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Thread: AC electricity questions

  1. #1 AC electricity questions 
    Time Lord
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    I'm having a hard time understanding whether AC is a self propagating current, or whether it's more like DC, and requires opposite potentials at the beginning and ending part of the circuit. (So far my coursework only extends to DC)


    What would happen if I used extremely high frequency AC electricity? Say 100 mhz in a length of copper wire 200 meters long. So the length of a full wave is 2 meters (C in copper is ~200 million meters/second)

    Once I'd sent a full wave length through, could I cut power, and expect the part that had been sent to go the full length of the wire?


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  3. #2 Re: AC electricity questions 
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    Quote Originally Posted by kojax
    I'm having a hard time understanding whether AC is a self propagating current, or whether it's more like DC, and requires opposite potentials at the beginning and ending part of the circuit. (So far my coursework only extends to DC)


    What would happen if I used extremely high frequency AC electricity? Say 100 mhz in a length of copper wire 200 meters long. So the length of a full wave is 2 meters (C in copper is ~200 million meters/second)

    Once I'd sent a full wave length through, could I cut power, and expect the part that had been sent to go the full length of the wire?
    Yes, it is similar to radio. You cut power, many times a second, but the transmission is already sent.

    That is how I make my little generators using lengths of wire. There are dangers though. Not impossible to overcome. But the great minds have to get together and work on a simple and safe solution.

    Because if you get a high voltage spike. It is now coming at you with the same amperage as your low voltage spike. Now the wattage goes from say 12,000 watts, 100 amps at 120 volts. To 3,000,000 watts, at 100 amps at 30,000 volts. That will create light.

    However basically yes, an AC current is nothing more then a DC pulse. Many years ago, when this was a hot subject before World War Two. The theory was to use high frequency and high voltage. This way you knew what you were dealing with.

    This is a cool circuit, I have seen them created accidentally when welding. Different length wire, and high frequency cause the effect you mention. This diagram is the diagram of the apparatus explained in the pages below. I think you can see how this might be a useful device.









    Here is the entire link.

    http://www.Rockwelder.com/History/WorldsFair/wf.htm

    The only difference in copper, is that from my experience, although you can get spikes and lows transmitted simultaneously with a carrier wave or DC pulse.

    I have found that, although different parts of the loop can be at different voltages, and create voltage potential between the loop itself at different points, my welding accident basically. The instant that reverse power is applied to the loop, the loop starts changing. With a very long loop, it just takes so long, you can make use of the polarity difference, between the loop and the loop. And never draw upon the power supply.

    Radio waves offer a similar possibility.


    Sincerely,


    William McCormick


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  4. #3  
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    So, I'm wondering, what would happen if I sent the 100mhz signal down two wires simultaneously, one 200 meters long, and the other 199 meters long (one half wavelength difference in length)?


    Would I get destructive interference?

    What happens at the other end? Does the wave simply cancel itself out and disappear? Does it reflect and come back to where it was emitted from? Or, does it just convert into a radio wave, and leave the wire?

    I'm really curious what happens, almost to the point of trying to test it, but it would certainly be cheaper to read somebody else's test results, or find a book that describes these sorts of things.
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    This has a pretty good description of how an antenna works.
    http://www.arrl.org/tis/info/whyantradiates.html
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    Quote Originally Posted by kojax
    So, I'm wondering, what would happen if I sent the 100mhz signal down two wires simultaneously, one 200 meters long, and the other 199 meters long (one half wavelength difference in length)?


    Would I get destructive interference?

    What happens at the other end? Does the wave simply cancel itself out and disappear? Does it reflect and come back to where it was emitted from? Or, does it just convert into a radio wave, and leave the wire?

    I'm really curious what happens, almost to the point of trying to test it, but it would certainly be cheaper to read somebody else's test results, or find a book that describes these sorts of things.
    At some point in time. You could actually create a current between, the ends of the two wires, for the difference in time created by the frequency. And never draw on the power source. Except for the creation of the initial high frequency pulse.

    I believe that Satan's Fork is symbolic of this principle.


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    William McCormick
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    Quote Originally Posted by kojax
    So, I'm wondering, what would happen if I sent the 100mhz signal down two wires simultaneously, one 200 meters long, and the other 199 meters long (one half wavelength difference in length)?


    Would I get destructive interference?

    What happens at the other end? Does the wave simply cancel itself out and disappear? Does it reflect and come back to where it was emitted from? Or, does it just convert into a radio wave, and leave the wire?

    I'm really curious what happens, almost to the point of trying to test it, but it would certainly be cheaper to read somebody else's test results, or find a book that describes these sorts of things.
    If the length of the wire doesn't match the wavelength (or a factor of) you get a standing wave. This is normally a bad thing and results in inefficiency and I guess in some cases heating. The wave should be able to complete one cycle in it's journey down the wire or else it will bounce at the end and collide with the next one. Well more or less. Fractions of a wavelength are also often used. In some cases external objects can help to tune the wire to make it appear longer or shorter.

    Honestly, your question is not a simple one. I've read a few FCC text books on antenna design, but it's been a while. I highly suggest hitting the net for some info, there is a ton of it to be had. Just never trust a single source for anything.

    http://en.wikipedia.org/wiki/Standing_wave
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  8. #7  
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    Thanks very much. Both these links (Harold's and Sanity's) are working out to be very informative, especially since my current physics class happens to be treating electric and magnetic forces right now (which makes the material easier to understand).


    So, do you mean that an antenna is supposed to be at a specific length for resonance to occur, or that non-resonant lengths are the ones that cause the antenna effect? Intuitively, I'm guessing an antenna doesn't need to be any specific length (because otherwise it would be hard to tune a radio transmitter without physically altering the antenna's length)

    So the problem if I ran a 100 mhz current down 2 lengths of wire, one cut to be half a wavelength different from the other, is that the cut one would become an antenna instead of carrying the current all the way to the end?

    What happens if I re-combine the two lengths of wire at the end, and attach them to a third wire, and ground the third wire? When I measure the amount of current reaching the third wire, will destructive interference cause it to be dead?

    So
    Code:
                                     199 meters
    
                        /--------------------\
    100mhz   ------                              -------- Ground
                        \-----------------------/
    
                                     200 meters
    Kind of like that.

    If the two lines are one half wavelength different in length, will I get destructive interference?
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    Quote Originally Posted by kojax
    Thanks very much. Both these links (Harold's and Sanity's) are working out to be very informative, especially since my current physics class happens to be treating electric and magnetic forces right now (which makes the material easier to understand).


    So, do you mean that an antenna is supposed to be at a specific length for resonance to occur, or that non-resonant lengths are the ones that cause the antenna effect? Intuitively, I'm guessing an antenna doesn't need to be any specific length (because otherwise it would be hard to tune a radio transmitter without physically altering the antenna's length)

    So the problem if I ran a 100 mhz current down 2 lengths of wire, one cut to be half a wavelength different from the other, is that the cut one would become an antenna instead of carrying the current all the way to the end?

    What happens if I re-combine the two lengths of wire at the end, and attach them to a third wire, and ground the third wire? When I measure the amount of current reaching the third wire, will destructive interference cause it to be dead?

    So
    Code:
                                     199 meters
    
                        /--------------------\
    100mhz   ------                              -------- Ground
                        \-----------------------/
    
                                     200 meters
    Kind of like that.

    If the two lines are one half wavelength different in length, will I get destructive interference?
    The wire that is grounded will have a severe impedance mismatch and will produce very low to no output radiation at all. Unless of course you put a shitload of wattage in it, then most of the radiation will be heat

    An antenna has a frequency based on it's physical length that it will have the lowest standing wave on, also known as standing wave ratio (SWR). Now there are ways to change this rule by using loading coils and ground planes. You may have noticed some older antennas with coils in the center. Those are in effect to artificially lengthen the wire through inductance. To answer a possible question you might have about that, it's not just the wire coiled up in a smaller package. This artificial length ends up being much long then the physical wire if stretched out.

    Coils can also be placed on the bottom as well. Theses also have the effect of changing the radiation pattern of the antenna producing "Gain".

    http://en.wikipedia.org/wiki/Antenna_(radio)

    http://en.wikipedia.org/wiki/Antenna_Measurements
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  10. #9  
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    So, in effect, there's no way to test for interference, because you can't use odd lengths of wire with AC.

    I had always wondered what the coils were about, with antennas. So, basically a transmitter that is able to transmit multiple frequencies using the same antenna has to use tricks to make the antenna behave as though it had different lengths?


    So, what, exactly does impedance mismatch do? Does it reflect the signal back? (Is it similar to how using materials with different refractive indexes together will reflect light?)
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    Quote Originally Posted by kojax
    So, in effect, there's no way to test for interference, because you can't use odd lengths of wire with AC.

    I had always wondered what the coils were about, with antennas. So, basically a transmitter that is able to transmit multiple frequencies using the same antenna has to use tricks to make the antenna behave as though it had different lengths?


    So, what, exactly does impedance mismatch do? Does it reflect the signal back? (Is it similar to how using materials with different refractive indexes together will reflect light?)
    Complex questions.

    A full wavelength antenna @ 100 Mhz is a half wavelength @ 200 Mhz. Both end up with very little SWR. If you run 101 Mhz you get a little bit, if you run 99 Mhz you get a little bit. Most radio antennas for a freq range use a happy medium and just assume some loss. It's far easier when receiving then it is when transmitting. Also fractions and multiples of the wavelength can result in a low SWR amount, say 1/4 wave, 1/2 wave, etc. What they call ground plane antennas such as a 5/8 wave can be very effective for radio communications. They are always found as vertical masts from my experience. The ground reflection makes up for the SWR. You will want to research this one a bit, it's a strange set of interactions that end up working really well

    Impedance mismatch pretty much causes a reflection and in turn lower efficiency. If the impedance is mismatched too much it can result in very very little output power. It's interesting to note that even coax cable should be a multiple of the primary wavelength you are trying to send down it. This practice is rarely ever practiced however (at least not with receivers).

    Receiving signals is far far more forgiving then transmitting.
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    Quote Originally Posted by kojax
    So, in effect, there's no way to test for interference, because you can't use odd lengths of wire with AC.

    I had always wondered what the coils were about, with antennas. So, basically a transmitter that is able to transmit multiple frequencies using the same antenna has to use tricks to make the antenna behave as though it had different lengths?


    So, what, exactly does impedance mismatch do? Does it reflect the signal back? (Is it similar to how using materials with different refractive indexes together will reflect light?)

    You may not remember, but years ago we all had CB radios. Some kids had the big rotating antennas. And some had the sticks. Some big some small.

    A lot of times if you were on the water. The moist air could boost your signal. So although others could hear me, I could not hear them.

    These CB antennas could light a florescent bulb, if you put the bulb near the top of the stick antenna. But only at the very top of the antenna. It was not really transmitting power from anywhere but at the top of the antenna.

    That means that a tiny point at the top of the antenna was outputting all that power. I thought that was pretty amazing. I also noted that it took a certain light creating kind of power to transmit a signal, even over a short distance.

    I would also speculate that while receiving, the same area of the antenna was used. Which means that the size of the antenna was really just for gaining signal while receiving, and it could be used to output more power while transmitting if you had the power to make it happen.

    A transformer or coil is also a capacitor. If you supply power to a capacitor, you know that often you can get a lightning like response or high amperage draw or output on the other side of the capacitor.
    It is often a very fast response. Not matching your input amperage or voltage. That is one reason from what I can see for the coil. The other is that if a magnetic field is created, and then you create a radio transmission within the magnetic field, the magnetic field will amplify the signal. Just like an EMP pulses magnetic field does.



    Sincerely,


    William McCormick
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  13. #12  
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    Quote Originally Posted by kojax

    So, what, exactly does impedance mismatch do? Does it reflect the signal back? (Is it similar to how using materials with different refractive indexes together will reflect light?)
    Impedance mismatch just causes a less efficient transfer from source to load.

    You can get the idea like this:

    Assume you have a voltage source and this voltage source has an internal resistance (As all really voltage sources do).

    This internal resistance can be represented as a resistance in series with an an ideal voltage source.

    If you apply a load to this voltage source, the equivalent circuit is a two resistors in series with an ideal voltage source. Call them RS and RL ( for source and load).

    It turns out that in order to get the most power to RL, it has to have the same resistance as RS.

    For example:
    Vs = 1 volt
    RS=1 ohm
    RL= 0.5 ohm

    I= 0.667 A

    and the power delivered to RL is

    0.667 x 0.5 = .222 W

    If we change the load to 2 ohms, then:

    Vs = 1 volt
    RS=1 ohm
    RL= 2 ohms

    I= 0.333 A

    and the power delivered to RL is

    0.333 x 2 = .222 W again.

    but if the load is 1 ohm:

    Vs = 1 volt
    RS=1 ohm
    RL= 1 ohms

    I=0.5 A

    and the power delivered to RL is

    0.333 x 2 = .25 W

    More power than when RL was either smaller or larger than RS.

    Now, impedance is the combination of resistance and reactance (capacitive (Xc), inductive(Xl) or both) in AC circuits and is given the symbol Z.

    The same thing that goes for what said above for resistance also goes for impedance (In fact, the above example is exactly what you get with an AC voltage source and purely resistive internal and load impedance). IOW, you get the most load power when the source and load have the same impedance.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Well stated Janus.
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    So what happens to the power that doesn't transfer when the impedances are mismatched? Does it get reflected back toward the sender, does it all just dissipate into heat, or is it simply never sent in the first place?

    I'm kind of wondering how conservation of energy asserts itself here.
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    Quote Originally Posted by kojax
    So what happens to the power that doesn't transfer when the impedances are mismatched? Does it get reflected back toward the sender, does it all just dissipate into heat, or is it simply never sent in the first place?

    I'm kind of wondering how conservation of energy asserts itself here.
    I would say you are on the right track. It would be dissipated in resistive heating and less energy would be absorbed from the source.
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  17. #16  
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    Quote Originally Posted by kojax
    So what happens to the power that doesn't transfer when the impedances are mismatched? Does it get reflected back toward the sender, does it all just dissipate into heat, or is it simply never sent in the first place?

    I'm kind of wondering how conservation of energy asserts itself here.
    It depends on the impedance mismatch.

    See the chart below. It plots power vs load resistance, assuming a 1volt source and 1 ohm source resistance. The lines plot the power of the source impedance, the load impedance and the total power.



    Note that while the load impedance is smaller than the source impedance, the total power is high, but the load power is low because the majority of the power is dissipated by the source impedance.

    When the load impedance is larger than the source impedance, its power is greater than that dissipated by the source impedance, but the total power is now low enough that even its greater share is less than the its power when the impedances matched.

    This happens because as the impedance of the load increases, it decreases the current of the circuit. Since power is proportional to the square of the current (P=IR), The decrease in current causes an overall loss of power.
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  18. #17  
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    Quote Originally Posted by (In)Sanity
    Quote Originally Posted by kojax
    Thanks very much. Both these links (Harold's and Sanity's) are working out to be very informative, especially since my current physics class happens to be treating electric and magnetic forces right now (which makes the material easier to understand).


    So, do you mean that an antenna is supposed to be at a specific length for resonance to occur, or that non-resonant lengths are the ones that cause the antenna effect? Intuitively, I'm guessing an antenna doesn't need to be any specific length (because otherwise it would be hard to tune a radio transmitter without physically altering the antenna's length)

    So the problem if I ran a 100 mhz current down 2 lengths of wire, one cut to be half a wavelength different from the other, is that the cut one would become an antenna instead of carrying the current all the way to the end?

    What happens if I re-combine the two lengths of wire at the end, and attach them to a third wire, and ground the third wire? When I measure the amount of current reaching the third wire, will destructive interference cause it to be dead?

    So
    Code:
                                     199 meters
    
                        /--------------------\
    100mhz   ------                              -------- Ground
                        \-----------------------/
    
                                     200 meters
    Kind of like that.

    If the two lines are one half wavelength different in length, will I get destructive interference?
    The wire that is grounded will have a severe impedance mismatch and will produce very low to no output radiation at all. Unless of course you put a shitload of wattage in it, then most of the radiation will be heat

    An antenna has a frequency based on it's physical length that it will have the lowest standing wave on, also known as standing wave ratio (SWR). Now there are ways to change this rule by using loading coils and ground planes. You may have noticed some older antennas with coils in the center. Those are in effect to artificially lengthen the wire through inductance. To answer a possible question you might have about that, it's not just the wire coiled up in a smaller package. This artificial length ends up being much long then the physical wire if stretched out.

    Coils can also be placed on the bottom as well. Theses also have the effect of changing the radiation pattern of the antenna producing "Gain".

    http://en.wikipedia.org/wiki/Antenna_(radio)

    http://en.wikipedia.org/wiki/Antenna_Measurements

    Suppose I want to measure the voltage at a point along the wire before it reaches ground.

    So I insert a transistor in the last part (represented by the [] )

    So
    Code:
                                     199 meters            
    
                        /--------------------\          
    100mhz   ------                              ---[]----- Ground
                        \-----------------------/      \
                                                               \
                                     200 meters           \  Measuring device
    Would I be able to detect anything, or do I actually have to create harmonic resonance using an antenna to get a strong enough result to read?
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  19. #18  
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    I assume your using a transistor to amplify the very small voltage in respect to ground that would develop that close to ground. The rules of parallel resistors would come in to play here with the internal resistance of the wire being a factor. For the sake of accurate measurement we'll assume your using an oscilloscope to measure the output of your transistor or some form of true RMS meter. In theory you would see voltage at any point along the wire up until you get so close to ground that you no longer can pick up a signal without massive amounts of amplification. Even then you still would if you can filter out noise.


    This may help with your calculations

    http://en.wikipedia.org/wiki/Paralle...llel_resistors

    You may find this device interesting.

    http://en.wikipedia.org/wiki/SWR_meter
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