Basic Advice.

Discussion in 'General Electronics Chat' started by djohnb, May 31, 2009.

  1. djohnb

    Thread Starter New Member

    May 31, 2009
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    When I was a lot younger, a wise old man told me "Remember, its the Volts that Jolts you, but the Current that Kills you".
     
  2. count_volta

    Active Member

    Feb 4, 2009
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    What that means is, you can have a very high voltage input to something, but if that something has a high resistance it wont kill you. But if you have a high current, it will kill you.

    Voltage causes current. Voltage is potential energy. A car on the edge of a tall cliff has a high potential energy, but it won't kill you. Now if that car begins to fall, the energy turns kinetic and you're dead. Then there is air resistance which slows the car down.

    Its all ohm's law. V=I*R

    For example if you have a 1.5 volt battery and you connect it to a resistor of 1 ohms, you will have 1.5 amps, which is high enough to kill you!!!

    But if you connect the 1.5 volt battery to your body, since your body has very high resistance, the current will be tiny, milliamps and lower, which won't kill you.

    Look for example at my avatar. It says Danger High Voltage. When people write these signs they mean the voltage is high enough to cause a current in the 1 amp and higher range, which can kill you. They mean the voltage is too high considering the resistance of the human body, which varies from person to person, but is generally the same. For a 200 mega ohm resistor that high voltage is nothing. ;)

    Umm, just dont mess with anything above like 15 volts or something. Read this before messing with anything above 5 volts. http://www.allaboutcircuits.com/vol_1/chpt_3/index.html
     
    Last edited: May 31, 2009
  3. Wendy

    Moderator

    Mar 24, 2008
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    There is also the issue of where the current path through your body goes. If it goes through your heart it is a lot more likely to be lethal.
     
  4. radiohead

    Active Member

    May 28, 2009
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    Here goes...I disagree with 'it's the current that kills you' theory because it's not the current alone, but many factors interrelated...voltage is an electromotive force, it can't move without current. Current is the quantity/amount of electrons that pass a through a conductor in a given time. If you multiply current by voltage, the product will be the power in watts. The watt is a rate of energy, equal to one joule per second. A joule is the energy exerted by a force of one newton to accelerate an object. A newton is the force required to accelerate an object that weighs one kilogram the distance of one meter per second. Anyways, a wire heats up when it has excessive current going through it. One example is an incandescent light bulb filament. You have a lot of current going through a very thin wire. Because its in a vacuum, it doesn't pop like a fuse. To prevent wasting energy in the form of heat (Kirchoffs law of thermal radiation), we can have large amounts of voltage potential (tens of thousands of volts) on a cable at a low current then step it down at substations, and step it down again at the power pole transformer before it goes into your home or business. The bottom line is even though the current is low and the voltage is high, I still respect a 35KV volt power line. Furthermore, I believe it's the Coulomb that kills. The Coulomb is the amount of stored energy in one second. So, one coulomb is the amount of electricity transported in one second by one ampere. There, I ran it up the flagpole. Let's see who salutes...
     
  5. Mike Mandaville

    Active Member

    May 27, 2009
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    Radiohead wrote:

    "If you don't want to stand behind our troops,
    feel free to stand in front of them."

    I like that. It reminds me of that fellow in utilities, with a cup of coffee and a smiling face, who basically says the same thing. I'm going to look around for that picture, and post it.
     
  6. count_volta

    Active Member

    Feb 4, 2009
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    Well you are right, it is the coulomb that kills you. ;) Current is the speed of the coulombs. But a slow coulomb won't kill you, only a fast one. So still, current wins.

    Along the same lines, its the watt which kills you, or actually the joule. Once again a slow joule is less powerful than a fast joule, so watts win. Haha.

    We should make a card game and call the cards, voltage, current, watt, etc. Call it electrical poker. ;)
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    It is definately the current that kills, but it is the voltage that injects. Allow me to insert electrodes, and I can kill you with a "D" cell.
     
  8. radiohead

    Active Member

    May 28, 2009
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    ...I'm all-in...Count Volta, like I said, the factors are all interrelated... haha I win
     
  9. electr

    Active Member

    May 23, 2009
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    I use a 5mohm resistor to measure appliances' current.
    When I have a 16Arms load connected, the resistor gets very hot since it consumes 1.3W that are spread over pretty small area, since the resistor itself is not that large (the resistor itself is 3W rated, so no damage is done to it).
    Anyway, there's a very experienced electrician where I do these measurements.
    He told me once that even when 16Arms current is flowing through the resistor, since its voltage drop is only 80mVrms, then its not dangerous to touch the resistor.
    He said something about only high current along with high voltage can harm you, but when one of the two is low enough, then it'd cause no harm.


    How do you explain it?
     
  10. radiohead

    Active Member

    May 28, 2009
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    My advice would be not to touch it on purpose, no matter what the ammeter or voltmeter says. Respect that which can kill you. It's a matter of mathematics, what's the input voltage, what's the current draw of the device, how strong is your heart, etc... either way, are you seriously willing to touch a known hot wire just to prove a point?
     
    Last edited: Jun 1, 2009
  11. radiohead

    Active Member

    May 28, 2009
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    Oh yeah, one more bit of advice...if you do feel the urge to test a hot wire, use the back of your hand, that way your fingers won't clamp around the hot cable preventing you from letting go. I made that mistake when I started out...I touched the yoke on a CRT television...lucky for me, all I got was a nasty burn on the back of my hand...
     
  12. electr

    Active Member

    May 23, 2009
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    bro, all I wanted to acheive was knowing whether its dangerous or not touching a high current carrying resistor with very small voltage drop, and understand the reason for this being dangerous of safe.
    i'm all about electronics.
     
  13. hobbyist

    Distinguished Member

    Aug 10, 2008
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    The 80 mv. is because the resistor is taking almost the full load. (current)
    The lower the resistance the greater the current and the lower the voltage drop across it.
     
  14. count_volta

    Active Member

    Feb 4, 2009
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    It only takes about .015 amps across the heart to kill a person. 15mA???!!! :eek:

    http://www.ohmcheck.com/human_electrical_resistance.htmFirst

    It has to get through the skin first though. If your skin is wet, you could have a resistance of only about 1100 Ohms. With dry skin, the amount is much higher at around 495,000 Ohms.

    Thats very high, so small voltages are safe. A 12 volt battery would cause (with wet skin as worst case scenario)

    I=V/R = 12V/1100Ω = 11mA. You are getting dangerously close at that point, but you also have to add internal organ resistance 85 Ohms up to 575 Ohms. There are many other factors to consider.

    If you are dry, V= 495,000Ω* .015A = 7425 volts. So in theory you could survive 5000 volts if you are totally dry.

    So the moral of the story is, changing a car battery in the rain could kill you, but 5000 volts in the Mojave desert won't, maybe. Ah darn you will still sweat...
     
    Last edited: Jun 2, 2009
  15. radiohead

    Active Member

    May 28, 2009
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    I am learning a lot from info generated from asking a simple question.
     
  16. electr

    Active Member

    May 23, 2009
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    So if I understood correctly, it is eventually the voltage that kills you.
    If i touched this 16A, 80mV resistor, nothing should've happened to me since 80mV would generate a very small current flowing through my body.
    However, if I touched a 1Mohm resistor, with 100V applied on it, then even though there's only 0.1mA flowing through the resistor, in certain environmental conditions, too much current would flow through my body.
     
  17. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    pish posh guys.LOL :D

    Let me tell you my version of this voltage, current joules and coulombs or whatever.
    The human body has resistance, the wetter the lower the resistance. It takes at least minimum of 24VDC or AV to generate a noticeable feeling through the body, so to say any thing higher than 24V is dangerous to humans :eek:

    OK Here is the deal. If you touching anything that has potential respective to the ground you are at has a higher value than 24V you are likely to get a shock.
    Now how severe depends on the amount of voltage present.
    How lethal depends on the amount of current the source can supply.
    If the source cannot supply enuf current to sustain the shock, you will just get kick ass shock and reflex action will take you away from it, but if the source is powerful enuf to supply curret and maintain the initail volatge then you are done for, you cannot let go and your muscle cramps and holds onto what ever it is you are touching.

    Now if the source is like 32KV but only with few mA. like a anode of a CRT, you'll just get a nasty shock cause the output of the Horizontal drive has very poor regulation and the voltage drops to zero giving the reflex action enuf time to act.

    So to say voltage is there to produce current.
    and it is the amount of Current that flow through the body that kills

    Rifaa
     
  18. count_volta

    Active Member

    Feb 4, 2009
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    You see, because there is a 100V drop across the resistor, it means if you touch the resistor (in parallel), 100V will be applied to you. But the 1Mohm resistor has a much much higher resistance than your body, so the 100V is nothing for the resistor, but is fatal for you. ;)

    Voltage can't hurt you. The bottom line is, voltage is potential energy, and just like a huge rock on a cliff can't kill you until it begins to fall (even though it has huge potential energy) electric potential energy can't hurt anyone until it causes electrons to move, i.e. current.

    Voltage doesn't do anything itself, it forces electrons to move, and they are the ones that hurt you.
     
    Last edited: Jun 4, 2009
  19. radiohead

    Active Member

    May 28, 2009
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    Charge stored:
    Capacitance in farads X voltage across the capacitor = charge in coulombs

    Energy stored:
    Capacitance x applied voltage squared divided by 2 = energy in joules
     
  20. djohnb

    Thread Starter New Member

    May 31, 2009
    2
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    Hell, I'm learning a lot from my initial post.
    So, would the guy in the following link have been saved by wearing rubber shoes ?
    http://www.forbezdvd.com/cod.php?v=MzY0Mw (WARNING ! One flash and you're ash) :(
     
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