Why free wire is'nt dangreous?

[panda1234]

Hi,
I have a fundamental question:
we know if we have a wire in air and free it can have any voltage from earth!for example sometime can be 0.1v and sometime can be 200000v but why we touch a wire in air don't worry about that?
tnx

 

[albbg]

There is no voltage between a free wire and earth. Energy is required to generate a voltage. It cannot be created from nothing.

 

[Nik_2213]

"There is no voltage between a free wire and earth."

Sorry, that may be wrong.

Check out 'Atmospheric Electricity', albbg, which can run at 100 V/m on a fine day due atmospheric processes.

Fortunately, a 'free wire' with many hundreds of Volts potential will have very little capacitance, so contain very, very little available energy.

 

[betwixt]

The real answer is the voltage is irrrelevant, it is the current flowing from it that makes it dangerous.
Current cannot flow unless there is a path for it to follow, if you are insulated (no path exists) you simply adopt the same voltage as the wire. The path can be resistive or capacitive. If the voltage is DC, the danger comes fom the current flowing in to the capacitance to charge it and how long it flows for depends on the amount of capacitance and the resistance of your body. If the voltage is AC, the same resistive flow can exist but the reactance of the capacitance comes in to play. The amount of reactance (resistance to AC) depends on the amount of capacitance and the frequency of the alternating voltage.

Brian.

 

[treez]

yes, you can see birds perched on the 33KV power lines and not getting shocked. I actually wonder if when they alight on the 33kv wire, they probably do get a "charging shock" as their bodies take in the 33KV potential.

 

[vGoodtimes]

This is more of an electrostatics question. to actually get the high voltage difference you would need an actual electrical field between the wire and etc. This means moving electrons to one side or the other.

In the dry air of winter, many metal elements (wire for the analogy) do develop this electron difference and result in small shocks.

 

[tggzzz]

There is no voltage between a free wire and earth. Energy is required to generate a voltage. It cannot be created from nothing.

In practical situations there are always energy sources.

For one simple example, I suggest you do some searching for electrostatic charges on helicopters and airplanes, and how they discharge. Consider particularly operation where flamable gasses can be present, people might be in the discharge path, and tanker aircraft refuelling opertions.

Understanding those points, you will understand how problems arise in more, ahem, down to earth situations - e.g. walking across a plastic carpet!

 

[albbg]

Yes, I didn't consider the atmospheric electricity. I'm not an expert but I think you need a very long wire, quite perfectly insulated from earth, to generate a remakable voltage.

tggzzz: your example is completely different. The electrostatic charge is generated by the rotation of the rotor of the helicopter or the motion of a plane. They behave like a Van der Graaf generator. The engine of the helicopter or of the plane provides the required energy. Electrostatic energy can be also generated on a stationary object by the wind. In this last case the energy is provided by the wind. But is not so easy to charge a pure metallic wire by the wind.

 

[jiripolivka]

Any "wire" or a conductor located off Earth, low or high, can get charged with respect of Earth. There are electrostatic charges due to moving ionized air, UV radiation,solar wind, moving dielectrics (air or other objects).

Natural electrostatic charges can be detected by a wire or rod antenna connected to a high-impedance (>100 MOhms) operational amplifier. Google "atmospheric electricity detector".

Small conductive objects like that can only hold a small charge, sothey are not dangerous Wire antennas, however, can generate a considerable charge if a lightning occurs in the vicinity. One can be killed from such event, so stay careful. Connect your wire to ground and do not touch it.
Even communication and power cable workers stop working when a storm is close. By induction,, the cables can generate killing discharges. In mine-blasting, wires laid to discharge explosives must also be treated with care if a storm gets close, same reasons.

 

[Tunelabguy]

"There is no voltage between a free wire and earth."

Sorry, that may be wrong.

Check out 'Atmospheric Electricity', albbg, which can run at 100 V/m on a fine day due atmospheric processes.

Fortunately, a 'free wire' with many hundreds of Volts potential will have very little capacitance, so contain very, very little available energy.

That is true. Years ago I saw an article in a radio controlled model airplane magazine that described a wing-leveler auto pilot based on this 100v/m electric field. The model airplane had electrostatic pickups at each wingtip. By comparing the relative voltages of these two pickups with a very high impedance amplifier, the on-board control circuit could determine if one wing tip was a few inches higher than the other, and send a correction to the ailerons to bring the wings back to level. As I recall, it only worked in very nice weather when the electrostatic field was nice and flat.

 

[Audioguru]

New radio controlled model airplanes for beginners have a gyroscope/accellerometer IC that makes the airplane fly straight and level (but slightly climbing) when an "oops" button is pushed.
Most of my RC airplanes use the IC to smooth turbulence.