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Current and Voltage Confusion

  1. Apr 18, 2009 #1
    Hi,

    I'm a little confused as to Current and Voltage, and I was wondering if someone could tell me if my understanding is correct and/or point me to a good tutorial on current/voltage and basic electronic circuit principles. As well as some good tutorials on circuit analysis.

    So this is how I understand it + my questions about it.

    1) Electrons travel in a conventional circuit travel from positive to negative (although in reality they travel from negative to positive).

    Q)I'm confused as to how this may affect the current/voltage output at different areas.

    2) Charge is a particle (electron) that moves around a circuit. The potential amount of energy is called the voltage. And the current is the quantity of charge passing through a point.

    Q)I'm confused about the relationship between current and voltage. How can an electron have a potential amount of energy? What exactly does the battery do? Does it like 'kick' the electrons to give them more power? If so, what would be an analogue for charge? I mean, what affects it... is it the width of the wire? I can't really see how a battery can increase the quantity of charge passing through a point. Also, can charge be lost? Or does it just get split up? And what happens to current when it goes to a battery... is it constant unlike voltage?

    3) Another question/analogue that's been plaguing me. In a lightbulb, when the tungsten filament resist's the electrons, is it resisting the voltage, or the current?

    Sorry about the large amounts of questions :(
    Thanks!
     
  2. jcsd
  3. Apr 18, 2009 #2
    exactly right. As to your question with this...im not too sure whats confusing you.

    the charges that travel through the circuit from positive to negative ARE teh current. The more charges per unit time that travel through the wire, the higher the current is. In other words, current is proportional to the number of electrons that pass through a cross section of a wire.

    The voltage is what "pushes" the charges across the wire. The higher the voltage drop between two points, the more "push" there is to make a charge go from one end to the other. This is why voltage is described as potential energy; the more "push" there is to get a charge from one place to another the more potential energy it has.

    This kind of leads into your second question. An electron has potential energy because of the Voltage that is pushing it. Again, the higher the voltage the more it "pushes" the electrons, and the more potential energy the electron has.

    A battery has 2 ends, with a potential difference across them. Because of the potential difference, it pushes a charged particle from one end to the other. The electron tries to go from the state of high potential to a state of low potential by traveling from one end of the battery to the other using the "push" created by the voltage drop along the battery.

    Charge and voltage are related through Ohm's law, V = IR, V is voltage drop (depends on the battery or power source), I is the current, and R is the resistance of the wire (dependant on the wire and its properties).
    So if you apply the same voltage drop across two different materials, the material with a higher resistance will have less current traveling through it.

    Resistance depends on the material the wire is made of, the cross section of the wire, and the length of the wire.

    Charge can not be lost. Charge physically exists in the form of electrons and protons, they are never lost or destroyed. They are however neutralized, when a proton and electron meet.

    As for 3...Resistance is a physical property of a material. It is how much a wire resists the flow of charge (current) per unit voltage. So, for any given material with resistance, more current will pass through it if a higher voltage is placed at the 2 ends.
     
  4. Apr 18, 2009 #3
    Let me give it a try.
    Electrons move from atom to atom, migrating from a more negative toward a more positive potential. You can think of it is the electrons moving from a place where there are more electrons to a place where there are less.

    You can think of voltage and current as pressure, and flow. An analogy would be water pressure and water flow. The voltage is the pressure, the current is the flow. Any time you restrict the flow, the pressure goes up. There is a set of equations called Ohms Law that equate the pressure (voltage) the flow (current) and the restriction to flow (resistance).

    You have it basically correct. Although some might question the notion that charge is a particle... Charge is a property of an electron, not the electron itself. Electric and magnetic charges are actually a condition of space as described by James Clerk Maxwell. The condition is possible because of two properties of space. These two properties are electric permittivity and magnetic permeability.
    When atoms possess the same amount of electrons as protons, their charge (potential, or voltage) is neutral. So the electrons are happy to remain in place. Most metal atoms have an electron or two that is loosely held by the atoms. These electrons can easily leave their atom, leaving it with an unbalanced charge. Electrons of neighbouring atoms jump in to fill the vacancy. Electrons thus move around in the metals.

    A battery is a container that provides a chemical reaction that separates electrons from their atoms and concentrates them at a terminal while providing an opposite terminal where the electrons can accumulate. An analogy for charge is pressure. You can have a lot of pressure without much ability to do work. Like a hundred psi in a small garden hose or a hundred psi in a six inch pipe. You will get more work out of opening up the six inch pipe. If you have nothing to maintain the pressure, the work in terms of flow quickly stops. It is the same in an electric circuit.

    The resistance is to current flow. The voltage appears across the filament because of the resistance to flow. Voltage, E, is equal to current, I, times resistance, R. So E = IR is the simple relationship between voltage, current and resistance.
     
    Last edited: Apr 18, 2009
  5. Apr 18, 2009 #4
    Hi swraman, you beat me to it:smile: Nice post. I can vouch for your comments.
     
  6. Apr 21, 2009 #5
    Thanks heaps! :) Think I understand now!
     
  7. Apr 21, 2009 #6
    Avoid "pressure and flow" analogies. They don't work and confuse the new student. Also, "potential" includes both voltage and current. In the *electric* domain of energy, the voltage is the potential. But in the *magnetic* domain, current is potential.

    Also, voltage does NOT "push" charge. Charges mutually push against or towards one another and this property is "Coulomb force". It is an axiom, not derived or constructed. Current and voltage are mathematical constructs, ratios derived from the more basic quantities, charge, time, and energy.

    Does this help? BR.

    Claude
     
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