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My son's science teacher had 15 fifth grade students stand in a semicircle holding hands. The student at each end held an electrode connected to some kind of variable contoller. The teacher increased the voltage until they could "feel" the electricity. He also had one student in the middle let go to demonstrate a "complete circuit" Is this dangerous?

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closed as primarily opinion-based by Matt Young, Leon Heller, Nick Alexeev, Dave Tweed, Anindo Ghosh Jan 12 at 3:40

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

    
Search for "van der graaf safety" to get the real story. This is actually very low risk. –  Scott Seidman Jan 11 at 20:42
    
I hope that the demonstration was done with a Van de Graaff generator but that's not at all obvious from the question. If the teacher used a variac or something similar he'd better have a damn good lawyer. –  Joe Hass Jan 11 at 21:29
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Why would anything else be assumed? These sorts of demos have been done for at least three generations in classrooms. –  Scott Seidman Jan 12 at 1:02
    
What concerns me is the phrase "some kind of variable controller". My recollection of VDG machines is that they typically are just on/off devices. "Science" teachers have done crazier things...see, for example, John Freshwater. –  Joe Hass Jan 12 at 1:06

3 Answers 3

From memory, I believe a current value of 65mA passing through the human heart has the potential to kill. According to Hyperphysics, 1 mA is the Threshold of feeling, 10-20mA causes a "can't let go response" where the muscles contract, and 100-300 mA can cause Ventricular fibrillation, and can be fatal. The linked pdf stats that 6mA for a woman is enought to cause a painful shock. For the sake of safety, let's say we do not want to go beyond 3 mA.

A typical resistance between two body parts is 1500Ohms Hyperphysics claims 100kOhms for the human body. Ohm's law dictates that if I held a potential difference of 300V across my hands, that the current could be dangerous.

The class was standing in series, so it would take 15*300V = 4500V to kill pose risk to the students. I would hope the teacher wasn't using a source anywhere near that value, which is not exactly the most accessible either.

From Wikipedia, it is said that 5mA is enough to feel, as opposed to 1mA.

In short, I'm sure the experiment was safe, but extreme caution should always be exercised in regards to electricity. Also the above values that I explained should not be interpreted to find absolutely safe zones when dealing with current.

[1] http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html

[2] http://www.hubbellpowersystems.com/literature/encyclopedia-grounding/pdfs/07-0801-02.pdf

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Both the safe current limit and typical resistance values are not correct. Safe current is about an order of magnitude lower, or microamps for microshock, and resistance may be much higher. –  Scott Seidman Jan 11 at 20:36
    
I'm not surprised my values were off. Luckily others will see your comment and realize the importance of researching the topic and approaching the situation with caution. I'll make an edit –  Nick Williams Jan 11 at 20:40
    
Much better. You should never exceed let go. –  Scott Seidman Jan 11 at 20:53
    
Assuming that the situation is safe if 15 students stand in series is incredibly dangerous. What if student number 2 leans against the plumbing on the lab bench? This kind of experiment should never be done. –  Joe Hass Jan 11 at 21:32
    
Then some of the students might not feel the shock. –  Scott Seidman Jan 12 at 1:04

These sorts of experiments are safe so long as the teacher takes care to follow standard guidance such as Van der Graaf Generator Safety which goes into some detail.

The electrostatic energy stored by the sphere should not exceed 0.5 J.

...

An enquiry to CLEAPSS has revealed no recorded incident of direct injury caused by shocks from the correct use of school Van de Graaff generators.

and so on.

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The plural of anecdote isn't data. Just because 100 of us answering the questions have done similar experiments doesn't mean that it's safe or recommended. I've personally received shocks in the hundreds of thousands of volts range (at very low current) and have been set on my ass from much lower voltage but higher current shocks and survived. It doesn't mean that what I survived was therefore safe or recommended. It means I was lucky, if anything. Now that I'm older and wiser I don't go out of my way to experience shocks, even if I'm sure they're safe. (The only thing I do still do is briefly tap a 9V battery to my tongue to see if it's got any juice left in it, but I've found that 9V batteries will "bubble out" on the bottom when they're dead and tend to trust that instead.)

In fact, I'd be very concerned about having students stand in a circle conducting enough current that they feel a (mild) shock. You never know if one of those students has an undiagnosed heart condition. Sometimes the damage caused by electric shock can take several hours to manifest, and it's doubtful that the people taking the victim to the hospital would connect the dots in such a case.

If the teacher wants to demonstrate the concept of a circuit he can do so with a moderately high frequency, low voltage AC signal and a sensitive detector which lights an LED. It'd be way safer than anything that you might actually feel and get the point across just as easily.

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I agree, it wouldn't matter how sure I was. I wouldn't feel comfortable having students line up in a circle like that! Perhaps the teacher was simply more confident than I. –  Nick Williams Jan 11 at 20:37

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