Trying to get intuitive understanding of voltage/current/resistance

Discussion in 'General Electronics Chat' started by fr0sty, Apr 1, 2008.

  1. fr0sty

    Thread Starter New Member

    Apr 1, 2008
    Hey guys,

    I'm slowly making my way through the first chapter of the All About Circuits book (awesome awesome stuff) and want to try and get my understanding of electron flow fairly solid.

    What I'm not sure about at the moment is why current stays the same through all parts of a simple circuit. Here's my understanding so far, it'd be great if someone could clarify or point out where I'm going wrong/missing something:

    Take a simple circuit with just a battery and a resistor connected to the battery. In my minds eye, this is how I visualise it from reading the first chapter of the book (sorry if this is a bit long but I'm trying to clarify things :) )

    - Voltage is the force that moves electrons between two inequally charged atoms until they're equally charged. This is because on the one hand, the excess electrons in one atom are being attracted to the protons in the other atom, while the other atom doesn't have enough electrons itself to push the new electrons from the first atom away. The stronger the imbalance, the stronger the force within the electric field to make the electrons balance out (is this because more electrons have to move if the balance is to be equalled out?).

    - A group of electrons moving in unison is a current. To measure a current, you count the number of electrons passing a single point over a certain amount of time (say one second). The more electrons that pass the point during that time, the stronger the current.

    Now here is the first point I'm not sure about. My impression is a resistor is harder to push electrons through than a wire. Do the electrons slow down as they go through the resistor, coming out the other end slower? If this is the case, is it because the resistor is causing 'friction' which converts some of the electron's voltage into heat (instead of ALL the voltage being converted into electron movement)? Is this the same as saying the resistor actually applies a 'reverse voltage' to the electrons flowing through it, essentially?

    Now the electrons come out the other end and go through the battery. My next question (and I think I might have the answer to it, but I'm not sure) is why does the current remain constant through the circuit?

    If I was to guess what was happening, I've got two expectations from my understanding. One is that the electrons get pushed through the battery quickly, then slow down at the resistor. So you'd get a higher measurement of current if you measured at a point between the negative terminal of the battery and the resistor and a lower measurement of current if you measured at a point after the resistor as the electrons were on their way back to the positive terminal of the battery.

    However, I realise this impression doesn't take into account that all the electrons in a circuit are affecting their neighbours (touching each other, so to speak). Taking this into account, my guess is what actually happens is that there are two opposing forces acting on ALL the electrons in the circuit. The first one is the voltage supplied by the battery, that makes the electrons want to move forward. The electrons moving forward push the other ones in front of them forward, giving the ones in front their voltage and moving together as a current. On the other hand, at the same time, the resistor is trying to push the electrons back in the direction they are arriving. This force is being transferred backward through all the electrons, which causes them all to slow down (even making them move slower through the battery). Since everything is linked together, the resistance causes the current to drop through the whole circuit.

    Now my final question, if this is right, what happens to the current inside the battery? Do the electrons inside the battery move slower, or does the battery try and move them through itself at the same speed (which I gues would cause a pileup of electrons at the negative terminal of the battery)?

    Hopefully I haven't bored you all to death. I know my intuitive understanding is probably wrong in a few places, would be nice to hear your feedback :)
  2. mik3

    Senior Member

    Feb 4, 2008
    Only God knows what is really going on! But imagine the battery as a water pump, the wires as the hose and the resistor as a hose with smaller diameter than the other hose (wires). Can you move a portion of water without moving all the water in the hose? No, you must move all the water the same time. Do you get it?
    Remember that you cant compress water.
    Thats the way electrons behave
  3. Whoops

    New Member

    Apr 1, 2008
    The analogy Mik3 gave is pretty good.

    My own analogy is to picture a container of water, with a hose attached. Imagine the container as being held up in the air. The number of feet from the water tank to the outlet of the hose, on the ground, is similar to the voltage. The higher the container, the more distance and energy/pressure which the water coming through the pipe has. The size of the pipe is similar to resistance. A bigger pipe has less resistance, so more water will flow, which the water flow is similar to the current.

    As to the technical phsyics of the electrons, which way they really flow, and all of that stuff, for the time being you don't need to be concerned with. You just need to know how high the container is, relative to the ground, which is the voltage. Then if you know either the size of the pipe, or how fast the water is coming out of the pipe, you can calculate the item you don't know.

    Hope that helps.
  4. RmACK

    Active Member

    Nov 23, 2007
    Electrons move at close to the speed of light. Their speed isn't what varies but the energy that the carry does. If they were coming out slower than entering there would be a pile up! Water is always a good analogy. The voltage that you mentioned across the resistor is called voltage drop and is the current times the resistance. Basically the difference in pressure across the hose fr0sty mentioned. The resistor will get hot, as energy is taken from the electrons. Don't worry too much about the electrons themselves, more how hard they push (voltage) and the rate they flow at which is more if there are 5 per second or 100000 per second travelling past a point. Either way they will travel the same speed. Think of a road where few or many cars may travel at the speed limit.
  5. Matsukaze

    New Member

    Apr 2, 2008
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    The electric field, rather than the electrons, is what moves so quickly. It is like a wave moving over the surface of a lake. The lake does not move, and the water in the lake does not move quickly. The wave does.

    The current does not "slow down" when going through a resistor. There is simply less current flowing in the circuit because of the resistor. There is no charge "piling up" on one side waiting to get through.

    Remember that a potential difference is always between two points - the one pole of the battery is "pushing" the charge and the other pole is "pulling" the charge with equal force. Also remember that current flowing inside the voltage source is the same value as current flowing in the circuit.
  7. Dave

    Retired Moderator

    Nov 17, 2003
    The random motion velocity of electrons in the absence of an applied electric-field approaches the speed of light. However as an electric-field is applied, the drift velocity of the electrons which constitutes the flow of current is very slow.

  8. r_sriraman

    New Member

    Apr 2, 2008

    I think, initially for a short duration (very short to be perceived) after power on, the electrons in the entire length of the circuit talk to each other to find out the entire resistance of the circuit, so only the current corresponding to the computed resistance is released from the battery.
  9. Wendy


    Mar 24, 2008
    They don't "talk" to each other. They are responding to a field, much like a chunk of metal responds to a magnet (probably a bad analogy though).

    Going to the water analogy, the water is responding to the gravitational field of the earth. The battery, through chemical means, causes the electrons within it to move from one pole to the other. The electrons in the wire and in the electron rich side of the battery are trying to fill the void. They are "falling" into the lower potential. If they succeed in filling this void the battery is dead and no current flows.

    If there is no current path then you have something very similar to static electricity, which is a slightly different animal.