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Ohm's Law Conundrum

Discussion in 'General Electronics Discussion' started by Flurng, Dec 29, 2012.

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  1. Flurng


    Dec 29, 2012
    Howdy, gang!
    So, I've got me a few questions regarding Ohm's Law. I know - what could be simpler, right? Well, actually it's more about how one value affects the others.
    I mean, I understand that E = I*R, I = E/R & R = E/I; easy kid stuff, really. What I DON'T get is - when you change one quantity, which other quantity ( quantities ) change & why?
    For example: let's say we have a circuit with 1 amp at 1 volt & 1 ohm resistance. Beautiful!- Ohm's law is happy & we're all good. But now, suppose we swap out our 1 ohm resistance for 2 ohms. Now, it seems to me there are 3 possibilities: A. Voltage changes ( doubled ) B. Current changes ( halved ), or C. BOTH values change to some degree.
    O.K.- so, here's my primary question: when we change one value ( either E,I, or R), which other value(s) change and, more importantly, WHY?
    Next question: can we predict and/or CONTROL which value(s) change? And, if so, HOW?
    You see, I've read plenty of electronics text forwards & backwards, but none of them really seem to address this question, which has been bugging me for years! Can anybody share some insight on this matter? Any information at all would be most appreciated! Thanks in advance, folks & s'long for now!
    Hasta, La Vista, Baby!
  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi flurng
    welcome to the forums

    well if you swap the resistance for a 2 Ohm resistor then you halve the current so you will only have 500mA. You didnt say you changed the voltage, so its still 1V so ...
    1V / 2 Ohms = 0.5A

    R isnt going to change unless you physically swap the resistor so.....
    changing V will change I
    changing R and keeping V constant will change I
    WHY? = you cant vary I unless you vary one of the other 2 parameters.
    I is totally dependant on the values of V and R

    yes you can control the current in a circuit by varying the resistance as the example above has shown you
    or less likely to be used you can keep the resistances fixed and vary the supply voltage and thereby vary the current flowing
    there are ways of using components like voltage regulator IC's to provide constant current supplies
    most regulator datasheets will show an example of that configuration

    Last edited: Dec 29, 2012
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Think of it like distance traveled, fuel used and amount of fuel used per kilometer.

    just like R = V/I for (R)esistance, (V)oltage, and (I) current

    R = V/I for (R) Km/l, (V) Km travelled, (I) Litres of fuel.

    Resistance is just a ratio that describes the current that will flow for a vertain amount of voltage placed across it. You can change it by (say) getting another resistor.

    For fuel consumption, that is determined by lots of things, but let's say it is mostly determined by the type of car. To change the fuel economy (resistance), you sell your Hummer and buy a Prius.

    As described above, Ohms law just describes a relationship between some things, they're not all independent.
  4. Laplace


    Apr 4, 2010
    Look at your circuit and determine whether the current flows from a current source or a voltage source. If the current comes from a current source, then the current will be constant and the voltage will change with the resistance. If the current comes from a voltage source, which is the usual case, then the voltage will be constant and the current will change with the resistance.

    Now if you have a variable power supply that can operate in either voltage mode or in current mode, then you can vary the voltage or current of the source. But once set at the power supply the source will remain constant. So that is how you determine whether the voltage or current remains constant in Ohm's Law.
  5. Flurng


    Dec 29, 2012
    Many Thanks!

    Thanks so much to all those who have shared their invaluable insights regarding this problem! ( And so quickly, too - Wow! ) I had already guessed that resistance would be more or less "concrete", being a specific characteristic of a physical component, but the whole "voltage supply" vs. "current supply" issue is still a bit fuzzy for me. At any rate, you've all certainly given me plenty to think about, and pointed me in the right direction! Thanks, once again, to each of you, and a very Happy New Year to all! S'long for now!
    Flurng :^)
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Yes, you're right that in most cases, resistance is determined by characteristics of a physical component, and is pretty much constant. A component of this type, such as a resistor (obviously), is called "ohmic", and its resistance determines the ratio between the voltage across it and the current through it.

    In a very simple circuit, such as a resistor connected across a battery, the applied voltage is relatively constant, and changing from one resistance to another will cause the current to change. For example, halving the resistance will double the current. (In the real world, the amount of current WILL affect the battery voltage somewhat, so it's not QUITE that simple, but that's a useful approximation in may circumstances.)

    There are also cases where the CURRENT, not the voltage, is determined by some factor external to the resistance; in this case, the current is (relatively) constant, and doubling the resistance will cause the voltage dropped across the resistor to double. This is not a very common situation.

    Edit: This situation DOES occur when the resistor has a relatively small value and is inserted in series with a current flow as a SHUNT so the current flowing in a circuit can be measured by measuring the voltage across the shunt resistor. In this case the resistance is chosen so that relatively little voltage is dropped across it, and the current is relatively unaffected by the presence of the resistor. In this situation, it's more accurate to say that the current is independent of the resistor value, and changing the resistor value affects the voltage across it.

    In most cases, there is an interaction between all three quantities, and other formulas are used to calculate quantities. A good example is a voltage divider; that is, two resistors in series, connected across a fixed voltage. The total (fixed) voltage is DIVIDED between the two resistors, according to the RATIO between their resistances, and the current flowing in the resistors (which is the same for both, as they're in series) is determined by the SUM of their resistances.

    I've created an analogy that I call the DTS analogy which may help you understand these three quantities in a more intuitive way. It may not, of course! But I'd be interested to hear your feedback on it. I described it in
    Last edited: Jan 1, 2013
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