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Heating a steel nail

  1. Apr 24, 2010 #1
    Hello everyone,

    I think this is very simple. I have very little knowledge about electricity and would like to understand a bit more about a simple experiment I was recently shown:

    Basically a steel nail was connected to an electrical circuit at both ends. The two electrical contacts to which it was connected were coming out of a wooden box (there must be a transformer or something like that inside right?). When the current started flowing, the nail got red hot really fast and then melted and broke. After that, a man touched both of the terminals and said he didn't feel a thing and it was safe to touch it even with the current flowing, yet it would melt a steel nail in ten seconds or so. If I understood correctly, the circuit was connected to a socket in the wall ( 220 V ).

    I have spent several hours googling for information on metal heating using electricity, but couldn't find anything I would be interested in. Mostly stuff about induction heating etc. I understand that to heat metal you need low voltage and high current? but isn't current deadly?

    I would appreciate any information. You don't have to try and explain everything carefully, just a rough explanation would be more than enough, I can use the internet and books to figure things out.

    Thanks in advance :D.
     
  2. jcsd
  3. Apr 24, 2010 #2

    vk6kro

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    The box contained a low voltage transformer which could deliver a lot of current at a low voltage.

    Because it delivered a low voltage, the man could not feel it. We cannot normally feel voltages that are less than 30 volts or so.

    The nail had some small resistance and there was a large current flowing, so there was enough heat generated to heat the nail to red heat and eventually melt it.

    The power generated in any resistance is given in watts and is:
    (current in amps) times (current in amps) times the (resistance in ohms)
    This applies to nails too.
     
  4. Apr 25, 2010 #3
    There's a common expression that the voltage doesn't kill you, the current does. That can be pretty confusing and it gives many people wacked out ideas.

    The bottom line line is that yes, there must be a certain level of current traveling through a vital area (usually the heart) to cause death. But, your body has electrical resistance. So, there is a minimum voltage required to generate the lethal current.

    Generally, 48 volts is considered the dividing line between safe and unsafe becaus of the Safety Extra Low Voltage, or SELV directive.

    Personally, I'm pretty sure that 48 volts is enough to sieze a heart if applied in some unusual fashion, but the likelihood of this happening are quite low.

    As for stun machines, such as Tasers, they develop extremely high voltages. Then again, sometimes you experience fairly high voltages walking across new carpet.

    With the carpet scenario, the current discharged through your body is very high, but very very short in duration. Thus, your nerves know that something attacked them, but it did so for such a very short duration that the effect was averaged out and had little overall effect.

    Tasers and other stun weapons are designed to build up high voltages, but they don't substain that voltage as current begins to flow. So once again, you have a high peak current that drops down quickly.

    The difference between the stun weapon and the carpet is that the stun weapon will attempt to maintain current flow after the initial discharge. Thus, the voltage doesn't drop to zero, but stays high enough to get the current delivered. The current is then cut off and the cycle repeated.
     
  5. Apr 25, 2010 #4
    Thanks a lot! This explains everything I wanted to know and more :).
     
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