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Author Topic: Does an electric cable store electricity?  (Read 6823 times)

Mike Huggins

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Does an electric cable store electricity?
« on: 08/09/2011 17:30:03 »
Mike Huggins asked the Naked Scientists:
   Greetings. 
 
Love listening to you on 702. 
 
My question. 
 
I attach a hose pipe to a tap and a sprinkler to the other end.  When I am finished and unplug the hose, it is full of water, which sometimes wets me. 
 
I attach an extension cable to a power outlet and a drill to the other end.  When I am finished and unplug the cable, it is NOT full of electricity and NEVER bites me. 
 
 
Why?
What do you think?
« Last Edit: 08/09/2011 17:30:03 by _system »


 

Offline JP

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Does an electric cable store electricity?
« Reply #1 on: 08/09/2011 18:29:56 »
Hi Mike,

Good question.  What happens when a hose wets you is that gravity is pulling on the hose and water.  But the hose itself is held fixed, but because it's a liquid, the water can move.  It's pulled out of the hose by gravity and wets you.

Electrons in a wire aren't free to move separately from the rest of the wire just due to gravity, so they can't spill out of a wire to get you.  They need an electrical force to push them.  The outlets in your house provide this force.  When you unplug the wire, the force is removed, so the electrons stay put.
 

Offline Geezer

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Does an electric cable store electricity?
« Reply #2 on: 08/09/2011 18:39:53 »
Hi Mike,

Good question.  What happens when a hose wets you is that gravity is pulling on the hose and water.  But the hose itself is held fixed, but because it's a liquid, the water can move.  It's pulled out of the hose by gravity and wets you.

Electrons in a wire aren't free to move separately from the rest of the wire just due to gravity, so they can't spill out of a wire to get you.  They need an electrical force to push them.  The outlets in your house provide this force.  When you unplug the wire, the force is removed, so the electrons stay put.

Well, not exactly  :D

The cable can actually store some energy because it has capacitance. Fortunately, the amount of energy stored is usually rather small, and the inductance of the cable limits the rate at which the energy can be delivered to you, so you don't feel a thing.
 

Offline syhprum

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Does an electric cable store electricity?
« Reply #3 on: 08/09/2011 21:17:27 »
I should think the 200KV DC cables that run under the English channel to keep French industry running store quite a fair amount.
Anyone who knows what the capacitance of HV power cables per meter could easily calculate how much.
« Last Edit: 09/09/2011 17:38:36 by syhprum »
 

Offline techmind

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Does an electric cable store electricity?
« Reply #4 on: 08/09/2011 22:57:41 »
I attach a hose pipe to a tap and a sprinkler to the other end.  When I am finished and unplug the hose, it is full of water, which sometimes wets me. 
 
I attach an extension cable to a power outlet and a drill to the other end.  When I am finished and unplug the cable, it is NOT full of electricity and NEVER bites me. 

Firstly, it's important to remember that in any water-analogy, the water represents electrons... which is not quite the same as what we call "electricity". Electric current is the flow of electrons, and electric potential (or voltage) is essentially the pressure of electrons. So when you disconnect the cable, you remove the 'pressure' which means you've got no voltage and you don't get a shock.
Also the electrons don't dribble out of a cable when you disconnect it as there's a preferred equilibrium density of electrons (to match the proton chanrge in the atoms making up the cable). In this way it's a bit more like a fine capillary-tube of water which wouldn't necessarily empty when you disconnect it from the tap.

If you disconnected a very well-insulated cable from a very high-voltage source (tens or hundreds of thousands of volts or more) then you might get a brief shock if you subsequently touched it. That would be because there was a kind of residual electron-pressure inside the cable (it was charged) - this is essentially static electricity. It's a little bit like having a compressed-air line - if you connect a pipe to a compressed air cylinder and use the air for something, but then seal both ends of the tube before isolating it from the cylinder then the tube still contains some compressed air which will give a "psssht" (and maybe make you jump) when you free the end of the pipe! *

*Note this is a somewhat crude analogy - the ability of conductors to store charge depends on capacitance, which relates to the geometry of the object, what other conductors it is near... (and isn't very closely related to the air-line analogy). Large surface areas with very thin (millimetres to micrometres) insulating gaps store huge amounts of charge compared to your unplugged domestic extension-lead!
« Last Edit: 08/09/2011 23:05:00 by techmind »
 

Offline syhprum

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Does an electric cable store electricity?
« Reply #5 on: 09/09/2011 17:37:49 »
Even if the cable had a high capacitance and was connected to a high voltage AC supply the the amount of energy stored in the cable would depend on the phase of the the ac cycle that could vary from above the nominal supply voltage to zero
 

Offline Geezer

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Does an electric cable store electricity?
« Reply #6 on: 10/09/2011 07:05:22 »
Even if the cable had a high capacitance and was connected to a high voltage AC supply the the amount of energy stored in the cable would depend on the phase of the the ac cycle that could vary from above the nominal supply voltage to zero

I suppose if there was a hefty inductive load attached to the cable it might even be possible to charge the cable to a voltage that was far greater than the peak line voltage, but I suspect it would be necessary to disconnect both ends of the cable almost simultaneously to make that happen, so it's probably a bit unlikely.
 

Offline simplified

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Does an electric cable store electricity?
« Reply #7 on: 11/09/2011 13:38:45 »
Even if the cable had a high capacitance and was connected to a high voltage AC supply the the amount of energy stored in the cable would depend on the phase of the the ac cycle that could vary from above the nominal supply voltage to zero

I suppose if there was a hefty inductive load attached to the cable it might even be possible to charge the cable to a voltage that was far greater than the peak line voltage, but I suspect it would be necessary to disconnect both ends of the cable almost simultaneously to make that happen, so it's probably a bit unlikely.
One cable should be charged by positive electric charge,second cable should be charged by negative electric charge [:0].Mike Huggins does not make such work,therefore he does not recieve any shock.Probability can not work there.
 

Offline Nitrobob69

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Re: Does an electric cable store electricity?
« Reply #8 on: 25/05/2014 03:20:38 »
I was wondering the same thing, but I was wanting to know if an electrical push button switch was controlling a 12 volt device and say there was 50 ft of 12 guage wire also attached to the switch but not to any thing else at the other end, would the wire store electricity while the button is pressed and then back feed into the device to keep it on for a few tenths of a second?
 

Online Bored chemist

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Re: Does an electric cable store electricity?
« Reply #9 on: 25/05/2014 10:40:11 »
The cable would carry a charge and it would take some time for that charge to leave the cable.
To a rough approximation the time taken would be about the same time that it would take light to travel the length of the cable, so with 50 feet of wire you would have a stored charge for about 1 twenty millionth of a second.
A tenth of a second would take a cable roughly long enough to go round the world.
 

Offline evan_au

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Re: Does an electric cable store electricity?
« Reply #10 on: 25/05/2014 22:03:37 »
The energy stored in a capacitor is E=CV2/2.
  • Energy E in Joules
  • C is capacitance in Farads
  • V is Volts
Short cables have a low capacitance, so the energy is normally too low to be noticeable.

Some older devices had a fairly significant capacitance across the mains voltage as a noise filter.
  • If you unplug the AC cable before turning off the device, the energy in this filter can keep low-power devices running for a few seconds (LEDs slowly fade out over a few seconds).
  • If you turn off the device switch then unplug the AC cable, this represents a significant amount of energy in the filter capacitor that could give you a shock. Safety regulations encourage capacitive filters to include a self-discharge circuit, reducing the capacitor voltage to a safe level within 1 second or so, minimising the chance that you will come into contact with a dangerous voltage in this short time.
The "socket" end of mains cable is well-insulated, to protect users when it is "live"; the main risk comes from touching the "plug" end, which has exposed conductors when unplugged.
 

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Re: Does an electric cable store electricity?
« Reply #10 on: 25/05/2014 22:03:37 »

 

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