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I don't understand the physics of Ohm's law. Please help

  1. Dec 3, 2003 #1
    I don't see how voltage (in a series circuit)is dropped by a resistor if there is more than one connected (divides the voltage) EX: 12V total and 2 resistors diving it into two 6-Volt sections. Wouldn't that drop the WHOLE voltage to 6V? AND Why would a resitor drop VOLTAGE? Shouldn't it resist current? And what exactly is YOUR definition of current and YOUR idea of what happens when current encounters a resitor? Also I need a more clear "analogy" of voltage other than "Potential". I can't seem to tie their relation together.

    I would appreciate any insight on what happens on the attomic level. I get the math of ohm's law, but I don't get the "why?".
  2. jcsd
  3. Dec 3, 2003 #2
    If you go from one end of a voltage source to the other, you always have to drop by the voltage of the source, no matter what else is going on: that's the definition of a voltage source! So if you put a 12 V battery, and then a bunch of random circuit elements like resistors or whatever, the voltage will have dropped 12 V across whatever elements you put in there.

    If you have two identical resistors, then you know by symmetry that the voltage drop across one has to be the same as the drop across the other ... if the total drop is 12 V, then the drop across one is 6 V.

    If you put a bunch of circuit elements in series, the current through each one of them has to be the same, it can't drop. That's because current is the rate at which charge is flowing, and charge has to be conserved: charge flows in one end at the same rate it flows out the other.

    I gave you the definition of current. Loosely, ignoring the quantum picture of how conductors and insulators work: when current encounters a resistor, the flowing electrons scatter around more off the atoms of the resistor than they do in the wire, and some electrical energy in the system is lost due to heat. That's what produces a loss in electrical potential.

    Potential difference just determines how much work is done on or by the charges...
  4. Dec 3, 2003 #3
    Its not right to say that "voltage is dropped by a resistor". There is a voltage drop on a resistor, but thats different, and implies series circuit. As well one could say that voltage appears on the resistor.

    As some very down to earth analogy, at first you could think of voltage as of pressure force applied to a water to get it through a pipe. If pipe is huge, no resistence, no pressure drop at other end. But if pipe is small, the resistence is higher, and at other end less pressure arrives. There is a pressure difference between ends of pipe (voltage), and pressure drops on that pipe. You can look at resistence as at mechanical friction. Note that this is not correct analogy, but only gives the "gist" of it.

    Current is like speed of motion of water molecules through the pipe. If there is narrow cross section of pipe, then there less molecules can pass concurrently, and they must speedup. Then they face more friction, and to force them through that narrow section, more force needs to be applied (voltage).
    So, voltage is like amount of force you need to get molecules going, current is like amount of molecules per second that flows into and out of pipe, and resistence is like friction of the pipe.
    When you close a valve on a pipe, resistence goes infinite, which is equivalent to infinite friction, and thus no matter what force you apply (voltage), no molecules would flow (current), and no work would be done.

    Thats abit confusing explanation. This leaves impression that with higher resistence more energy goes into heat, no matter what voltage, which is false. Current does not exist without application of force (voltage). To get same current when there is lack of charges, you need to force charges to move faster, and for that you need more force, voltage potential. Faster charges have more energetic interactions with matter, and produce more heat as a byproduct. Resistence is lack of free charges.
    Good conductor is that which has alot of free charges that can each move slowly while together providing large current thanks to their large amount per unit time.

    Correction, ... work done per unit charge against the electric field. Also confusing as leaves wrong impression that whenever there is potential difference, then there is work done.
    Potential difference just determines how much Force is applied to the charges in given section of conductor...
  5. Dec 6, 2003 #4
    I will try and make this simple, If you had two 5 gallon buckets with a hose connecting them on the bottom, on that hose were a couple of valves, and one bucket was full of water and the other empty. The bucket that is full of water is your voltage the more full it is the more potential it has to push water through the hose. If the valves are closed nothing happens because the water can't get through. So you are like a battery sitting there with potential to do something fully charged per say. Think of the gallons as volts, so we have a five-volt battery. Now open the valves all the way and what happens? The water will rush through the hose into the other bucket until the water is at the same level. Why? Because they are at the same potential now and there isn't any reason for the water to move. Ok now fill the bucket again and empty the other. Now only open one valve a quarter of the way and then the other half. If you had a pressure gauge and measured the pressure after each one it would equal total pressure, which your pressure is directly proportional amount of water in the bucket. The water flowing through is your current and the valves are your resistors and the pressure is your voltage. I don't know if I made this more complicated or less:) oh well hope it helps.
  6. Dec 10, 2003 #5
    Wish i knew more in this area

    This is just a guess, but arent the terms resistors, and conductors somewhat self explanatory?

    It would seem to me, that a resistor is that which "resists." In terms of the subject matter before us, a resistor would be anything that somehow resists the flow of something, which is i suppose current, and current, is like a flow of force of some kind. But as for voltage is concerned, i have no idea how to understand that since, it seems somewhat similar to current, but relates more to the amount of power behind something.

    I dont know. I have wanted to learn electronics, but i could never really get around to actually understanding anything.

    Anyone know of a good site on this subject?
  7. Dec 10, 2003 #6


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    I hate to say it, HeavensWarFire, but if you so freely admit that you have no idea what you're talking about -- why bother posting a response?

    - Warren
  8. Dec 10, 2003 #7
    How old are you?

    Like 16?

    Why bother? Hmmm, sounds to me like a philosophical question. Why anything? Why live? What not just kill yourself? Why anything? To be, or not to be?

    Should, i, or should i not take a shot at asking a girl out?

    I think, you need to live life, before you can use your fingers to type things on a screen.

    I choose. I can could have chosen not to post, but i chose to post. Heres a noval idea: free will. Imagine that. Can you dig it man. can you really grove to the contortions of the mind as it tries to travel through the waves of thoughts? Can you dig it man? Can you groove to it?


    And if you hate to admit anything, then why admit it, since it only brings you hate? ahahahahahahahahahahahahahahaha....

    Its alright man, you can be small if you want, and block all thoughts from entering your mind.

    Ahahahahahahahahahahahahahahahahah......you are small.

    Tell me, or great Master of Nothing, can you see beyond your death?

    Are you GOD? The laws of Science, and morality, and i suppose luxury, are they all of your creation?

    What is freedom, if one ever uses it?

    can you dig it man? Can you ride the infinite waves of time, and thoughts?

    ahahahahahahahahahahahahahahahahahaha.....you must have been what Albert Eistien was talking about when he said something about "how great minds have always encounted great opposition from mediocre minds."

    And didnt he also said something about how imagination is better than knowledge? You do respect Albert, dont you? Isnt he like after all one of your GODS?

    lol.........you are small man, way small. LOL. I feel sorry for you. You shouldnt be afraid of thoughts. But you can always choose to lock yourself up in a basement, if you dont want to expose yourself to the contamination of other people's points of view. Its all right bro. You can be small.
  9. Dec 10, 2003 #8


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    Yep, that settles it. I think you are definitely the dumbest new member we've gotten here in a while.

    - Warren
  10. Dec 11, 2003 #9
    Well, seeing

    as how you cant think, I guess your opinion doesnt matter, or count. First prove that you are intelligent, and maybe, just maybe, i will consider anything you have to say.
  11. Dec 15, 2003 #10
    Ok calm down, your question is about volt or voltage so here are several classical definitions from www.dictionary.com

    volt1 n. Abbr. V
    The International System unit of electric potential and electromotive force, equal to the difference of electric potential between two points on a conducting wire carrying a constant current of one ampere when the power dissipated between the points is one watt.

    \Volt\, n. [After Alessandro Volta, the Italian electrician.] (Elec.) The unit of electro-motive force; -- defined by the International Electrical Congress in 1893 and by United States Statute as, that electro-motive force which steadily applied to a conductor whose resistance is one ohm will produce a current of one amp[`e]re. It is practically equivalent to 1000/1434 the electro-motive force of a standard Clark's cell at a temperature of 15[deg] C


    n : a unit of potential equal to the potential difference between two points on a conductor carrying a current of 1 ampere when the power dissipated between the two points is 1 watt; equivalent to the potential difference across a resistance of 1 ohm when 1 ampere of current flows through it
  12. Dec 16, 2003 #11


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    Damn, how did I miss this thread?
  13. Dec 16, 2003 #12
    Hey, I thought I was the dumbest new member. Anyway.

    Here is what I was tought as a way to visualize V=IR.

    Voltage is electrical pressure
    current is moving electrons
    resistance is the opposition to moving electrons

    There have been a few examples using water, let me give you another.

    You have a garden hose, one end connected to a valve the other end is open.

    With the valve shut there is a large amount of pressure(voltage) on the valve but because the valve is shut(infinate resistance) there is no water(current) flowing. This says we can have voltage without current flow.

    When I open the valve water startes flowing through the hose because I lowered the resistance (I=V/R). This current flow is now only limited by the hose diameter(wire resistance) and water pressure(voltage)

    Now put your foot on the hose midway. You will notice the harder you step on the hose(resistance) the less water flows out the open end. If you step on the hose real hard(infinate resistance) then you will stop the flow of current yet there is still pressure on the side of the hose connected the the valve but not on the other side of your foot where the hose is open. You could say that your foot dropped pressure the same as a resister drops voltage.

    Hope this helped.
  14. Dec 22, 2003 #13

    Would "two" resistors "divide" a circuit into "two" sections? Seems more like three sections. I don't know either, and the replies here are of no help, so I will go and look it up and get back to you!

    What I can tell you is: voltage is the name given to the electric field that occurs when there is a charge imbalance. There are more electrons in a storm cloud than on the earth; that's an imbalance. There are more electrons than protons on the negative terminal of a battery... Voltage is another word for electric field. It's strength is tallied up by finding the difference between the potential energy an electron has at the ground end and the potential energy it has at the beginning (or any two points inbetween) and is expressed in units of "volts". If it helps: you have, say, 9 volts of potential speed standing on the roof. You have zero volts of potential speed standing on the ground. What is the voltage between the roof and the ground. 9 minus zero is 9 volts! What is the voltage between the roof and half way down (say, a ledge you might land on)? 9 minus 4.5 is 4.5 volts!

    Current is the same everywhere in the circuit because applying resistance to one part of the circuit is like applying resistance to one part of a spinning wheel; the entire wheel slows down, not just the part around the brake.

    Hope that helps.
  15. Dec 22, 2003 #14
    Whether a material is a good conductor or resistor of current depends on the valence shell of the atoms in that material. The valence shell is the outer most shell of electrons in an atom. If the valence shell has few electrons (1 or 2) such as copper then the electrons are not held tightly and the atoms give up electrons easily. This is why copper, gold, aluminum are good conductors.

    Conversely, the atoms of an insulator have valence shells which are full or nearly so. The electrons in this situation are held tightly and thus current (movement of electrons) is not easily accomplished.

    Voltage is most often referred to as a difference of potential. In my opinion this is the best name. You may run across the term electro-motive force as another name for voltage but this is a misnomer. Voltage is not a force as defined physically. A battery or cell has an excess of electrons on one terminal (negative) and a deficiency of electrons on the other (positive). Thus the term potential difference. The electrons on the negative terminal will be attracted to the positive terminal so that a neutral state will be achieved. Just as in a static charge, if one material has a negative charge (excess of electrons), the electrons will be compelled to go to where there is a lack of electrons (positive charge) to achieve an overall neutral charge.

    Current, simply put, is the flow of electrons. If you have more electrons(Higher voltage/potential difference) more electrons will flow and current will be higher given the same resistance. The higher the resistance the lower the current because less electrons are able to move in the circuit. In a series circuit there is only one path for the electrons to flow and so the current is the same anywhere along that circuit. The current multiplied by the total resistance will equal the source voltage. (E=IR).

    A resistor is made out of a material that has less free electrons in the valence shell of the constituent atoms. That is to say, the outer shell of the atoms have more electrons and are held more tightly then is the case with conductors. They fall between conductors and insulators. Ideally conductors have 0 resistance and insulators have infinite resistance. In reality, conductors and insulators do have a definite resistance but normally, in small circuits, they can be ignored.

    As stated before, a resistor does not drop the voltage as in lowering the value. Voltage drop in this sense means a difference of potential is developed across the resistor. In the case of one resistor, the entire voltage of the source is seen across the resistor. This is easy to see because the single resistor is parallel with the voltage source. If you measure the voltage across the resistor you are measuring across the source as well. In the case of 2 or more resistances in the circuit, each resistor will develop a difference of potential depending on the value of each resistance. The sum of all the 'voltage drops' must equal the source voltage because the source voltage is applied across the entire circuit. If you measure the voltage across all the resistors you are measuring the source voltage.

    In the case of a parallel circuit, say three resistors in parallel, the current is not the same in all parts of the circuit. The current in each parallel branch will depend on the resistance of each branch. The voltage in each branch, however, is equal since the source voltage is applied across each resistor. The total current will be the sum of the individual currents in each branch. The total resistance will be lower as more resistors are added in parallel simply because there are more paths for the current to flow.

    Mathematically: Rt=1/(1/r1)+(1/r2)+(1/r3)...

    If all the resistors are the same value, then the resistance of one resistor divided by the number of resistors will give Rt. Rt=R1/n

    Hope this helps a little.
  16. Dec 22, 2003 #15
    Voltage drop in this sense means a difference of potential is developed across the resistor. In the case of one resistor, the entire voltage of the source is seen across the resistor. This is easy to see because the single resistor is parallel with the voltage source. If you measure the voltage across the resistor you are measuring across the source as well. In the case of 2 or more resistances in the circuit, each resistor will develop a difference of potential depending on the value of each resistance. The sum of all the 'voltage drops' must equal the source voltage because the source voltage is applied across the entire circuit. If you measure the voltage across all the resistors you are measuring the source voltage.

    This post made sense to me; but there still some things I haven't straightened out. Well, I can hear someone saying, "if there were no resistors and you measured the volts across the same two points then you would get 12V. Put a resistor between those two points and you will measure 12V. How does placing a resistor of equal value somewhere else on the circuit now make your measurement less?"

    Here is how I imagined explaining it:

    Think of the entire voltage drop. Falling through the whole thing is falling through one resistance. Falling through two resistances is falling half way through, to 6 volts in a total 12 V drop and then falling the rest of the way through the last 6 volts. If one resistance is greater than the other, it's like having to fall a greater distance, say, 8 volts. Hence the larger value resistor sees a greater voltage drop than the smaller value resistor. It helped me to think of the voltage between the sky and the earth during a storm (or anytime, I guess).

    Does that make sense? Many thanks.I still feel I could elaborate more, but I haven't worked it out, yet.
  17. Dec 23, 2003 #16
    I think are getting confused because there is no difference in measurements between 0 resistors and 1 resistor. That doesn't mean if you add more resistors, the voltage will stay the same across one resistor. If you have two, the voltage stays the same across both taken together but is divided between them individually.

    When you add a second resistor, the difference of potential on the first resistor will have to decrease because you now have a voltage across the second resistor that you have added to the circuit. If the voltage (12V) didn't decrease across the first resistor, then that would mean you would be measuring 12 volts across both resistors. If the voltage didn't decrease across one resistor when you add more resistors, then you might as well say that the voltage would not decrease across any resistor. This just doesn't happen. You would effectively have two potential differences of 12V in series which would total 24V. That would be quite amazing as you have created an extra 12 volts out of nothing.

    Look at it this way: When you add the second resistor, you will have less current. There will be less electrons flowing in the circuit. That means that the potential differences across each resistor will be less. E=IR. If you have less current through the same resistance, the voltage will be less.

    I guess the real question is 'why do resistors develop differences of potentials in the first place?' Well consider a simple circuit with a battery, a wire, and a resistor. In the wire there are more free electrons than in the resistor. When you close the circuit, electrons will flow. Because less electrons can flow through the resistor, you end up with an excess of electrons on one side of the resistor than you do on the other side.
    Last edited: Dec 23, 2003
  18. Dec 23, 2003 #17
    I see. With just one resistor, the electrons on one side are those electrons that would be on that side anyway without a resistor. With two resistors you could still say that the potential difference between the source side of the resistor and the ground is 12V, but the potential difference between the source side of the first resistor and before the second resistor is different because one side is 12 and because, I suppose, of the excess of e's at the second resistor, the potential there is, say, 4 so that the potential difference across the first resistor is 8. Hmmm?
  19. Dec 23, 2003 #18
    That sounds right to me. I was hoping more people would join in on this little discussion. It's been about 15 years since I studied this in school. My memory was a little cloudy. It's been a good refresher for me. I enjoyed it.
  20. Dec 23, 2003 #19

    Right, this has been great. I arrived at the understanding I wanted and it's extremely rare to find someone in the know who is willing to bear with you particularly when you're way of asking questions isn't exactly orthodox (too many "schooled" people as oppose to "together in the head" ones). In other words, I'm much obliged. I started looking at this stuff seriously last about 2 years ago now. It takes time to both learn and unlearn all the false notions one has about how things work (for example, how many folks think electricity is caused by electrons zipping through a wire at light speed rather than just waves moving through the sea of electrons in the conductor...? This is taught in school with all rigor!) Since I'm keen on engineering stuff, I want to know the physics of things. I want to build a Theremin which means getting the hand of how circuits work, how they're engineered so that a particular combination of components does this and that to the electric current... I figure that by teaching myself to build this instrument I will teach myself more and faster than just perfunctorily poring through a text book. I use the text books, to be sure, but not in the way schoolers use them. But I ramble...
  21. Dec 23, 2003 #20
    I tried to make my answers as uncomplicated as I could while maintaining essential correctness. I understand the math behind electronics/electricity very well but to be honest, I had to really stretch my brain to visualize what is actually happening when a voltage develops across resistors. Especially when dealing with more than one. I searched all evening on the internet and in books I own for a physical description but just couldn't find any good information. Just the basic E=IR and the typical info one finds when doing a search for resistance and pontential difference.

    I'm glad you found it helpful. Have you studied AC theory at all?
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