Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dangers of electricity!

  1. Apr 25, 2007 #1
    im just trying to find out what the dangers are...ok so what kills people, high voltage or high current? or is it both? does this question even make sense? well whichever one could kill people, how much of it is needed?

    im pretty sure current can kill people by stopping the heart and possibly burning the insides
  2. jcsd
  3. Apr 25, 2007 #2


    User Avatar
    Homework Helper

    High power is usually what kills. Simplifying things, power is the product of voltage and current. You need either large voltages and a substantial current , or a substantial voltage and large currents.

    Cases of high voltage but not enough current to kill are that of your typical static shock. (as when you get a shock touching your car door after a long drive in winter). More serious cases of this corona discharge would be getting hit by a lightning strike. Owwie.

    The thing is that with low voltages, it's pretty much impossible to get you killed, since your body's internal resistance is relatively high. If there's not enough voltage, no current will flow.
  4. Apr 25, 2007 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you consider the body as purely resistive (which is more or less true, the human body's capacitance is rather low), then Ohm's law holds, and current is proportional to voltage. You cannot have one without the other.

    - Warren
  5. Apr 25, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    As I understand it.

    At high power you can literally get cooked.
    High current passing thru the heart can cause an uncontrolled contraction causing rupture of the heart muscle.
    Lower current passing thru heart can disrupt the hearts timing circuits sending the heart into a mode called fillibration.

    The last one is probably the most common cause of death.
    You can recover from this, although intervention is often required.
    Intervention in this case often means sending another current pulse thru the heart as depicted on TV medical shows, but CPR can work.
  6. Apr 25, 2007 #5
    Electric shock cause heart and/or lung paralysis. This is why the first aid to give is artificial respiration and heart massage and in some cases defibrillation.

    It is currently assumed that a current below 30 mA doesn't present a hazard to healthy people. The results from a shock due to a greater current can be anything: from death to just a bad souvenir.

    Current can also tetanize the muscles and people get "stuck" to the electrical wire.

    Now what determines the current? As it has been said in others posts the body is mostly resistive. But the body is just a very good conducting electrolyte wrapped in an insulator bag made of (human) leather. Leather is a god insulator if it is dry. Take an ohmmeter and measure the resistance between your hands. You will find something like 100 kOhms. If you wet your fingers you will get ten times less. Values depend on peolple.

    Never play with hot conductors when there is a risk of wet skin.

    When voltage is over a thousand volts or more, the current can be so high that no only there can be heart and lung failure, but people can get burnt. If the current go through an arm and the leg on the same side, people can survive with serious injures in the members.
  7. Apr 25, 2007 #6
    thanks all...i zoned out in class when my teacher was talking about this and its been annoying me since.
  8. Apr 26, 2007 #7


    User Avatar
    Science Advisor

    Fatalities have occurred with voltages as low as 16 volts. All you need is 50 milliamps to produce cardiac ventricular fibrillation. It depends on a combination factors of course. The body's resistance in ohms varies from a mere 300 to 100,000 depending on the site.

    The mechanism of fatalities are multiple but the most common form of injury from electrocution is actually from the arc flashes, this accounts for 80% of the injuries/fatalities we see in the ER. The injury is caused by the intense heat, light, and pressure wave (blast) caused by electrical faults. If you ever watch a welder, the arc flash is similar in heat and energy to that from a Tig or Mig welder. The heat produced may cause severe burns, especially on unprotected flesh. The blast produced by vaporizing metallic components can break bones and irreparably damage internal organs.

    The other forms of injury include a direct burn......Damage due to current is through tissue heating. Usually this is from high voltage (> 500 to 1000 V) and DC current usually produces this type of injury. .In some cases 16 volts might be fatal to a human being when the electricity passes through organs such as heart.

    Last but not least, there is ventricular fibrillation more commonly caused by ac current rather than dc current. The interesting thing is that even though ac current tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation, AC electrocution at higher frequencies tends to run along the skin rather than penetrating and touching vital organs such as the heart. While there may be severe burn damage at these higher voltages, it is normally not fatal.....
    Last edited: Apr 26, 2007
  9. Apr 26, 2007 #8
    thats interesting :bugeye:
  10. Apr 27, 2007 #9
    Skin effect is relevant only at frequencies higher that 10 kHz. At 50 or 60Hz (the mains frequency), the skin effect in human body has a thickness of about 10 meters. That is, it does not exist.
    I think that very few people is exposed to risks concerning high power at high voltages and high frequencies.
    Most current risk is AC mains which, from the skin effect point of view, is identical to DC.
  11. Apr 27, 2007 #10
    Arcs, in arc welders, whether ordinary or TIG or MIG (it is the same thing as much as voltage and current are concerned), are very different from arcs of high power, high voltage devices.
    Arc welders work at low voltages (around 50 volts). You can not imagine an ordinary worker with several thousands volts in the hand in an outdoors yard. You cannot create an arc on a human body with one of those devices.
    When you are victim of an arc from a high power, high voltage source, it is not the heat or the surface burns that will kill you. It is the tens of amperes that traverse your body.

    Usually, people do not have a lot of metallic components in their body (androids do, of course).
    There is something that can vaporize and explode in the body: it is water. But when you reach this temperature, all your sorrows are ended.
  12. Apr 27, 2007 #11


    User Avatar
    Science Advisor

    That is true about the water. Don't forget that many people have spinal rods that are made of metal, implantable defibrillators where the electrodes are imbedded in the muscle itself, old bullet wounds where the bullets or shrapanel have stayed lodged in a body cavity, old surgical clips ,( we see this a lot on incidental xrays.), cardiac pacemakers, prosthetic heart valve, prosthetic hip or knee joint, implanted infusion pump, intrauterine device (IUD), cochlear implant, aneurysm clip or vascular clips, hearing aid, metal monitoring device, metal plates, pins, screws etc.
    Last edited: Apr 27, 2007
  13. Apr 27, 2007 #12
    Happily, most people dont.
    And before a metallic part evaporates, all organic material would have been charred.
  14. Apr 27, 2007 #13


    User Avatar
    Science Advisor

    More people than you think, consider that almost all people with congestive heart failure with less than 35% ejection fraction have implantable defribillators etc. I have metal rods in my spine. Everyday I order cat scans instead of MRIs because so many patients have metal in them that precludes getting the more sensitive medial imaging such as an MRI>

    On autopsies I have seen lightening injuries that followed the path of least resisistance ( the metal implant) and spared the other tissues. Can't figure that one out. ( this happened to a young patient with idiopathic cardiomyopathy who was found dead and GBI ( georgia bureau of investigations) found that lightening had travelled along his pacing wires and fried his heart without doing much damage to his chest wall!
  15. Apr 27, 2007 #14
    I do believe that most people and corpses that you see have health problems and metallic parts inside. But there are other people with better health that you don't see.
    Did the metal parts vaporized?
  16. Apr 27, 2007 #15


    User Avatar
    Science Advisor

    The particular patient I just mentioned his plastic coating around the wires melted and the distal pacing wires were gone ( vaporized) and the battery melted. the poor fellow ( he was only 49)
  17. Apr 27, 2007 #16
    At this level of currents nothing could have saved him, even he had no metallic parts.
  18. May 2, 2007 #17
  19. May 2, 2007 #18


    User Avatar
    Gold Member

    I always understood that one can withstand VERY high voltages (a typical harmless shock can be thousands of volts), but even a small amount of current can kill (a few hundred milliamps passing through your body that crosses the heart will kill you).

    Has this changed?
  20. May 2, 2007 #19
    You can receive a discharge from an object a high voltage. I have myself received sparks about 40 cm long from a small electrostatic generator. This means voltages of about 300 kV. But I have no withstood 300 kV. Most of the voltage drop was in the arc. In my body the drop must have been about 100 volts (maybe 200). I repeated the experience several times. If there have been 300 kV across my body, well, I would not be here. The current, of course, was very small. maybe 1 or 2 mA.
  21. May 2, 2007 #20


    User Avatar
    Science Advisor

    In the lower voltages, ACis still more dangerous than DC on the heart and requires much less voltage to do so.

    DC current has the greater tendency to induce muscular tetanus quite readily. AC alternately reverses direction of motion and provides brief moments of opportunity for an muscle group to relax between alternations. Becoming frozen is thus more dangerous with DC current.

    However, AC's alternating nature has a greater tendency to throw the heart's pacemaker neurons into a condition of fibrillation, whereas DC tends to just make the heart stand still. Once the shock current is halted our "frozen" hearts have a better chance of resuming a normal beating pattern. That is why our defibrillators use direct current to shock people out of fibrillation By just inducing the heart to stop versus fibrillation the heart's chances of resuming normal sinus rythem or normal rythem is much better.

    ( side note, our hand flexor muscles are stronger than extensor muscles which is why when our hands get tetny from DC current we tend to grip harder rather than extend our fingers and let go!)
    Last edited: May 2, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook