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What exactly is electricity?

  1. Jan 5, 2012 #1
    I dont 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....
     
  2. jcsd
  3. Jan 5, 2012 #2
    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.
     
  4. Jan 5, 2012 #3
    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:



    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).
     
    Last edited: Jan 5, 2012
  5. Jan 5, 2012 #4
    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.. im having hard time with this topic...
     
  6. Jan 5, 2012 #5
    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
     
  7. Jan 6, 2012 #6
    Er.. I dont 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?
     
  8. Jan 6, 2012 #7

    cmb

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    (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.
     
  9. Jan 6, 2012 #8
    RA,
    Do you understand/know about "free" electrons and the difference between a conductor and an insulator?
    DC
     
  10. Jan 6, 2012 #9
    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 dont 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....
     
  11. Jan 6, 2012 #10
    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
     
    Last edited: Jan 6, 2012
  12. Jan 6, 2012 #11
    What exactly is electricity?

    If I knew the answer to that question I'd be practising my Nobel Prize acceptance speech!
     
  13. Jan 6, 2012 #12

    sophiecentaur

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    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.
     
  14. Jan 7, 2012 #13
    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... :confused:
    Thank you...
     
  15. Jan 7, 2012 #14
    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?


    What kinda interesting things? yes im interested....
    Thank you..
     
  16. Jan 7, 2012 #15
    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
     
  17. Jan 7, 2012 #16

    Bobbywhy

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    In the first sentence above you say "I dont 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!
     
    Last edited: Jan 7, 2012
  18. Jan 7, 2012 #17
    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...
     
  19. Jan 7, 2012 #18
    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
     
  20. Jan 7, 2012 #19

    nsaspook

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    Last edited: Jan 7, 2012
  21. Jan 7, 2012 #20

    sophiecentaur

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    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?"
     
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