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Confused about some details of potential difference

  1. May 23, 2015 #1
    I understand how to do calculations involving potential difference but I've never been able to find any information that helps me understand what is physically happening to cause it.

    My assumption is that for a PD to occur, there must be a build up of charge to cause the difference in potential. But if the charge is physically there to cause the PD, what stops is flowing? If a 220V wall socket can deliver enough current to be fatal, how can a 1,000,000V stun gun only cause a shock? Assuming the internal resistance of the individual experiencing the shock is the same in both situations.

    Am I right in asking whether the stun gun does have such a high PD because of a build up of charge it posses? How does this result in a low enough current flowing to make sure its not fatal? Is it due to the way the charge is delivered, possibly its duration of discharge? Why is it even necessary to have a device possess such a large PD, what benefit would it have over a 100,000V version?

    Now that I've typed all this, it sounds really stupid. I can't think of a better way to explain my uncertainty though.
     
  2. jcsd
  3. May 23, 2015 #2
    Hi smulc,

    I think that is actually that is a really good question.

    First off, a wall socket can supply 220v with several amps of current since it comes from a large generator.
    The stun gun current is much much less ( possibly in the microamps ).
    So the two energies in the "shock" are much different.
    So that is why you will hear an expression that it is not the voltage that kills, but the amps.

    The stun gun, not sure exactly how they work, but it could be similar to how a high voltage can be made in your car for the spark plugs from a 12 volt battery.
    Simply, that is just the energy stored in the electromagnetic field surrounding a coil of wire to which the 12v is connected. When the 12 v is disconnected, another coil will have a current induced in it by the collapsing electromagnetic field. The second coil has many windings, so by transformer theory, and conservation of energy, the voltage will be higher and the current much less.

    So how do we make a PD. By using charges and their separation. If one has 2 electrons far enough apart, they do not exert much force on one another. If the distance between them is decreases by a distance d, one has to do work to move them together. The closer they get to one another the harder it becomes, or the greater the force one has to exert to move the distance d. The same thing for 2 opposite charges ( electron and proton ), but here one has to use a greater force to separate the particles when they are closer together.

    The work is measured as joules/ coulomb ( J/C ) or as V, volts. The more amount of charge separated, the more work is involved. The ratio V, though, stays the same going from point A to point B. V, is the amount of work,, one can recover per coulomb from the separated charges.

    Hopefully that answers at least part of your inquiry.
     
  4. May 23, 2015 #3

    sophiecentaur

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    Remember the definition of Potential Difference (Voltage). It is Joules per Coulomb. The Energy from a stun gun is arranged to be non-fatal but the voltage is high in order to get over the resistance of clothing, dry skin etc.. If you charge up a Capacitor to a very high voltage, the charge will be small and the energy can be arranged to be as low as you like by choosing a low enough Capacitance.
    Getting to grips with Potential Difference (and all the rest of EE, aamof) can really only be achieved by applying it. There are a lot of apparent paradoxes which you can resolve by calculations. Doing the sums for numerical examples just can't be beat as a way of learning about EE. Alternatively, people reckon to get a lot out of using simulation software. There is a lot of free stuff about.
    Arm waving about the subject can be pretty fruitless, I'm afraid.
     
  5. May 23, 2015 #4

    CWatters

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    In addition to the resistance of the individual there may also be resistance (or something equivalent) in the source that limits the current.

    Consider a 6V lead acid motorbike battery and a 9V PP9 Alkaline battery. If you try and "suck" 10 amps out of both the 6V lead acid motorbike battery might manage to maintain 6V where as the voltage delivered by the 9V PP9 might fall to just a few volts if that. The 9V motorbike battery is nearer to being an Ideal Voltage Source than the PP9.
     
  6. May 25, 2015 #5
    Now we're getting to the point. Or two points if talking about a Taser or stun-gun. (Sorry ;-) ) By the way, they're different devices.

    Current limiting resistor? Some of the stun guns I've seen demonstrated are downright vicious. i.e., if the points are touching the flesh, the voltage is sustained to such an extent that the skin burns - badly. A reasonable self-protection device will have a current limiting circuit of some kind so the initial voltage drops in a fraction of as second. (or indeed, never rises to the claimed unloaded PD) Remember, the PD is only across the skin and some flesh between the points. Having said that, an aircraft engineer wrote up how he felt after trying one on his thigh. I resolved at that moment never to brave out such a test.

    Old black and white TVs with Line fly-back EHT generation would give a nasty shock (the tube helped as it was also a capacitor but not very large.) On colour TVs the voltages were higher and more powerful in the first place, and in addition to this, the capacitance of the tube is huge and would help sustain that voltage as it made the luckless techie leap about. Some died, but I don't have any references anymore.

    At this point, we could thread drift off into the way the voltages disperse and scatter around the heart, but it really is a different subject. Suffice it to say, a lot depends on the two (or more) points of the PD. In the UK and the US domestic voltage supply Neutral is tied fairly closely to Earth/Ground. Stick your finger on the L and the voltage will try to reach any point the victim is touching on its way to ground. I feel I have to mention this. I nearly lost my teenage daughter because a smoothing iron was grounded/earthed. The springy wire holder had rubbed through and she touched that and the grounded iron. Right across her chest. It really hurt her but mostly the muscle contraction which saved her life.

    Sorry, but another drift. GFIs are perhaps one of the best inventions since the wheel. In the UK we have whole-house protection and it saves people and buildings. Typically a flow of 35mA from live to ground will cut the power. However, touch L and N and it won't help one jot if there's no current flow to ground.

    A meaningful anecdote: one of my lecturers in a long-gone TV manufacturer in the UK was working on a PYE TV which had an enormous transformer with an EHT winding. I recall c 9,000 volts. He told me how he woke up in hospital and was later taken to his mini-lab. His rubber heels had made marks across the ceiling. So, this was a classic example of getting zapped by a sustained voltage. It was thought the very contractions that had hurled him across the room had saved his life, because in the old days several techies have been killed by CRT EHT.
     
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