: Oct 27, 2013
  18. Oct 27, 2013 #17
    STOKED on all the interesting responses.

    i think i have a good enough idea now to understand the answer to my original question so, mission accomplished. =)

    however, the answer has led to new questions!

    i dont know that i can formulate them clearly yet though...

    im sure i'll be asking them soon enough =)
  19. Oct 27, 2013 #18

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    No worries - let them cogitate for a while - and try to apply the mathematical tools rather than some sort of intuitive construct or analogy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook