Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conductance and resistance

  1. Jul 4, 2011 #1
    Hello EE forum !

    I would like to talk about conductance and resistance. I understand both terms. And formulas that they derived from.

    First newbie question: Each element has conductance and and resistance(admittance)? Probably yes, just checking.

    Second question: In process of transforming current generators to voltage generators, conductance term is used. Here is what i mean:
    [PLAIN]http://pokit.org/get/bdeffacfa7c89d3782ea7e987f740473.jpg [Broken]
    [PLAIN]http://pokit.org/get/05ecbd73c068c173060cf294371735fb.jpg [Broken]

    Now I get the analogy of this but, why is conductance used in the first place? I've seen this everywhere, where current generators are written like this, with conductance multiplied by voltage. Especially in the transistor models.

    Now why is this written like this? Why not simply with resistance and voltage?

    I mean this bothers me probably because I am new to this term, and I find it pointless because the relation between conductance and resistance is simply inverse proportion. Is there any physics behind it? Because current generators work like this? Any thoughts?

    Thanks
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 4, 2011 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    For a start, it's totally arbitrary whether one uses Resistance or Conductance in any problem. We happen to use Resistance, mostly because it fits many situations. However, where one is dealing with parallel circuits - say a mains distribution system, it could be a great advantage just to add the conductances of all the connected loads without launching off into the 1/R calculations.
    Most electrical power supplies tend to approximate to Voltage sources so that goes in favour of looking at most circuits in those terms.

    It tends to be a matter of horses for courses but the courses you come across early on tend all to favour Vs and Rs rather than Is and Gs.

    In complex antenna design it is quite common to use an Admittance Matrix rather than an Impedance matrix. (At least, I had a colleague who was always hopping from one to the other when feeding and matching multiple VHF transmitting elements for tailored broadcast antenna radiation patterns.) HE was a wow with the Smith Chart!
     
  4. Jul 4, 2011 #3
    I think i understand. Conductance gave me a headache. I know its not anything complicated but I am so fond of resistance and voltage that I found it difficult to accept. Thank you for your reply.
     
  5. Jul 4, 2011 #4
    {Conductance and Resistance} ; {Admittance and Impedance}

    are reciprocal pairs.

    Use the first for DC the second pair for AC. The second pair are frequency dependent and reduce to the first at zero frequency.

    Conductances are usually used where vey low resistances are involved as this avoids working in milli or micro ohms.
    You will find them in earth conductance measurements, solution chemistry, fish tanks and so on. that is they are not usually used by electrical engineers.

    Admittances on the other hand are used frequently as many AC formulae which involve division by an impedance can be more conveniently rewritten as multiplication by an admittance - hence the admittance or Y matrix that SC mentioned.

    Bear in mind that we generally wish to avoid division by very small numbers on numerical calculations.

    go well
     
  6. Jul 4, 2011 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I hate to suggest using water to help you with this but this scenario may be worth thinking about. As you add outlet pipes to the bottom of a tank of water, more water will flow. Adding litres per second per kPa would make sense, wouldn't it? That's the equivalent of adding Conductances which have the units of Amps per Volt.
     
  7. Jul 4, 2011 #6
    Interesting, I never looked at the parallel resistors that way. I know that resistors in parallel make a smaller equivalent resistor. You just made me realize a lot of things. Thank you ! You just gave me a lot of to think about.
     
  8. Jul 4, 2011 #7
    Yea that is mainly why I stumbled upon those admittances. Because of the matrix calculation. If I may say that I really don't like that part of EE. I just failed my exam in electric circuits, because I had a error in calculation.

    We got a 3x3 matrix, complex admittances, using method of potential difference between nodes, find currents in all branches.

    A lot of places where u can make small but important calculation errors.

    Thank you for your reply, now I distinguish what I learned.
     
  9. Jul 4, 2011 #8
    The thing to take away from admittance matrices is two port networks. Have you have come across these yet?
     
  10. Jul 4, 2011 #9
    This?
    http://en.wikipedia.org/wiki/Two-port_network

    we call it "Quadripole" here in my country :).

    Yes I am doing some problems as we speak, but I didn't come across any problems. We are doing Z-parameters and Y-parameters. We didn't have time to cover hybrid with some examples but we learned it in theory.

    I find them very fun to solve!

    Third title we are doing is magnetically coupled circuits. They also aren't hard to solve, but as future EE, I will over the summer vacation, do a little research, where I attack this title in deep. I find those coupling very interesting, but I don't have time right now to go into deep physics behind it. I know the basics.

    And those Y and Z parameters, yes I learned that by heart, and didn't bust my head thinking over what is what, why is that that way etc etc. Time is not on my side.
     
    Last edited: Jul 4, 2011
  11. Jul 4, 2011 #10
    Yes that's it, they are very important.

    However find a more useful text than Wikipedia. I think you have already done this from what you have said.
     
  12. Jul 4, 2011 #11
    But You as electrical engineer, can you tell me what's the point of finding these Y or Z parameters? What are they used for? in practice i mean.
     
  13. Jul 4, 2011 #12
    Almost every circuit has an input and an output.

    What is that if not a two port network?

    You can analyse everything from micrcircuits to power stations by cascading such building blocks they ae so basic.

    This was why I was saying that from a circuit point of view you don't need in depth knowledge of the physics of device action you describe it in terms of Z and Y parameters.

    there was an excellent New Zealand textbook by Gatland

    Electronic Engineering Applications of Two Port Networks.

    The part contents read like this

    Small signal paramters
    Two port models
    Matrix reduction
    Tandem connection of two ports
    Feedback amplifiers
    Feedbck oscillators
    Capacitive effects on transistor analysis
     
  14. Jul 4, 2011 #13

    Thank you for your recommendation. I will check that out for sure. I am still in a phase where I mix up what is exactly electrical engineering. I am still obsessed with physics behind everything.
    I always think if you understand something, you must be able to explain it to your grandfather so that he understands too.(I think this is Einstein's quote)

    I am still overwhelmed with tons of questions why, how, when etc.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Conductance and resistance
  1. Ballistic conduction (Replies: 1)

  2. Conductivity Speed (Replies: 4)

  3. Metal conductivity (Replies: 2)

Loading...