Low Current High Voltage

Discussion in 'General Electronics Chat' started by rperea, Apr 7, 2009.

  1. rperea

    Thread Starter Member

    Apr 7, 2009
    15
    0
    Hello,
    I am always concern about how high voltages are not high current if we keep the resistance constant (V = IR). I know the electrical potential or voltage is a difference in charges and the current is the movement of the charges....so how can we upgrade the low current to high current...should we change the resistance ? Now this brings another doubt......so the more time it takes for a resistor to change this electric potential difference or voltage the higher the current?
    How is this related, how is this possible and how can it be changed?
     
  2. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    I = V/R so to increase current one can either raise the voltage or lower the resistance or do both. Time has no bearing in pure resistance circuits.

    Lefty
     
  3. Terp

    Active Member

    Jun 6, 2008
    32
    1
    "high" is a relative term. There has to be a boundary that distinghuishes between "high" and "low" current. Given that you are dealing with a linear, ohmic material, yes, V = IR. Increasing V will result in increasing current.

    I am not so sure if "difference in charges" is what defines electric potential or voltage. Potential at a point is the amount of work that the field does to move a coulomb of charge from a reference point (the ground, where V=0) to the point of concern in space. Potential at a certain point due to charge distributions is the difference of potential due to each distribution.
     
  4. rperea

    Thread Starter Member

    Apr 7, 2009
    15
    0
    Thank you for the response. So lets suppose I receive a discharge of 20kV, I know this is not fatal and I am assuming its because current is low, is this because the body resistance is small (which I know its not....:cool: ) or why is this not fatal......also is there any case when the electricity if fatal so someone ? why and how?
     
  5. fish hat

    New Member

    Apr 7, 2009
    1
    0
    the human body is generally modeled as having a resistance of 1.5kΩ across your wings. as little as 65mA across your chest will give you a heart attack. depending on your weight the muscles in your arms and hands will contract involuntarily between 10mA and 20mA.

    i.e. if you grabbed a 120VAC wire from the wall with both hands you're probably going to die.
     
  6. Terp

    Active Member

    Jun 6, 2008
    32
    1
    When the body receives a voltage discharge, the electrons are pulled from the ground in the upward direction. When you say low current, this implies that the electrons are moving slowly upward through you. Consequently, those tiny electrons stay inside your body for a longer duration and cause increased damage, thereby increasing the fatality.

    Now, larger the current through you, faster the electrons move, hence less damage to your internals, and a lower fatality.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I have to disagree with this -
    - as the information is incorrect.

    Electrons move from a relatively negative point to one that is positive. With AC, this polarity changes at 60 or 50 Hz, depending on your utility distribution. The points with the difference in voltage will has current between them, but not in a predictable path through the body.

    20 KV at a high frequency may not be fatal as it may travel over the skin and only cause burns. At low frequency or DC, this is a lethal voltage. It will cause fatal current through the body. That is pretty close to that used in the electric chair - definitely a fatal application. But -
    - is by no means a correct statement.

    Small currents involve fewer electrons flowing than large - hence large currents will do more harm than small. Any current of 20 ma or larger has the potential of freezing your heart muscles.

    Standard advice - always assume any exposed wire is live and carries a lethal voltage. Verify with a meter before touching it.
     
  8. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    A 20 KV exposed wire could be part of the power grid, 20 KV
    @20 amps could Zap a meter and holder. (Call 911) Let poster state
    the enviroment of 20KV. Some indusdry complex might
    have this type of voltage,I have heard they use a bang
    stick,an instrument 3 foot long to avoid arc shock.

    Look at spark plug voltage,12 volt battery,starting amps appox 250 amps.
    When the ignition coil reaches max voltage at cylinders,the magnetic
    force of the coil collapses. This creates high voltage spark,if your
    hand is near that plug so you get a quick shock. In an Instant it will
    shock you again. Everybodys system is different that why most people
    will not get near anything electric or or if they think it electric.
     
    Last edited: Apr 12, 2009
  9. b.shahvir

    Active Member

    Jan 6, 2009
    444
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    It's always the amps that kill, not voltage. Voltage or potential difference is just a force which drives charged particles around a closed circuit. The amount or quantity of charged particles passing thru the human body decides the extent of fatality. :eek:

    This is analogous to a water pump which drives certain quantity of water around a closed pipe circuit. It is the volume or quantity, as also velocity of water impacting a person (who presents interference to the path of water within the closed pipe circuit) which would prove fatal and not the pressure developed by the pump itself! ;)

    Regards,
    Shahvir
     
  10. b.shahvir

    Active Member

    Jan 6, 2009
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    Could you plz explain the concept of high frequency currents being less fatal? Do the high frequency currents travel over the skin due to the 'skin effect' phenomenon? But at 60 hz the frequency is not that high!

    This concept is a bit difficult for me to grasp and would be grateful if someone can make me understand it in detail. :)
    Thanx.
     
  11. rperea

    Thread Starter Member

    Apr 7, 2009
    15
    0
    So by the relation V =IR..... a battery that produces a direct voltage greater than 97Volts will create a direct constant current greater than 65mA and thus it will be fatal? And ....if the current is AC will this be fatal as well?
     
  12. italo

    New Member

    Nov 20, 2005
    205
    1
    it is a fact that the skin will limit the current it is also a fact that current not voltage kills. As matter of fact a 'D" battery can kill and that is a fact. how break your protective skin get to the blood which is liquid with a lots of sodium a metal and a "D" CELL CAN provide 2 amps in a short circuit the voltage is zero but the current is 2 amps if it goes though your heart you are history. On a final note pole electric climbers if they get the misfortune to die the path is trough his private parts because the blood concentration is there . INTERESTING
     
  13. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    I recall a good discussion on this topic about 2 years from now. IIRC, The reason as to why small currents sometimes prove more fatal is that the muscle contraction tends to be more at this range which can lead to heart attack. This perhaps is not the case with larger currents.

    Pressure can kill as well.;)

    rperea,
    P= V*I and VI =R. This does not change. All transformer applications revolve around these two.
     
  14. b.shahvir

    Active Member

    Jan 6, 2009
    444
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    Pressure can only kill if it is large enough to force fatal currents thru the human body. If human body is considered to possess 1.5KΩ, apply Ohm's law to find the pressure (voltage) which can force above 65mA thru the human body. It's the amps which causes abnormalities in the body which prove fatal! :eek:
     
  15. rperea

    Thread Starter Member

    Apr 7, 2009
    15
    0
    Now....a quick question, we use a resistor to measure current (charge flowing) by the relation between electric potential and resistance but lets say we have a circuit where a piezoelectric materials generates opposite charges in the top and bottom surfaces when compressed, if we short this circuit....is there a current flowing through the wire that shorts the circuit? how does it change if we have a resistor int he middle of the wire, is the current the same in the wire? does it change?
     
  16. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    People have a high "tolerance".
    Some people are strong and almost nothing will hurt them.
    Other people are fragile and almost dead. Looking at them might kill them.

    If you give a big shock to a strong person then he laughs.
    If you give a tiny shock to a fragile person then they die and you are sued for murder.
     
  17. italo

    New Member

    Nov 20, 2005
    205
    1
    the concept being that hi frequency current travels on the skin as opposed to dc .
     
  18. b.shahvir

    Active Member

    Jan 6, 2009
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    But 50 or 60Hz is not that high a frequency to give rise to skin effect! :confused: Also, the discussion is based on fundamental frequencies of 50 or 60Hz.
     
  19. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That is very true. It is also true that the condition of the skin can make important differences in getting a shock, and the magnitude of it.

    That is why it is never safe to imagine "that won't hurt me" in any encounter with a wire carrying a voltage. The absolutely safe limit is very small.
     
  20. b.shahvir

    Active Member

    Jan 6, 2009
    444
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    But, in my opinion, condition of skin can also affect DC conduction equally. Then why is AC more prone to conduction thru the skin? :confused:
     
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