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Please explain Ohm's law

  1. May 30, 2014 #1
    I play music on occasion so I am familiar with using Ohm's on amplifiers. However, I have never quite understood how they work or what they are. Without taking a course in electronics and physic could someone help me understand what an Ohm is and how it functions in the electrical world.

    Also, Ohm's law I=V/R...how could I the "current" possibly be the DIVISION of voltage over resistance? And not only the division in this formula but all formulas, I've always been mystified how many formulas use division when it seems out of place.

    Thank you
  2. jcsd
  3. May 30, 2014 #2

    Larry Gopnik

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    Firstly, Ohms and Ohms Law are different things. A Ohm is merely the unit of resistance.

    Ohms law essentially means that in an "Ohmatic Conductor" - say a normal wire - resistance is the same, so that the the current in the wire and the voltage in the wire are directly proportional, so when VOltage goes up in the wire, the wire's current also goes up. Ohms law does "break" after a certain time in a wire, that's when the wire heats up so the resistance goes up.

    V=IR, (or I = V/R) shows mathematically that when current goes up, so does voltage. I don't understand your problem with division, but that's just me. I am sure that other members on this forum can easily explain why division is so important in Physics - I just think "It just is!" haha

  4. May 30, 2014 #3


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    Per wiki: Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship: I=V/R.

    In other words, Ohm's law states a relationship between current, voltage, and resistance in a circuit. Changing any of the variables necessarily changes at least one other. For example, increasing the voltage while keeping the resistance the same also increases the current. The "division" here is a means of finding the current by dividing the voltage by the resistance in ohms. You have to divide voltage by resistance because resistance opposes current. So if you increase the resistance of the circuit while keeping the applied voltage the same, the current decreases. Mathematically this is represented by division.'

    Note that this relationship is about finding quantities. Ohm's law does not define what resistance, voltage, and current are. It merely states that the quantities of each are related. So the definition of current isn't voltage divided by resistance, it is the flow of electrical charges. The quantity of charges passing through a point in the circuit per second is measured in amperes. This quantity is what the I in Ohm's law represents.
  5. May 30, 2014 #4
    Since you are an amplified musician who "uses ohms on amplifiers" I'm confident I can help you understand better what is going on but perhaps unfortunately for you it is not as simple as Ohm's Law. When you say you "use ohms" I'm betting you are referring to speakers and labelled connections on the back of your amplifier.

    Let's start with basics. Ohm's Law is a beautiful expression of the linked interaction between all the variables in the most basic of circuits - A driving force or pressure (voltage), a sort of volume in motion making work possible (electronic current), and a resistive load, doing the work. Change any one of these elements and the others change too. Ohm's Law lets us calculate what those changes will be.

    Now for your amp/speaker system and how it varies from a basic circuit. A straight wire presents a fixed resistance at a given temperature based on it's composition and also it's geometry - ie: a solid wire will have a different resistance than a stranded wire, largely because electrons tend to not travel through a wire but over it's exterior surface, it's "skin", if you will. More surface area translates into more electrons available thus reduced resistance to flow. This last bit is oversimplification but it should do for the basics.

    How your speaker system is different is that the load, is a wire (actually in most cases a very special kind of edge-wound ribbon wire) that is wrapped in a coil. This coil which is an electromagnet when current flows through it, surrounds a permanent magnet. When the poles align it moves one way. When they are in opposition it moves the opposite way. Since the coiled, electromagnet is connected to some sort of diaphram, usually a cone, this structure acts as a piston creating oscillations in the medium, usually air, that mimic the oscillations of the current flow in your amplifier, which mimics the oscillations in your instrument. Thus the speaker can be said to reproduce the sound, the frequency and amplitude variations of your instrument.

    The problem is that the coil not only has the fixed resistance of the same wire laid straight, but once wrapped into a coil and surrounding a magnet the system becomes considerably more complex. Without getting too deep just yet, suffice it to say that this form of resistance (called inductive reactance or simply, impedance) varies with frequency.

    So you may have a speaker system that is labeled 8 Ohms. If you measure it with a DC voltage as all basic VOMs do, you will perhaps be surprised to read it commonly somewhere between 5 and 7 Ohms. The 8 Ohm label is a functional average. An 8 Ohm speaker may be at 20 Ohms below 50 Hz, which is one reason why it takes more power and larger speakers to reproduce bass frequencies at a given sound pressure level, commonly and incorrectly referred to as "volume".

    I could write for days on this subject and some people actually have, but I should probably stop here and await some feedback from you. I hope this makes sense to you because it is truly a peaceful pleasure to understand how your instruments actually work and that often includes amps and speakers since in musical instruments exact reproduction isn't always desirable. Sometimes that esoteric "color" and "feel" is more important. These too can be expressed in mathematical terms by people who comprehend both the Art and the Science of musical instruments. I encourage you to continue in your quest to understand yours more fully. An artist should have intimate knowledge of his tools.
  6. May 30, 2014 #5


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    Yes. This is just one of the many formulae we use in Science and Engineering, linking three quantities. If you know two of them then then we can work out the third one. (That's just Maths)

    PD = Current X Resistance
    Force = Mass X Acceleration
    Power = Force X Speed
    Distance = Speed X Time
    Force = Spring Extension X Stiffness
    etc. etc.

    Ohm's law, itself, is a result of the fact that the Resistance of Metals (only) is the same over a range of currents, just as Hooke's Law only applies to springs made of certain materials (steel - luckily).
  7. May 30, 2014 #6


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    If you think of Voltage as the "pressure" of the electricity then it might make more sense.

    The greater the pressure (voltage) the higher the current flow.

    The higher the circuit resistance the lower the current will be.
  8. May 30, 2014 #7
    It might help to think of the resistance as a "derived" quantity, defined as the reciprocal of the conductance.
    I = VG, where G is the conductance.
    Current and voltage are "real" quantities in that current is actual charges in motion, and voltage is a difference in the electric potential. G is just defined in such a way that the equation is true, and R is defined such that
    I = V/R is true
    We define R from I and V, so it is always true the I = V/R.
  9. May 30, 2014 #8
    Very interesting!!! thank you all. I am still left wondering about the idea of division in formulas though. I may move this question to a different section of the site but how can an abstract concept be divided by another abstract concept? Numbers that divide, perfectly logical. Concepts that divide, however, do not make sense to me. Please elucidate.
  10. May 30, 2014 #9
    Numbers are a concept. But as you say, this is probably best for a different discussion.
  11. May 31, 2014 #10
    There is nothing to elucidate. Perhaps a more familiar example will help. The area A of a rectangle is the length of the base B times the height h

    A = Bh

    which gives us

    B = A/h.

    There is the division. If you're OK with multiplication of concepts than you're necessarily also OK with division of concepts.
  12. May 31, 2014 #11


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    If you go back in history, 'Science' was discussed without the use of Maths at all. the relationships between tangible quantities (that is to say, familiar things that were already accepted) were described in qualitative sentences. This proved to be very limiting and the language of Maths was then introduced as a shorthand way of describing the relationships in more detail. Then, by the logical process of symbolic manipulation (with algebra etc.) other relationships were noticed.
    To ask for a 'physical' interpretation of many of the mathematical statements that are made about Physics is to take a step backwards. It's not all bad, though. Once someone clever than you (/us) has done a bit of smart manipulation, you often find a new relationship emerges, with a simple formulae and we can then start to appreciate. But we can't always expect to back track and link to the really obvious stuff.
    It may be a useful exercise to examine some of the more familiar three-term equations that you know and ask yourself whether there is not an equivalent to 'Resistance' in there - but you just accept it because you know it so well.
  13. May 31, 2014 #12


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    Concepts that divide? I don't know what that means. V, I, R are not "concepts". When you do a simple Ohm's law experiment in intro physics labs, you are not plotting concepts when you measure V and I at various values. You are actually plotting actual numbers! Ohm's law gives the relationship between those three quantities at particular values!

    There's also a puzzling situation here. Since this wasn't brought up, I presume that you have no conceptual issues with "concepts that multiply". Does that mean that you don't have a problem with V = IR? After all, there's nothing dividing here.

    Secondly, if you have a problem with I = V/R, what if I introduce a new "concept", called it S, defined as S = 1/R, the conductance (units: Siemen). Then your equation becomes

    I = VS.

    Nothing is dividing here, because I can measure S directly in experiments. So are you now conceptually OK with this SAME, identical equation that you had a problem with earlier?

  14. May 31, 2014 #13


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    I really don't think it's as easy as that - especially if you want to feel that you are getting things from scratch. All three of them may be measurable on a 'Meter' but not one is as tangible as a Force or a Temperature. They are, in fact conceptual things, rather than concrete quantities.

    You and I (and all the old lags) are so familiar with Volts and Amps - and Resistance, that we can take them for granted. But, when you think of the number of posts with titles like "What are Volts?". "What is Electric Current?" then trying to get comfortable with:

    (Something you don't quite get) / (something you don't quite get) = (Something else you don't quite get)
    can be quite bewildering.
    The only answer is to be educated in such a way that you either just accept those three things and deal with the 'problems' later OR your path through the subject takes you step by step and everything gets defined for you in a sensible order.
    Frankly, I had to do it the former way because the latter would have just been too boring.

    The bottom line is that you need to accept that Maths will give you the answer. Then sheer familiarity will take you along to the next step. Despite what people say on this forum, that pragmatic approach will take you a long way. And practice makes perfect.
  15. May 31, 2014 #14
    Perhaps some of the disconnect here is in the definition of "concept". It is not necessarily equal to "abstract" and certainly should never be assumed to be merely imaginary (in the real world sense of having no substance nor connection with reality , not like "imaginary numbers" which are sort of real). Concept just means how humans can visualize things, often meaning breaking systems down into component parts.

    That the longest line in a Plane Geometry triangle is the hypotenuse and that it squared is equal to the sum of the other two sides squared is a concept, but it came about from observation of all, real, right angle triangles.

    Voltage is a differential, a relationship between two points so within a complex circuit there are many voltages based on which two points one chooses for reference. That pressure may also be a concept but it is by no means imaginary. It can do work and one can calculate exactly what work it can do and be correct. Even if it is not employed it still exists as a relationship, a potential. The reason, or at least a fundamental block in that structure, we can calculate so accurately is Ohm's Law. He didn't pull it out of some nether orifice. He observed and measured it repeatedly until a pattern emerged.
  16. May 31, 2014 #15


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    In this case perhaps think of it as...

    I is proportional to V
    I is proportional to 1/R

    "Resistance" is a reasonably simple concept. Take football, the more resistance the defensive team provides the lower the score by the offensive team. The more resistance the brakes on my car provide the slower my car goes. In all cases the relationship is similar. The larger the R the smaller the outcome. Division is just a way of indicating an inverse relationship.

    Perhaps it's clearer to write..

    I = V * 1/R


    I = V/R

    although it amounts to the same thing.
  17. May 22, 2015 #16
    Ohm law simply explains relationship between Voltage and Current. There is a direct relationship between both these quantities if we keep the Resistance constant. In case resistance is not constant, then the amount of current flowing through any conductor is directly proportional to voltage over that resistance value. Also, have a look over plot of Ohm law.http://eecottage.blogspot.com/2015/05/ohm-law-experimental-verification-of.html
  18. May 22, 2015 #17


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    Old thread alert.
  19. May 23, 2015 #18


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    LOL indeed. I wish people would look at the age of the thread before posting
  20. May 23, 2015 #19


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    For an old chestnut like this, I can't see that it matters, hugely, that threads are resurrected. Nothing has changed since the very first ever PF post on the topic. All my text books are a lot older than PF yet they have the same message about such a fundamental subject.
    It could be different for a thread on Dark Energy or the latest Solid State Electronics technology.
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