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Is it incorrect to say when the voltage arrives to point X

  1. May 12, 2012 #1

    Femme_physics

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    Is it incorrect to say "when the voltage arrives to point X"

    To my understanding, voltage is potential difference. It can't be defined as "arriving" to some place in the circuit. Current can arrive. Voltage is "just is". It's there. It doesn't move. It creates the borders for where the currents flow.

    The reason I'm asking is because I saw a classmate writing that, and it's supposed to be a legit solution supposedly. What do you think?

    Am I right, or what?
     
  2. jcsd
  3. May 12, 2012 #2

    wukunlin

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    Re: Is it incorrect to say "when the voltage arrives to point X"

    you're right, voltage doesn't move. It is the charge carriers that are moving
     
  4. May 12, 2012 #3
    Re: Is it incorrect to say "when the voltage arrives to point X"

    Actually in beginning classes, we assume the circuit dimension is very small and the voltage variation is very slow, voltage and current are just "there". But if you look at it in RF world, current and voltage do take time to move from one point to the other.

    Without making it more confused, on a pcb, the voltage move about 1.5X10EE8 m/sec, so if you apply a voltage step on one end of the trace, it is going to take a nano second to travel a few inches. You literally say the voltage arrive at a point after a certain amount of time.
     
  5. May 12, 2012 #4

    Femme_physics

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    Re: Is it incorrect to say "when the voltage arrives to point X"

    Oh, I misread his explanation...he wrote "when the voltage arrives to the angle that sets off the SCR"... And in AC it is possible. Sorry :)

    I appreciate the replies at any rate
     
  6. May 12, 2012 #5

    vk6kro

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    Re: Is it incorrect to say "when the voltage arrives to point X"

    In English, "arrives at point B" is more correct than "arrives to point B".

    Someone may travel "to Cairo", but they arrive "at Cairo" if they arrive by train since the train only stops at the train station.
    They drive a car "into Cairo" since Cairo is much bigger than a car and a car may stop at many places..

    This must be very confusing for people learning English.
     
  7. May 12, 2012 #6
    Re: Is it incorrect to say "when the voltage arrives to point X"

    Hello, FP.

    Since you are studying mechatronics you will be doing a significant amount of digital electronics.

    Here the digital signals are in the form of voltage pulses, which travel along circuit board tracks rather as yungman has noted.

    If the tracks are of different lengths these pulses will take different times to travel from their source to destination.

    This time difference can be significant in digital electronics if, for instance, the the two pulses are inputs to a circuit that requires synchronisation.

    We saw the fuss recently at CERN where a pulse (light in that case) took too long to travel a path and made folks think neutrinos were travelling faster than light.
     
    Last edited: May 12, 2012
  8. May 12, 2012 #7
    Re: Is it incorrect to say "when the voltage arrives to point X"

    You can very well compare voltage level to pressure level linguistically. A pressure does not arrive to point A from somewhere. The pressure at the point A increases to the target value.
     
  9. May 12, 2012 #8
    Re: Is it incorrect to say "when the voltage arrives to point X"

    The charge difference that gives rise to voltage is the product of information exchanges at the quantum level. As such, the speed is limited to light speed. So, rather than voltage "arriving" somewhere, it emerges from the exchanges at a given rate.
     
  10. May 12, 2012 #9
    Re: Is it incorrect to say "when the voltage arrives to point X"

    This is a contradiction. If you hooked up a battery, a switch, and a light bulb in a huge wired circuit (e.g., the battery and switch is on Earth while the bulb is on the moon), then according to your reasoning, the bulb wouldn't light up instantly because the current hadn't arrived yet, but the voltage across the bulb would "just be there" instantly. George Ohm would not approve of this situation.
     
    Last edited: May 12, 2012
  11. May 15, 2012 #10
    Re: Is it incorrect to say "when the voltage arrives to point X"

    I have noticed a lot of times people will use "when the voltage arrives" not as a location reference, as in reaching a point on a wire, but more as a time reference; and so it has less to do with the physics of how the signal propagates, and more to do with the timing relations of the circuit operation. In digital, I could say "when the next clock cycle pulse arrives, the data will be buffered", and I mean it with reference to time rather than the location on the IC's clock pin where the pulse's voltage is.

    In analog, a timing reference example could be: "This transistor turns on when the comparator's output high level voltage arrives" implying that a comparator will detect a threshold at its input at some time, and the transistor is turned on with reference to this event.
     
    Last edited: May 15, 2012
  12. May 15, 2012 #11
    Re: Is it incorrect to say "when the voltage arrives to point X"

    In high speed digital, clock timing is one thing, propagation delay become a major factor in design. I designed a lot of ECL logics and we sure have to take into consideration of the propagation time of each signals. It is very common to use coax delay lines to control signal propagation.

    This is nothing new, When I was working for LeCroy ( digital scope) in the early 80s, we designed a lot of transient recorders that capture a single event. There would be only ONE single pulse event( like a nuclear test explosion!!!), we need to use that one single( you don't get another one as everything vaporized!!!:rofl:) pulse to trigger a series of event to capture the information. For example, first trigger that sample & hold to hold the signal, then after waiting for the output of the S&H to settle then trigger the ADC to capture the data. Then after a delay, trigger a latch to capture the data. In my days, we don't have even 8 bit ADC that run over 100MHz, we design subrange ADC using two 4 bit fast ADC that involve summing and cancelling in a few steps sequence to get an eight bit ADC. The one pulse has to go through coax delay lines to trigger each and every event.

    These are all transmission line propagation design even it is digital.
     
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