Why is it dangerous for a person to stand near a very highly charged object?
Basically, a highly charged object has (effectively) a lot of bits of charge on it with the same sign, and since charges of the same sign repel, most of them would rather be someplace else, and if your body happens to present a good way to leave the charged object, the charges will travel through you, and give you a shock. Normally, you might expect this to happen if you touch something dangerous - not if you're just near it. However, if there is enough charge, it is possible for the charges to create an arc, and jump to your body through the air.
For a description of electric arcing, see: https://www.physlink.com/education/askexperts/ae402.cfm
Gregory Ogin, B.S., Student Researcher, Los Alamos National Laboratory
Providing that you are not charged to the same magnitude as the "object," the difference in your charge magnitude and that of the object causes a potential, also known as voltage. That flow of voltage, if large enough could cause a current between the object, yourself then back through the ground to the object. If the current was large enough, serious bodily harm could be the effect of standing too close to a super charged object.
In real life, an example of the precaution taken due to differences in potential of two objects can be seen in the repairing of high tension, high voltage lines. The courageous men/women who decide that they want to take on this risky job (which usually requires them to sit on the edge of a platform, connected to a helicopter) slowly "charge" themselves up to the same potential of the wires they are working on, which sometimes can be in upwards of 150,000 volts (and more), so that no potential is created between them and the wire. Because their initial charging is done very slowly, and their separation from any grounds (which could complete a circuit and kill them), being charged to 150,000 volts isn't entirely dangerous.
Jon Yimin, Junior EE student, Pennsylvania State University
Like most other things in nature, electric charge (electrons) tends to prefer the lowest possible energy state. Thus they will tend to move from a place with high electric potential to a location of lower electric potential.
An analogy would be the gravitational potential. Objects at a higher location from the surface of the Earth have a higher gravitational potential while the surface of the Earth can be viewed to have a gravitational potential of 0. Therefore, objects in the air will fall to the ground unless there is something supporting them.
The electric potential of the Earth (or "ground") is defined to be 0. Therefore for a highly charged object, the electric charge will attempt to move into the ground. So if you were to connect, say, a wire from the object to the ground, you would send all of the charge into the ground through the wire (thus discharging, or "grounding" it).
Unlike a mass in the air, electric charge requires some sort of conductor to propagate through. Let's say your charged object was not connected directly to the ground, what would happen then, since air is not a good conductor? Well, the charge still wants to get to ground, so if the potential difference is great enough (for example, if there was enough charge on the object), the air itself surrounding the object would be ionized, and the electric current would flow to the ground through the ionized air (this is the principle behind lightning).
However, it just so happens that a human being standing on the ground near the object serves as a perfectly good conductor as well. If the unfortunate individual stood close enough to an object with a high enough charge, it is quite possible that the electric charge would ionize the air between the human and the object allowing the current to flow to the ground, right through the (now very unhappy) human. As the human central nervous system relies on electrical impulses to transmit signals, a large external current can be quite catastrophic in interfering with normal body functions.
For more information on electric potential see: http://en.wikipedia.org/wiki/Electric_potential
For a somewhat morbid look at the electrical resistance of the human body and the effects of electrocution: http://www.allaboutcircuits.com/vol_1/chpt_3/4.html http://en.wikipedia.org/wiki/Electric_shock
Loren Chang, B.S., Aerospace eng. grad student, U. Colorado, Boulder
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