Voltage Drop Mystery

Discussion in 'General Electronics Chat' started by WEdgar, Mar 12, 2016.

  1. WEdgar

    Thread Starter New Member

    Mar 2, 2016
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    What controls Voltage Drop?
    1- If you have a single bulb 28volt light circuit the voltage drop thru the bulb is 28 v (in essence all the voltage is consumed thru the bulb).

    2- If you have a series circuit with two 28volt light bulbs, the voltage drop at the first bulb is less than 28 v and the remaining voltage passes on to the second bulb in the series (how does the voltage know to go past the first bulb and what tells some of the voltage to go on to the second bulb in series). This all happens very fast, there must be some triggering mechanism that alerts the Voltage that there is a second bulb.
     
  2. #12

    Expert

    Nov 30, 2010
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    Voltage doesn't flow, current flows. The voltage is a measurement similar to pressure. You might think of pressure drop being similar to voltage drop.
    Water flows from the meter to my water heater to my shower head. The pressure, "drops" across the distance and the restrictions, but the current flows. Voltage is the symptom of pressure changes caused by flow through obstacles..
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    You are not the first person to think it works that way, but it doesn't. The light bulb has a characteristic called resistance. If it is the only thing connected to the power source, then its resistance is the only thing limiting the current though the circuit, according to Ohm's Law:

    E - I x R -- voltage across something equals the resistance of it times the current through it.

    The equation can be rearranged to I = E / R and R = E / I. Each of these versions is useful depending on what you know and what you are trying to calculate.

    So, with a 28 V source, you can measure the current through the bulb and calculate its resistance: I = E / R

    If you double the resistance by putting two bulbs in series, the current will be cut in half. Assume the bulb has a resistance of 14 ohms (to make the arithmetic easy) and play with 2, 3, and 4 bulbs in series.

    Now that you know the current and the total voltage, you can calculate the voltage "drop" across each element in the string individually because for each element in the string, E = I x R.

    The thing that makes this work is the fact that when several circuit elements are in series, all of the current goes through all of the elements all of the time. Common sense says it has to, because there is nowhere else for it to go. Even if the elements are different resistances, the overall current is based on the total resistance, and the individual voltage drops are proportional to the individual resistances.

    ak
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!

    Voltage drop is determined by Ohm's Law. Assuming each bulb has a similar resistance; with two in the circuit, they each drop about the same voltage.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Learn Ohm's law as AK described.
    Once you understand that, you will be able to answer your own question.
     
  6. PeterCoxSmith

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    Feb 23, 2015
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    ..yep, electrons move pretty fast...
     
    Last edited: Mar 15, 2016
  7. anhnha

    Active Member

    Apr 19, 2012
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    The velocity of the electrons in the wires is very slow but the propagation of the electric field is fast.
     
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  8. WEdgar

    Thread Starter New Member

    Mar 2, 2016
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    The question is not that the Voltage is allocated, we know the Voltage allocates exactly as set out in Ohm's Law. THE QUESTION IS: what causes the Voltage to allocate?
    In the 1st circuit all the Voltage is dropped at the first and only light bulb in the circuit.
    In the 2nd circuit the Voltage is allocated between the two light bulbs in the circuit.
    So the question becomes what causes the Voltage to allocate between the two bulbs in the second circuit, remembering all the Voltage was dropped at the first and only bulb in the 1st circuit. Something is causing this allocation--does anyone know what is causing the allocation? Is it current or resistance or a combination of the two or something else?????
     
  9. #12

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  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Its worth pointing out that series bulbs must have the same current rating, or the lowest rated bulb will get all the voltage and blow.

    The concept is well illustrated by heater chains on valve (tube) equipment. In countries with 220VAC; 300mA heaters were common in TVs, smaller items like record players often had 100mA heater chains - IIRC: US setmakers had a range of 600mA heaters to chose from.

    Power valves in the scan sections require more heater power, so the heater voltage at 300mA was higher than the smaller signal valves - as long as they all have the same current rating, they all drop the right amount of voltage.
     
  11. WEdgar

    Thread Starter New Member

    Mar 2, 2016
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    For all of us trying to learn and understand voltage allocation your efforts are appriciated-THANKS
    Lets try a different approach:
    I ask you why a rock thrown up in the air comes down; you respond, Joe's Law states if you throw a rock into the air it will come back down. Now, this is clearly WHAT happens, but it is also clear this is not WHY the rock comes back down, we all know it is gravity that cause.

    Now when it comes to Voltage there is no question Ohm's Law tells us WHAT happens, but the question is WHAT causes the Voltage to allocate between the bulbs; it's probably not gravity but it must be something.
     
  12. WBahn

    Moderator

    Mar 31, 2012
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    That "what" is the electric fields that are set up by the charge distributions. More fundamentally, it is the net attractive force on every charged particle due to every other charged particle in the universe, though the dominant effects are due to those that are very close by.
     
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  13. ramancini8

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    Jul 18, 2012
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    This surprised me, but AnalogKid's analysis is incorrect for light bulbs because filament resistance is non-linear.
     
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  14. WBahn

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    I'm pretty sure AK knows this and is ignoring it to make things simpler to understand at this stage.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    The "what" is the simple fact that voltage drop across any resistance is proportional to the current flow through that resistance according to Ohm's law.

    First, it should be apparent that the applied voltage sees the total resistance in the circuit.
    This resistance is caused by the force required to move the electrons from atom to atom in the material and it doesn't make any difference if the total resistance is from one physical resistor or many in series.
    This total resistance and the applied voltage determines the current that flows through all the series resistors according to Ohm's law.
    The voltage drop across the various resistors is then this current times the resistance of any particular resistor resistance, again according to Ohm's law.

    Thus the voltage drop across each resistor is "magically" proportional to the value of the resistor compared to the total resistance.
     
    Last edited: Mar 13, 2016
  16. WEdgar

    Thread Starter New Member

    Mar 2, 2016
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    Applied voltage determines circuit Total Resistance
    This is very good and certainly appreciated by anyone trying to understand the mystery of Voltage Drop-THANKS.
    Where did you find this information?
     
  17. crutschow

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    The resistance is always there and not affected by the applied voltage (to a first order for linear resistance materials).
    The applied voltage determines the current through the resistance, not the resistance value itself.

    The information is basically just from my understanding of Ohm's law.
     
  18. WBahn

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    The applied voltage most definitely does NOT determine the circuit total resistance. The total resistance of a 1 kΩ resistor is 1 kΩ regardless of the voltage applied across it (until so much voltage is applied that destructive changes take place due to arcing or other phenomena).
     
  19. crutschow

    Expert

    Mar 14, 2008
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    Perhaps this is just an ambiguity in English semantics.
    The applied voltage is used to "determine" (as in calculate) the resistance by the amount of current that flows but it does not "determine" (as in cause) the resistance.
     
    Last edited: Mar 13, 2016
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  20. PeterCoxSmith

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    Feb 23, 2015
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    ...there is a confusion caused by language as the emf in a circuit is called "voltage" the same word as the "voltage" drop across a resistor but they are not the same thing.
     
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