How does it complete the loop?

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I know that current can flow in loop only. I mean circuit must be complete for current to flow.
So my question is:
If someone is hanging in the air holding power transmission line, he doesn't get shock. Because circuit is incomplete.
But if he touches ground in same position then he receives shock.

As per my assumption, this should not happen as a loop is not completed. I mean how can current come back if it flows in ground?

Where am i going wrong, please explain.
 

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  • #2
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I know that current can flow in loop only. I mean circuit must be complete for current to flow.
So my question is:
If someone is hanging in the air holding power transmission line, he doesn't get shock. Because circuit is incomplete.
But if he touches ground in same position then he receives shock.
So like all good scientists when faced with practical evidence that your theory is incorrect you question it. Good stuff.

What is incorrect is the assumption that there must be a loop for current to flow. Or, if you like, the assumption that a circuit must be a loop.

Current will flow wherever there is a path and a potential difference along that path.

To harness electricity we generally arrange for loops, as such a configuration makes things predictable and controllable.

In general current in open (ie non looped) paths only flow for a short time and are often to do with redistribution of charge.

Here are some examples.

Your man hanging from the power wire
Lightning
When a conductor is placed in an electric field a current will flow for a short time to establish an internal counter potential.
A Van der Graff generator.
An electron beam from a heated metal. ( This may or may not be part of a loop)
 
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  • #3
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Thanks for your answer!!

But now i have another question :)....that's what always happens with me!!!!

If current can flow in open path then is it possible that a bulb will glow when the wire coming out from +ve terminal is connected to one terminal of the bulb and other wire coming out from another terminal of bulb is inserted into ground or lets says connected to any other metal?

Is it possible? if not then why? I always get confused :confused:....Please help
 
  • #4
davenn
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I know that current can flow in loop only. I mean circuit must be complete for current to flow.
So my question is:
If someone is hanging in the air holding power transmission line, he doesn't get shock. Because circuit is incomplete.
But if he touches ground in same position then he receives shock.

As per my assumption, this should not happen as a loop is not completed. I mean how can current come back if it flows in ground?

Where am i going wrong, please explain.
Studiot didnt completely answer you question...

whilst hanging onto the power line then touching the ground... you get electric shock because you are completing the loop back to the source of the power ... the power station.

many power line systems dont carry the neutral line on the poles ... often there are just the 3 phase lines or for the huge transmission lines there may be 4 or 6 phase carriers and the neutral is commoned with the earth at the power station, the intermediate substations and finally at your home or place of work.

cheers
Dave
 
  • #5
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@Davenn

Thanks a lot for your clear answer :)
 
  • #6
davenn
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Thanks for your answer!!

But now i have another question :)....that's what always happens with me!!!!

If current can flow in open path then is it possible that a bulb will glow when the wire coming out from +ve terminal is connected to one terminal of the bulb and other wire coming out from another terminal of bulb is inserted into ground or lets says connected to any other metal?
Is it possible? if not then why? I always get confused :confused:....Please help
if you are referring to your 110/240VAC (depending on your country) power in your home

yes it should light up as long as you provide a good connection to ground/earth ...

if you are referring to a battery, no it wont, unless of course you provide a really good ground/earth connection for the negative terminal of the battery.

Dave
 
  • #7
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@Davenn

As you said my experiment will work in case of home power supply. So does it mean current flows all the way back to neutral terminal of source (power station) through ground?

But in case of car battery even if i provide earth connection the negative terminal will not have any contact. hence no path for current to return.
Then what happen in this case?
How i imagine this is power source is like a water tank and water should flow through the pipe ( wires in our case) then flow through some instrument (like bulb in our case) and come back to water tank. is it correct? Do you know any website where i can find animation or video explaining the same. Because i really find it difficult to imagine and relate it to the electricity laws.

I really appreciate your efforts to answers my question.
Thanks.
 
  • #8
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Sorry to disappoint you ,Dave, but connection to earth does not 'complete a loop'.

Touching a live mains supply conductor, whilst standing on a block of granite many miles thick between you and the power station, will fry you just as effectively as if you held onto an earth spike into wet ground.

The earth acts as an (effectively infinite) current sink, just as with the lightning.
 
  • #9
5,439
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Thanks for your answer!!

But now i have another question :)....that's what always happens with me!!!!

If current can flow in open path then is it possible that a bulb will glow when the wire coming out from +ve terminal is connected to one terminal of the bulb and other wire coming out from another terminal of bulb is inserted into ground or lets says connected to any other metal?

Is it possible? if not then why? I always get confused
Yes it is definitely possible.

This is how mains testing screwdrivers work. The ones with a little bulb in the handle. Have you seen these?
However you have to have enough voltage to make them work, they will not work at normal battery voltages.
 
  • #10
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Sorry to disappoint you ,Dave, but connection to earth does not 'complete a loop'.

Touching a live mains supply conductor, whilst standing on a block of granite many miles thick between you and the power station, will fry you just as effectively as if you held onto an earth spike into wet ground.

The earth acts as an (effectively infinite) current sink, just as with the lightning.
But in this case, with DC supply all electro in source will flow in earth and the source will be depleted of electrons. What exactly happens? (In AC and DC also) please explain.
 
  • #11
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Whenever some charge moves or is moved from point A to point B that is, by definition a current.

Now you will have to help me here because I do not know your level of knowledge.

Have you understood all the examples in my posts 2 & 9?

If so I can use them to explain further.
 
  • #12
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Whenever some charge moves or is moved from point A to point B that is, by definition a current.

Now you will have to help me here because I do not know your level of knowledge.

Have you understood all the examples in my posts 2 & 9?

If so I can use them to explain further.
I actually am Instrumentation Engg. I have studied all these theories in shcool & colleges. But still i am confused.

How i imagine this is power source is like a water tank and water should flow through the pipe (wires in our case) then flow through some instrument (like bulb in our case) and come back to water tank. is it correct?
Do you know any website where i can find animation or video explaining the same. Because i really find it difficult to imagine and relate it to the electricity laws.
 
  • #13
5,439
9
How i imagine this is power source is like a water tank and water should flow through the pipe (wires in our case) then flow through some instrument (like bulb in our case) and come back to water tank. is it correct?
Yes this is a workable model for some purposes.

But it has it's limitations.
So I would not recommend going too far with it.

For instance if you disconnected the return pipe and dangled it into the river, your tank water would just flow away until the tank was empty.

That is exactly the same situation as lightning from a cloud to ground.
The lightning carries on striking until the cloud is discharged.

But why would the water flow away?

Because the tank is above the river so there is a head or elevation.
Notice I do not say pressure.
This elevation is directly equivlent to voltage in and electric circuit.
Again you should not take this model too far.
There is one fundamental difference between water flow and electricity.

When water flows it does not build up a 'reverse charge'

When electricity flows, the more charge that moves from A to B, the harder it becomes to move it as the charge build up at B.

This is why we have a return path, to prevent build up at B so we can keep the current flowing.
 
  • #14
Drakkith
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Remember all that there must be a difference in electric potential for current to flow. When you generate power you are causing a large negative potential to build up which causes current to flow down the line. This potential will cause the current to go anywhere there is LESS negative potential than the line as long as the resistance is low enough.

In power lines, the resistance in the air is so high that current cannot flow to the ground, however if you connect yourself to both the resistance in your body is MUCH lower than air.

Also, remember that mains power lines are AC power, not DC. This means that the direction of the current is constantly switching back and forth. The generators providing power to the gird are not batteries where the charge is simply stored. They actually provide the required forces to generate the electric potential in the lines. An electron at a distance of 10 ft down the power line does NOT travel through the entire line and end up going in a big loop. Each electron is pushed one way first, which pushes another electron and ETC all the way down the line. Then it is pushed back the other way. In this way the current "flows" down a line, as it takes a finite time for the "push" to travel.
 
  • #15
Drakkith
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many power line systems dont carry the neutral line on the poles ... often there are just the 3 phase lines or for the huge transmission lines there may be 4 or 6 phase carriers and the neutral is commoned with the earth at the power station, the intermediate substations and finally at your home or place of work.
True, but a ground doesn't complete a circuit. The cable the current is flowing through in the grid is. A ground is there for things like buildup of voltages and shorts and such to keep stray current from damaging equipment in houses, businesses, ETC. If anything happens the current should flow to ground instead of through your house and computer and such. (In general. It's more complicated than that however.)
 
  • #16
AlephZero
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The water flow analogy often gives people another wrong idea, which is that when a current flows, electrons move very fast and travel right round the circuit, because when you press a light switch, the light comes on instantly.

This is completely wrong. With a typical household electricity supply, the electrons only move along the wires at a speeds of few millimeters per second. But the electrostatic force between the electrons is very large compared with other forces like gravity, and if one electron moves a small amount, the change in the forces travels fast round the whole circuit, rather like a mexican wave travelling round a sports stadium.

So where there is a return path "through the earth", what really happens is that some electrons get pushed out of the wires in the circuit into the earth, and a "mexican wave" of force spreads out through the whole of the earth. At the same time, some other electrons are being pulled out of the earth into the wires at the power station.

It is not true that electrons somehow travel at high speed "through the earth", or along the wires, all the way from your house to the power station.
 
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  • #17
Drakkith
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Exactly AlephZero.
 
  • #18
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So where there is a return path "through the earth", what really happens is that some electrons get pushed out of the wires in the circuit into the earth, and a "mexican wave" of force spreads out through the whole of the earth. At the same time, some other electrons are being pulled out of the earth into the wires at the power station.

It is not true that electrons somehow travel at high speed "through the earth", or along the wires, all the way from your house to the power station.
It is making sense now.
But in case of DC supply, since current moves in only one direction, all electrons will flow into the earth and the source will be depleted of electrons, is it true?(In one of my above posts, i had asked that is it possible for bulb to glow when one of its wire is connected to +ve of battery and other wire is inserted in ground. No wire is connected to -ve of battery.)

And in case of AC current the neutral wire is inserted into ground, but since electrons flow back and forth, electrons are pushed in ground and pulled back in a complete cycle, is it correct?

Thanks guys for taking efforts to clear my confusion.
 

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