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Strange questions about electrical circuits

  1. Jul 11, 2015 #1
    What's up people? I'm new in this wonderful forum. I have several questions about circuits, and they are not really common, I mean, I find them somehow complicated, but I would really come to enlightenment if you guys gave me some clues.

    1.- So, let's suppose we got the simplest circuit, a battery, two cables and a resistor in the middle of them, obviously everything is connected. The resistor is supposed to consume the whole voltage of the battery, no problem with that. However, the problem is that I can't understand how electrons can get back to where they started, I mean, it's a circuit, but there's a point in it where electrons loose their whole energy but still they have energy to get to the positive end of the battery. Why don't they just get stuck in the resistor?

    2.- If I place an LED in the previous circuit, order doesn't seem to matter. It's irrelevant wether I place it before or after the resistance. Why is that?

    3.- How can an LED light on in the circuit when the whole voltage is consumed by the resistance? Does the resistance in the LED matter? Does it act like a voltage divider then?

    4.- if it was possible to follow an electron from the beginning to the end of the circuit, does it recover it's lost voltage when it gets back to the battery?

    Thank you in advance guys, your answers would be really helpful.
     
  2. jcsd
  3. Jul 12, 2015 #2

    Svein

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    1. You do not consume voltage. Think of voltage as an analogue to height. If you place a ladder from the ground to your roof, the height of the roof is not consumed. What happens is that the resistor allows a current to flow.
    2. The LED lights up when current passes through it. In the circuit it acts as a fixed voltage drop of about 1.5V
    3. The voltage is not consumed. The current through the LED is determined by (VBattery - VLED)/R. VLED is ≈ 1.5V.
    4. The battery acts as an electron lifter. Using the height analogue: If a ball falls from the roof, it loses potential energy. The fall itself is analogue to the current through the resistor. If you take the ball and place it on the roof again, you act as a battery.
     
  4. Jul 12, 2015 #3

    davenn

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    yes, as Svein says , you need to stop using the words voltage is consumed
    and with that keep in mind ... voltage DOESN'T flow :smile:

    Voltage is dropped across components be they resistor, capacitors transistor etc
    because you have now added another component in series with the resistor ( namely the LED), there is now a voltage drop across
    each component. if you know from the datasheet what the voltage drop across the LED will be say the around 1.5V that Svein used
    then you know what voltage will be dropped across the resistor is say the power supply is 10V

    Yes it becomes a voltage divider :smile:

    you do some working out for more than 1 resistor in series and work out the voltage drop across each resistor
    for simplicity lets use resistors of the same value

    1 resistor across a 10V supply will drop the full 10V ( 10/1 = 10)
    2 resistors across a 10V supply will drop 5V each (10/2 = 5)
    3 resistors across a 10V supply will drop 3.33V each (10/3 = 3.33 recurring (9.99999V ))


    see the pattern ? :smile:


    for different value resistors another step needs to be taken
    wont go into that for now


    Dave
     
  5. Jul 12, 2015 #4

    meBigGuy

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    The power consumed by a component is the current through it times the voltage across it. The same amount of current will flow through both the resistor and the LED.
    The LED will drop some voltage (consume some power), as will the resistor. The voltage drop of the LED plus the voltage drop of the resistor equal the voltage of the battery.
    It doesn't matter where in the circuit the LED is, it will have the same current going through it. That current causes a constant voltage drop (determined by the LED's characteristics).

    If there is no LED, all the voltage is dropped across the resistor. Add the LED, and some voltage is across the LED and some is across the resistor (and the current is less because the LED plus resistor present more total resistance to the battery)

    I guess I'll commit the hydro analogy sin.

    Think of a hose feeding an orifice. Pressure (voltage) forces water (current) through the orifice.

    Now place a flow meter in the system. The flow will be the same whether before or after the orifice (even though the pressure is different). The flow meter needs some pressure across it in order to work.
     
  6. Jul 12, 2015 #5
    Voltage is force (electromotive force). Current is moving charges (electrons). Power is the force times the movement: Power = Volts times Amps.

    Resistance is where the power goes (heat): Volts divided by Amps = Resistance.

    Batteries provide electrons at one end with some force. (i.e. power) The other end consumes electrons. (They come from and go into chemicals.)

    An LED takes some of the power and converts it to light instead of heat. So when you put it in the circuit, it takes a certain minimum force (1.5V) and whatever current flows through it: VLED times AmpsLED = PowerLED
     
  7. Jul 12, 2015 #6
    Wow! That was quick! Thank everyone for your replies. But the matter goes on. Damn! Electricity challenges me so bad!

    Ok, voltage is not consumed, got it.
    So a resistor allows a current to flow, what happens when I plug both ends of a cable to the ends of the battery without any resistance? There's an infinite current circulating, and there's still a current. So a resistor just avoids currents from being infinite then? What happens in terms of height when there are two or more resistors?

    So, voltage doesn't flow, but if I place a voltmeter between the ends of a resistor it measures some voltage through it, is that the voltage drop? What's that exactly?


    You got the same current across every component, but in the case of a battery using cables plugged to a resistor, how can the current be the same when you got cables with cero resistance in some parts of the circuit and the resistor itself in the middle? How is that characteristic supposed to remain fixed when the conditions in the different locations are so different from one another?

    Thank you guys!
     
  8. Jul 12, 2015 #7
    Thank you for your reply bro. So, if voltage is an electromotive force, Can we say a resistor requires more of this force to get electrons through it? Is it truth that it is the electric field that provokes this force? Can we say that the electromotive force is caused by the electrostatic forces of the ends of the battery? I mean, is this force caused by the fact that electrons are repelled from the negative end of the battery and attracted to the positive one?
     
  9. Jul 12, 2015 #8

    Svein

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    There is no "voltage through it". There may be a voltage across it, as a result of current passing through it.
    There is an internal resistance in the battery, which limits the current. This internal resistance can be very low, however, so don't try this with a 12V car battery!
     
  10. Jul 12, 2015 #9
    Yes, that's mostly true.

    I was playing fast and loose with the definitions. Voltage, Electromotive Force, and The Electric Field are all closely related concepts. They have slightly different definitions (Voltage is a unit for example) but are basically the same thing. As you come to understand the concepts you will need to go back and restudy them with an eye to their actual definitions.

    Resistance doesn't need more force. What it does is slow the electron flow. So increasing the resistance will decrease the current for the same voltage. Or, as you say, we can get the same number of electrons to flow (through more resistance) by raising the voltage.

    V=IR: This is Ohm's law. It is the most basic equation in electronics. Learn it. Manipulate it. Love it. As you get deeply into the EE field you will use it in more and deeper ways. It works in vectors. It works with time variations. It works with space variations.
     
  11. Jul 12, 2015 #10

    CWatters

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    The voltage (height) is divided between them.

    Yes it's the voltage drop across that resistor. If you like the height analogy.. it's a bit like measuring the height between the floors in a building. If you measure the height of all the floors they add up to the height of the building.

    How can the current be different? Consider charge leaves the battery and flows down a wire and through the resistor. Where else can it go? There are no "leaks" or extra sources in your simple circuit. If there were two resistors in PARALLEL then some would take one path and some the other. The current wouldn't necessarily be the same in each path. Add up the amount taking each path and its the same as the total leaving the battery.

    Reminder.. Voltages appear "across" a component or "between" nodes. Currents flow "through" a component.
     
  12. Jul 14, 2015 #11
    Ok, I got two problems in here. First of all, what I still don't get is the before and the after when it comes to current, there are points in the circuit with different resistance each one, for instance, the cables got zero resistance and the resistor has its own value. If the value of current depends on resistance , then there should be more electrons going through the cable before the resistance than the number of electrons after the resistance, but still we asume current is the same in every part of the circuit, and this includes the battery doesnt it?
    Second: Through and across, there's little difference to me. As I know, across is less specific and means getting from one point to another without necesarily getting inside something. Am I right?
    I'm sorry for being such a headache. I'm really understanding things better in here; in fact, I'm excited because this is the first time I'm really feeling free to ask this kind of questions, and the first time someone actually takes the time to answer them. Thanks a lot again!!!!!
     
  13. Jul 14, 2015 #12
    It seems the quote I did above is incomplete sigh o_O
    Anyway, there's something else I want to ask.
    What if I have very little voltage and very long conductors? Resistance is also directly proportional to length. Will electrons still make it? I mean, will they akways reach the opposite end regardless of this condition?
    Namasté, my fellow and wise colleagues:smile:
     
  14. Jul 14, 2015 #13
    Yes, they will make it, but it could take a while.
     
  15. Jul 15, 2015 #14

    davenn

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    the number of electrons leaving the battery will equal the number of electrons returning to the battery
    No electrons were lost or harmed in the process :wink:


    we don't assume anything ... Ohms Law tells us the current flowing with a given voltage and resistance

    I = V / R


    In a DC circuit, yes they will .... electron drift is very slow

    In an AC circuit, you are likely never to see the electrons from the generator wire windings reach you home
    as they are just oscillating back and forward about a fixed position

    Dave
     
  16. Jul 15, 2015 #15
    I think of resistors as friction to the flow of electrons, could someone clarify this as being true or false?
     
  17. Jul 16, 2015 #16

    meBigGuy

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    True or false doesn't really apply to an analogy. Wikipedia says "Electrical resistance shares some conceptual parallels with the notion of mechanical friction." https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance.

    The hydraulic analogy would say it is more like an orifice causing a pressure drop.

    It's kind of hard to visualize a material with fewer free electrons (therefore higher resistance) as exhibiting friction.
     
  18. Jul 16, 2015 #17
    good to know, read and saved the link. should have realized that by remembering the makeup of carbon comp resistors.
     
  19. Jul 16, 2015 #18

    meBigGuy

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    @ Guidestone

    Please review the hydraulic analogy in https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance. Then you can ask questions about how that analogy breaks down. (read about the hydraulic analogy in other places also).

    Note that when you start a hydraulic flow, (in a system with no air gaps) that water comes out of the end immediately upon applying pressure, not just after the input water reaches the output. The water flow is the same all along the system (but the pressure can change). No more can go in than comes out (unless there are storage elements)

    It doesn't seem like you are really thinking about or digesting what others are telling you. Maybe reread some basic electrical theory. You are totally missing the basic points.
     
  20. Jul 16, 2015 #19

    f95toli

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    Just a word of caution to add to what meBigguy wrote above: The answers above are great, but It is important to remember that the water analogy is just that: an analogy. Electrons do not actually "flow" in a circuit in the way it is often explained (i.e. electrons leaving and then returning to a battery) which is why the water analogy breaks down if you think too hard about it and take it too literally. The water analogy e.g. breaks down if you try to understand why it is that while electrons actually move very slowly (cm per seconds), electrical signals travel very quickly (~3e8 meters per second); the analogy does not really tell you anything about the underlying physics.

    None of what I wrote above is very important if you are only trying to understand the basics (where the water analogy works well), but I've seen people get severely confused because they did not realize the limitations of this analogy.
     
  21. Jul 16, 2015 #20

    sophiecentaur

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    Emf is not a Force. The notion of a Force in Electrical circuits was discarded many years ago but the term 'emf' still hangs on. It is a 'special' term that 'we' use with our fingers crossed. Also Resistance is not 'a form of friction inside wires'. Beginners with Electricity are always tempted to grab onto mechanical analogies but they really don't help. Difficult as it may seem, the best thing to do is to rehearse, again and again. that Voltage is the Work Done in transferring a unit charge. It is the Joules per Coulomb. Then Resistance is just a ratio between Volts and Current. 'Fraid that both these definitions are not very friendly but they help to keep your mind away from dodgy mechanical analogies. Just get into the world of Electricity and don't expect Mechanics to hold your hand reliably. This advice may be tiresome but it would be really worth your while to take it (in the long run) - if you want to understand the subject and not just get on with it for the purposes of making electrical gadgets work or repairing them. Water analogies are sometimes used on technician courses and in the armed services but they will turn out to be a snare and a delusion if you want a deep knowledge.
     
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