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Electricity: conceptual understanding help

  1. May 1, 2012 #1
    I've been able to do all the calculations and whatnot well in my physics class, but I recently stopped to think more about the concept of electricity and how it fundamentally works, but quite frankly I'm a bit stumped and there are some gaping holes in my understanding.
    Would someone please be able to give me as succinct an explanation as possible to explain some more about electricity?

    Things such as:
    - What it is that's actually flowing in a circuit
    - The difference between charge 'flow' and electron 'flow'
    - A definition for some different derived SI units: [Voltage, Current, Power, etc.]
    and so on..

    One thing that I don't really understand:
    - The number of electrons that leave a power source (from the -ve terminal) is the same as the amount that return to the source (through the +ve terminal).
    If these amounts are the same, and each electron has the same charge (1.6e-19 C), then what is different about the electrons that leave the power source and those that return?
    What exactly is 'lost'?

    Thankyou so much to whomever can help me, the textbook definitions that I've come across just present statements without actually explaining the more fundamental concepts of electricity and it's really bugging me.
  2. jcsd
  3. May 1, 2012 #2
    - What it is that's actually flowing in a circuit?

    Answer: Electrons

    - The difference between charge 'flow' and electron 'flow'?

    Answer: A charge 'flow' is a flow of charged particles (ions) whilst an electron 'flow' is a flow of electrons.

    - A definition for some different derived SI units: [Voltage, Current, Power, etc.]
    and so on..

    Answer: Voltage, current, power aren't units. They're quantities.

    The SI units: Voltage is measured in volts and it determines electric potential, current is measured in amperes and it determines the rate of flow of electrical charges through a medium. Power is simply joules/second (energy per second).

    - The number of electrons that leave a power source (from the -ve terminal) is the same as the amount that return to the source (through the +ve terminal).
    If these amounts are the same, and each electron has the same charge (1.6e-19 C), then what is different about the electrons that leave the power source and those that return?
    What exactly is 'lost'?

    Answer: The charges do get used when flowing through a circuit.
  4. May 1, 2012 #3
    Wait, is an electron not a 'charged particle' ?

    Ok thanks I was just referring to this: http://en.wikipedia.org/wiki/SI_derived_unit
    Should I have said Volt instead of Voltage? and so on..

    I don't mean to offend or anything, but that's the kind of explanation that textbooks give and the kind that doesn't really enhance my understanding..
    I probably shouldn't have used succinct in my original post, I just meant precise rather than truncated

    Could you perhaps elaborate on the last question a bit? What is different from the electrons that leave the power supply and those that return?
  5. May 1, 2012 #4
    Hopefully someone can understand my dilemma with the following:

    First I read:
    "Electricity is the flow of electrons.."

    Then I read:
    "Electrons do not travel anywhere near c within a wire.. Electricity travels at c, while electrons do not."

    Is one of these statements incorrect?
    If not:
    If electricity travels at c, but electrons don't, then what exactly is travelling at c?
  6. May 1, 2012 #5
    A more accurate answer would be the net displacement of electrons, as each individual electron is not really flowing, but rather the net displacement of all the electrons in a material is in the direction of flow.

    A charge flow can be charged ions as Nomad91 says, such as in a particle accelerator or ion beam, but charge flow means it can be the flow of any charge carrier, and so it can be positive or negative. Charge flow can be the flow of electrons since they also have charge. A third convention (the kind used in circuit theory) is using holes as charge carriers. A hole is simply the absence of an electron, so when an electron moves away from a location, it has left a hole with a charge equal but opposite of the electron, this is why we can use currents in circuits that travel from positive terminal to negative terminal, like a positive charge would do, even if the physical flow is actually electrons. For basic understanding, this is important in understanding current direction convention, but you will want to remember this for when you start to study semiconductor devices like diodes and transistors.

    So to your question, a charge flow is a more general term that can refer to several different kinds of charge carriers, while electron flow specifically states what the charge carrier is (electrons).

    - A definition for some different derived SI units: [Voltage, Current, Power, etc.]
    and so on..

    A volt is defined as joule/coulomb by considering a uniform electric field of some chosen field strength, and if you placed a single charge in that electric field and allowed the electric field to do work on the charge (move it from one point to another in the field), the displacement of that charge over a certain distance, dependent on the field strength, will transfer 1 joule of energy to the charge. If the electron is not allowed to be displaced, it has a potential energy from its position to another position. In circuits, this is like picking a point at the voltage source + terminal, and a point at ground, and the amount of work the electric field generated between these two points can exert on a charge defines how many volts it is. It is good to try to think of the electric field's role in the definition of a volt as soon as possible, even if you can get away without considering its presence, since the voltage is enough information by itself.

    If you consider the above explanation of the volt, then you can see how V*I = power, because a volt is how much energy is transferred to the charge as it passes through a voltage drop, and I is the time rate that the charge passes through the voltage drop, so the product is the amount of energy transfer through that voltage drop over time. Where that energy goes and what happens to it is dependent on the circuit element that the voltage drop is through (resistor, LED, capacitor, etc.).
    Last edited: May 1, 2012
  7. May 1, 2012 #6
    See my post, the key to accepting that electricity can flow at the speed of c without any electron moving nearly that fast is that we are considering the net flow, not actual displacement of individual electrons. The electricity flows with the electromagnetic field, which travels at c.

    A similar but not equivalent analogy would be to consider a sound wave traveling through a wall. The sound wave energy is moved through the wall at the speed of sound, but each wall atom only vibrates with this energy, it does not actually move out of its position very far. If the wall atoms traveled with the sound wave energy, you'd have a lot of holes in your walls.
    Last edited: May 1, 2012
  8. May 1, 2012 #7

    Take a look at waterfall. Same amount and same speed of water flow at upstream and downstream river. Water do work at waterfall. gravitational potential energy which water has is transformed to energy of sound, vibration, temperature, etc. Look like your question, does it?
  9. May 1, 2012 #8
    Ya.. what he's saying is the electrons transfer energy stored in the electric field by moving between the electric field at the high potential to the electric field's lower potential. The electrons' potential energy has changed as they return to the negative terminal.
  10. May 1, 2012 #9
    Here is another way to consider this:

    The same number of electrons leaving the power source at a positive voltage terminal is the same number of the electrons (not necessarily always the same electrons, but always the same number, again remember net flow) that return to the source's low voltage point.

    So you are right, the amount of charge is the same, but remember the definition of the volt: joules/coulomb, so the same amount of charge at the higher voltage has a different amount of energy associated with it than the same amount of charge at the low voltage return point.

    Edit: It may help if you rearrange the terms in the definition of the volt, to get an expression for charge, which would be coulombs = joules/volt, so for a constant amount of charge (constant coulombs), the energy associated with the charge will vary depending on what the voltage is.
  11. May 1, 2012 #10


    Let me draw your attention to more advanced view of this issue, Poynting Vector. Electron or current do not itself contain energy but EM field has. Energy come from both poles of battery and move to resister for consumption. http://amasci.com/elect/poynt/poynt.html It is much different from waterfall interpretation that is easy to understand.

    Last edited: May 1, 2012
  12. May 1, 2012 #11
    Yes, the energy is contained in the electric field, but this energy is transferred with charge. In absence of charge, the electric field's energy cannot change over time unless it is radiating. The charge is transferring the field's energy.
  13. May 1, 2012 #12
    Hi. More precisely, current is required to generate magnetic field. Surface charge is required to generate electric field. Thus generated magnetic field and electric field are required to generate Poynting Vector that expresses energy flow.

    Does this help you or confuse you?

  14. May 1, 2012 #13
    That helps :)

    When a charge carrier has a mass associated with it, this energy is then transferred to the mass, yes?
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