What is the Science Behind Electric Shock and Grounding?

[RiddlerA]

I don't quite understand this concept, i know that electricity is the motion of electrons through a conductor. what exactly is the potential difference between two points in a conductor? Is it similar to the Bernoulli's principle where the air flows from high pressure to low pressure area?

And also why we experience shock when we are grounded(touching the floor) and not when wearing any insulator(not touching the floor)? Floor is generally an insulator right? Then How it Sucks current from the source through our body to earth?

Thanks in advanc3...

 

[silverdiesel]

Sounds like you understand it pretty well. The analogy between pressure and voltage (potential), is a good one. In fact, they are almost the same thing because pressure is the product of the electrical repulsion between electrons bound within atoms, and potential is the product of electrical repulsion of electrons bound to a metal (a cluster of atoms).

The floor probably is not the best insulator. If it were, then you would not get shocked. In any floor, there are going to be paths for the electrons to reach ground (the earth), and therefore the floor cannot really be described as a true insulator.

 

[Naty1]

That analogy with air pressure is ok to start...
check here for more:
http://en.wikipedia.org/wiki/Electrical_potential

The force on a charge q is F = qE where E is the electric field...the vector field maps out the different scalar potentials...and since W = Fd, if you multiply both sides of the first equation you end up with W = Fd =qEd. This says moving (a distance d) a charge q in an electric field E requires work...or work is done.

An electric potential is a scalar (not a vector) and has the ability to change an electron's PE, for example, to KE by moving it. So in the article it mentions:

Force and potential energy are directly related. As an object moves in the direction that the force accelerates it, its potential energy decreases.

Note that an electric field is a convenient method of modeling electric potential...one that works well in a wide variety of applications so we can set up math models and solve them.
Note that electric potential, like all PE, is a difference using an arbitrary (convenient) reference...like the difference in potential between two battery terminals. So an electric
field can do work on a charged particle analogous to how gravity can do work on any particle with mass (charged or not).

 

[RiddlerA]

Thank you guys for your reply.. i have 1 more doubt.. this may sound dumb but bear with me...

Imagine a conductor with 3 points A→B→C... Now if A is connected to a current source, there will be no current flow unless the circuit is closed right? Does this mean that there will be no electron flow at all in the conductor regardless of the voltage of power source?
Or the electron will flow upto point C and then stops?
Im aware that electron wil flow from -ve to +ve while current flows in opposite direction.
I just need some analogy to be able to visualise the current mechanisms.. I am having hard time with this topic...

 

[DarioC]

RiddlerA,
(this is fun) Imagine a mile long large diameter copper conductor that is connected at one end by a switch to one terminal of a 100 volt battery. The other end is not connected to anything. When you close the switch contact applying 100 volts to one end of the conductor, what happens at the other end? What happens along the length?
Think about that. See, I told you it would be fun.
DC

 

[RiddlerA]

RiddlerA,
(this is fun) Imagine a mile long large diameter copper conductor that is connected at one end by a switch to one terminal of a 100 volt battery. The other end is not connected to anything. When you close the switch contact applying 100 volts to one end of the conductor, what happens at the other end? What happens along the length?
Think about that. See, I told you it would be fun.
DC

Er.. I don't get it.. Could you explain a little bit more?
100 volt is too low for a conductor that large, so there will be no current flow?
Then what happens if the other end(of Conductor) is connected to the other terminal of the battery?

 

[cmb]

Imagine a conductor with 3 points A→B→C... Now if A is connected to a current source, there will be no current flow unless the circuit is closed right? Does this mean that there will be no electron flow at all in the conductor regardless of the voltage of power source?

(This was answered in another thread, but I don't seem to be able to find it.)

ALL conductors have a capacitance. A conductor's potential may be described by the total charge on it and its capacitance. [Circularly, you could equally say the capacitance of a conductor is a function of the potential on it for a given charge, etc..]

Anyhow, your wire has a capacitance and when you connect another conductor to that wire, they will share their charge until their potentials are equal. IF they have the same potential, no charge will flow.

If you were to connect the wire to an isolated battery, the wire would tend towards the potential of the battery terminal you connected it to, but equally the battery's whole potential will lift to that of the wire. Imagine a 500kV distribution line was isolated instantaneously with 500kV (with respect to ground) on it at the moment of isolation. It is acting like a capacitor, it has a potential of 500kV. If you bring a teeny battery up to it, the battery will get pulled up to the 500kV potential.

If you were to now bring a current source that was tied to ground up to your wire charged to 500kV, then which way do you think current would flow, how much charge would flow, and what would the end potentials be? What happens if it were charged to -500kV?

So your question is incomplete, because you've said nothing about either the current source, or the charge on the wire before connection.

Treat everything as a capacitor and this will make sense. Most 'wires' have a capacitance so small it is ignored. But this is the way you need to think about it if either a) you have a very long wire, and/or that runs very close to an earth, and/or b) are discussing high voltages. If neither a nor b, then you can naively say that no current flows.

 

[DarioC]

RA,
Do you understand/know about "free" electrons and the difference between a conductor and an insulator?
DC

 

[RiddlerA]

RA,
Do you understand/know about "free" electrons and the difference between a conductor and an insulator?
DC

Free electrons are electrons that are not bound to the nucleus of the atom and hence can freely move through from one atom to another...
Conductor is a material in which there are considerable amount of free electrons to make current flow possible whereas insulators don't have sufficient free electrons for the current flow... Resistance and Conductance depends on the amount of free electrons present in the material...

Are the above statements correct?
Correct me if i got that wrong...
Thank u...

 

[DarioC]

Appears you have that part nailed.

The simple answer to your original question is that the charge from the source will distribute itself along the wire, however huge/long, until the entire conductor is at the same potential/voltage as the terminal of the source (battery.)

My take on this, and I am pushing my knowledge, is that the removal of electrons in the end of the conductor when it is switched on will create a very concentrated "area" of charge in that area of the conductor.

From my experience with ignition system noise (and other practical RF experience), I believe the area of severe absence of electrons will travel down the conductor to the end and then return back to the terminal. It will travel back and forth, getting more evenly distirbuted with each pass, until the resistance of the conductor absorbs the energy. The "shortage" of electrons will then be spread evenly over the entire length of the wire giving it a positive charge (absence of electrons) equal to the source (battery) voltage/potential.

The are some very interesting things going on as this happens, if you are interested.

It is a little weird, but I have worked with this kind of thing all my life, but never had reason to reflect on it exactly this way. It actually sheds a different light on other phenomena I have seen in my occupation.

DC

 

[ColinW]

What exactly is electricity?

If I knew the answer to that question I'd be practising my Nobel Prize acceptance speech!

 

[sophiecentaur]

Please don't let him think that the electrons are zapping around the circuit. In fact, they move at less than 1mm per second on average. Nothing like water flowing round a pipe.

 

[RiddlerA]

In fact, they move at less than 1mm per second on average. Nothing like water flowing round a pipe.

OMG if this is true, it changes my whole visulaization about the electricity... To this day i thought electrons flow at around 10% of the speed of light... Dont ask me where i got that from.. Its just a misintrepretation from some book...
Thank you...

 

[RiddlerA]

The simple answer to your original question is that the charge from the source will distribute itself along the wire, however huge/long, until the entire conductor is at the same potential/voltage as the terminal of the source (battery.)

My take on this, and I am pushing my knowledge, is that the removal of electrons in the end of the conductor when it is switched on will create a very concentrated "area" of charge in that area of the conductor.

From my experience with ignition system noise (and other practical RF experience), I believe the area of severe absence of electrons will travel down the conductor to the end and then return back to the terminal. It will travel back and forth, getting more evenly distirbuted with each pass, until the resistance of the conductor absorbs the energy. The "shortage" of electrons will then be spread evenly over the entire length of the wire giving it a positive charge (absence of electrons) equal to the source (battery) voltage/potential.

So that means there is no such thing as currrent.. it is actually the absence of electrons right? Current and electrons are like the two sides of a coin?
But i have one more question.. While troubleshooting some hardware problem in my Computer without turning off the power source, i have experienced shock several times(only when grounded i.e touching the floor).. But on wearing shoe i never experienced shock(low voltage levels as inside Computer).. And also one day i touched inside the power socket(230V) when not grounded(not touching the floor), then i got the shock again...
So i figured higher voltages shocks us nevertheless... (correct me if i m wrong)..
But my question is that why low voltages are experienced only when we are grounded?

The are some very interesting things going on as this happens, if you are interested.

It is a little weird, but I have worked with this kind of thing all my life, but never had reason to reflect on it exactly this way. It actually sheds a different light on other phenomena I have seen in my occupation.

DC

What kinda interesting things? yes I am interested...
Thank you..

 

[DarioC]

SC has a good point. My explanation is a bit sloppy, but I was trying to keep it simple.

The process is more like you have a very long open tank of water (like a trough) and you somehow suddenly remove several gallons of water from one end. The water molecules at that end don't go anywhere, but the wave that is formed will. In the end the tank will settle down with an overall lower level of water. I wouldn't push the analogy much further than that.

The interesting part with the long conductor is that the "pulse" that travels back and forth on it can be detected and depending on the characteristics of the "source," the switch, and the conductor, some energy can be radiated as a radio frequency signal that will be heard in a radio receiver as a snap or pop. The "pulse" travels on the conductor roughly at the speed of light, back and forth until it is damped out and all settles down to the connected voltage. This is on a conductor that is not connected to anything at the other end.

Some one can chime in here with how many gazillion available electrons there are for conduction in the cross-section of a typical conductor (wire) so when you pop a few in or out one end it still doesn't amount to a "flow" when some pop out the other end; but the effect of the imbalance still travels along the open conductor or closed circuit at the speed of light.

Holy crap. No spelling errors, not bad for 12:30.

Sack time.
DC

 

[Bobbywhy]

I don't quite understand this concept, i know that electricity is the motion of electrons through a conductor. what exactly is the potential difference between two points in a conductor? Is it similar to the Bernoulli's principle where the air flows from high pressure to low pressure area?

And also why we experience shock when we are grounded(touching the floor) and not when wearing any insulator(not touching the floor)? Floor is generally an insulator right? Then How it Sucks current from the source through our body to earth?

Thanks in advanc3...

In the first sentence above you say "I don't quite understand this concept, i know that electricity is the motion of electrons through a conductor. what exactly is the potential difference between two points in a conductor?"

This contains a major misunderstanding of physics or your English is not so good. There is NO potential difference in a conductor. When you touch one end of a conductor (a wire, say) to a voltage source and the other end is not connected to anything (open) then almost instantaneously that same potential is present in every part of that conductor. And, no current flows.

Now, for basic Direct Current (DC) theory: connect a wire to the positive side of a battery. Connect the other end of the wire to one side of an electric lamp. Connect the other side of the lamp to the negative side of the battery. Now you have a complete circuit with current flowing and the lamp lights! The total voltage of the battery can be measured across the lamp. Avoid mixing Bernoulli's Principle with electricity: they do NOT mix!

 

[RiddlerA]

In the first sentence above you say "I don't quite understand this concept, i know that electricity is the motion of electrons through a conductor. what exactly is the potential difference between two points in a conductor?"

This contains a major misunderstanding of physics or your English is not so good. There is NO potential difference in a conductor. When you touch one end of a conductor (a wire, say) to a voltage source and the other end is not connected to anything (open) then almost instantaneously that same potential is present in every part of that conductor. And, no current flows.

Now, for basic Direct Current (DC) theory: connect a wire to the positive side of a battery. Connect the other end of the wire to one side of an electric lamp. Connect the other side of the lamp to the negative side of the battery. Now you have a complete circuit with current flowing and the lamp lights! The total voltage of the battery can be measured across the lamp. Avoid mixing Bernoulli's Principle with electricity: they do NOT mix!

Sorry English is not my primary language so it may not be good...
But I understand the mechanism of DC current lighting a lamp and i also know that voltage should be measured across lamp... But wait..

Imagine a HYPOTHETICAL conductor with two ends A and B where end A has more potential than B(Hypothetical conductor).. Now inorder to balance the potential throughout the conductor, something must flow across the conductor from end A to end B right?
How is this different from the air pressure analogy mentioned in the original question?
And also please tell me what is that something that flows across the conductor... Is it electrons?


Thanks for your time...

 

[DarioC]

In a conductor the "energy transfer" is by electrons.

What kind of floor do you have? Second question: You do know that most of the metal parts of the computer are grounded. If you have one hand touching almost anything on the computer and you touch a voltage source with your other hand you will get a shock.

Please be careful. Mains voltage in the U.S. (120) can kill you and 220 will do it even easier.

DC

 

[nsaspook]

In a conductor the "energy transfer" is by electrons.

DC

Electrical energy transfer is not by individual electrons in a conductor. Electrons carry charge and the movement of electrons is current but energy is moved in fields.

https://www.physicsforums.com/showpost.php?p=1886913&postcount=5

 

[sophiecentaur]

Exactly. This is where the water through pipes and waterwheels analogy falls down. The electrons are in no way equivalent to this and, if you want to get a real handle on this stuff, stick to Charge and Potential Difference. Frankly, when I was at School, no one mucked about with analogies or even with bringing electrons into electrical circuits. We all did fine and didn't 'fall over' later due to flawed models in our heads.
It's pretty much a non-starter to give a proper answer to the original question for someone who has not gone through the process with no short cuts. (Hasn't or is not prepared to.) Some things just don't have an easy answer - like "How do you win at Chess?"

 

[RiddlerA]

So is it okay to think of current as is the following video where the cards fall down one by one? The electrons move in a similar way? The rate of cards falling is the rate of the wave propagating through the conductor?

http://www.youtube.com/watch?v=840NbiFF1zM

 

[Bobbywhy]

Imagine a HYPOTHETICAL conductor with two ends A and B where end A has more potential than B(Hypothetical conductor).. Now inorder to balance the potential throughout the conductor, something must flow across the conductor from end A to end B right?
How is this different from the air pressure analogy mentioned in the original question?
And also please tell me what is that something that flows across the conductor... Is it electrons?


Thanks for your time...

Everywhere inside and on a perfect conductor, the electric field is zero. This goes for solid conductors as well as hollow, empty shells of perfectly conducting material. This means that the work done by the electric field on a test charge that is moved from one point in or on a perfect conductor (consider this to be a thought experiment), to another point in or on the same conductor, is zero. This means that the difference in the electric potential between any two points in or on a perfect conductor must be zero. This means that the electric potential at every point in and on a perfect conductor must have one and the same value. The surface of a conductor is always an equipotential surface, that is, the entire conductor is an equipotential.

 

[davenn]

100 volt is too low for a conductor that large, so there will be no current flow?

where did you get that assumption from ?

ahhh not at all ... even 48VDC in a telephone circuit will work over many many 10's of km's - several 100km

Dave

 

[DarioC]

I perceive a communications problem cropping up here. Riddler, I think, continues to question about a dynamic situation, that is, when the circuit is first connected (even in a DC circuit), versus amost everyone else talking about a "established" current condition. There is a definite and measurable difference in the two situations.

SC, good point, I have to admit the last time I thought about this (covering 50+ years in electronics) was in high school, when I was first starting to play with radios and questioned everything. That is what motivated my involvement here. As in, wow, I haven't thought about that in a long time!

DC

Edit: Let me add. Actually I think both situations are implied by his questions.

 

[Bobbywhy]

Imagine a HYPOTHETICAL conductor with two ends A and B where end A has more potential than B(Hypothetical conductor).. Now inorder to balance the potential throughout the conductor, something must flow across the conductor from end A to end B right?
How is this different from the air pressure analogy mentioned in the original question?
And also please tell me what is that something that flows across the conductor... Is it electrons?


Thanks for your time...

RiddlerA, As for your HYPOTHETICAL conductor question above, here is an explanation:

Consider a situation in which two points on the surface of a conductor are at different potentials. In this situation free electrons at one point have more potential energy than at the other point and so they will move. The movement of charge will stop when all points on the surface are at the same potential.

The following statements apply when considering static conditions in a perfect conductor:

1. All points on the surface of a charged conductor are at the same potential.
2. There can be not electric field in a conductor.

Suppose we bring a positive charge near a conductor. For a very short moment there will be an electric field inside the conductor. However, this field will act on and move the free electrons. The electrons will move closer to the positive charge, leaving net positive charge behind. The conductor’s charges will continue to move until the “external” field is canceled out. At that point there is no remaining field to move them so they remain fixed, not moving.

A more accurate statement of this rule is “After a very short time, there is no
electric field inside the conductor". How short a time is it? Recall that in cgs units,
resistivity (which tells us how good/bad something conducts electricity) is measured in seconds. It turns out that the time it takes for the charges to rearrange themselves to cancel out the external electric field is just about equal to this resistivity. For metals, this is a time that is something like 10^-16 to 10^-17 seconds. This what is called "almost instantaneously". This time is so short that we can hardly say that the original statements aren't precise enough!

 

[DarioC]

I just did a "little" calculation.

If one uses a one millimeter square wire (square for simple calculation), which is rather small for the current, and applies sufficient voltage across it in a closed circuit to produce 1 Ampere of current flow it would require only the free electrons from the first 6 microns of wire length to supply the current for the first second.

That is a square 1 mm X 1 mm X 6 microns thick/long of a typical conductor would supply enough electrons for 1 Ampere of current flow for 1 second.

Correct me on this one, but I just did a second calculation and it says, considering the above, that it would take roughly 46 hours for a free electron to move from one end of a 1 meter wire to the other end if the current flow was 1 ampere.

DC

 

[cupric oxide]

Electricity is the interactions that occur between charged particles.
Charged particles create the EM fields. By controlling how they move and where they are stored you can get them to do mechanical chemical or electrical work.
To power a home, usually mechanical work is converted to electrical current. Fuels are burned to create steam, which is used to turn a turbine, which turns an EM motor, which move a magnet across looped coils. Moving a magnet across looped coils generates a current. Why? Nobody knows. For the same reasons why like charges repel and unlike attract. No one knows. That's just how nature is. Look up the Biot-Savart Law to learn more details.

 

[sophiecentaur]

When someone asks "what, exactly, is ...?", they are really asking "what familiar thing is it most like?".
For Electricity, there isn'a any really good answer.
There is hope, however. Maths does a very job of telling you 'how electricity behaves'. It's asking a bit of indulgence if you really want to appreciate something like Electricity but don't want to involve Maths. Maths is the only model that gets anywhere near.

 

[rcgldr]

What exactly is the potential difference between two points in a conductor?

Previous posts mentioned that if a conductor is a perfect conductor with zero resistance, than all points in the conductor have the same potential. In a real world situation where a conductor has resistance, then potential will change with distance along the conductor.

Electrical potential is called voltage, wiki article:

http://en.wikipedia.org/wiki/Voltage

If you're looking for an analogy, you could compare voltage to gravitational potential, wiki article:

http://en.wikipedia.org/wiki/Gravitational_potential

To this day i thought electrons flow at around 10% of the speed of light.

The electrons move at high speed, but the direction is almost completely random, at about 1570km/s at room temperature according to the wiki article below. The net flow is called the drift rate and that is very slow. Wiki article:

http://en.wikipedia.org/wiki/Drift_velocity

The field propagation rate is very fast:

http://en.wikipedia.org/wiki/Speed_of_electricity

 

[sophiecentaur]

As mentioned in previous post, the definition of a conductor means that all points in the conductor have the same potential.

?? Only a conductor with zero resistance or one that is not in a cicruit.

 

[rcgldr]

As mentioned in previous post, the definition of a conductor means that all points in the conductor have the same potential.

Only a conductor with zero resistance or one that is not in a cicruit.

I left that out by mistake. I updated my previous post. I was thinking of conductor as used in an idealized circuit diagram, not a real world conductor.

 

[sophiecentaur]

But every element in a circuit is conductor except the insulation and the dielectrics! Even the 'Semi'conductors.

 

[sophiecentaur]

If you want to avoid misunderstandings and misconceptions you need to be scrupulous when you are trying to 'explain' something. Uninformed people can pick up a mistake and run with it, right down a blind alley.

 

[DarioC]

RiddlerA,
Are you still with us?

DC

 

[RiddlerA]

Yes I am, Reading all the posts repeatedly until they all make sense...

And in the meantime please watch this video and tell me is it okay to think of current as is the following video where the cards fall down one by one? The electrons drift in a similar way? The rate of cards falling is the rate of the wave propagating through the conductor?