Current

Discussion in 'General Electronics Chat' started by tahir51214, May 6, 2013.

  1. tahir51214

    Thread Starter New Member

    Nov 11, 2011
    29
    1
    In all my text books current is categorized as base quantity. They define it as rate of flow of Charge. So should it not be Charge, that has to be categorized as base quantity instead of current. Another thing Current is defined in Scalars although in all literature we are taught that net direction of flow of charges is defining current. It is not an issue whether it is conventional or electronic. But my question is why current isn't a vector and is a base quantity?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    How does the textbook define "base quantity". Certainly Charge is what is transported in a Current, but Current is fundamental to virtually all electrical devices.

    A Vector is a 3-dimensional value in space. Current is generally confined to 2-dimensions so is considered a Scaler.
     
  3. tahir51214

    Thread Starter New Member

    Nov 11, 2011
    29
    1
    sometimes vectors are two dimensional quantities like Area Vector used in electrostatics and electromagnetism not confined to space... I may be wrong but this is what I have studied..
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Scalar has only one dimension. Theoretically current can be a vector, for example mapping current flow through a conductive object. But since we usually only talk about wires and similar point to point connections, then current is treated as scalar.

    Take a river for example, you have different velocities and directions of water flow near the river bed than near surface, but the net flow is one number, say 300m3/s.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    The first and most obvious question is what subject are we talking about in your textbooks. Electrics/electronics or physics?

    To an engineer charge is not a very useful quantity, current is.

    To a physicist charge is a more fundamental quantity, but many effects only occur when charge moves. And moving charge constitutes a current.
    So both often take current as the fundamental quantity.

    Can you think of effects that require moving charge?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    Of course you are correct. It's been too long since I've thought about scalars and vectors.:p
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Charge is a scalar.

    Direct current (DC) is a signed scalar

    Alternating current (AC) is a vector.
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    As others have pointed out, in general current is a vector, but when talking about current that is confined to a conductor it is nearly always sufficient to treat it as a scalar in which the direction is taken care of by the sign and any further details pertaining to the direction can be obtained from the routing of the conductor.

    Current is a base quantity that is what we measure to produce a standard. This is (or was) done in the SI system using a current balance. The standard reference thus produced a current as the fundamental quantity and the unit of charge is defined in terms of that, namely 1C = 1 As (ampere-second), or the amount of charge that flows past a given point in 1 second if the current current is 1 ampere.
     
    tahir51214 likes this.
  9. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    As others have pointed out, current is flowing along a wire and therefore only the scaler needs to be accounted for; the direction can be positive or negative.

    A base quantity is the smallest division of a unit, such as grams for mass or Newtons for force (although it is defined in kg/m2). Current may be defined as the flow of charge, but it is a little more than that. You can charge a capacitor and say it has so many coulombs of charge. If you discharge the capacitor through a high resistor, you have a small current. If you discharge the capacitor through a low resistor, you have a large current. The amount of charge (alone) will not define the current. I know this is confusing, but I can't quite get the right thoughts together.
     
    Last edited: May 6, 2013
  10. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    If grams are the smallest division of mass, then what is a milligram or a microgram?

    And notice that the OP said that "current" was a base unit, not "amperes". I take from that, and the OP can correct me if I'm wrong, that he is referring to the base units of a measurement system, most likely the SI system in which the base units are meter for length, kilogram for mass, second for time, and ampere for current. All other physical quantities can be expressed in terms of combinations of these four units (things at the quantum scale possibly notwithstanding.
     
  11. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    OK, I shouldn't have stated "smallest" division. And no wonder I was confused, you are right in that the base quantities are length, mass, time and current.

    See http://en.wikipedia.org/wiki/Physical_quantity for a list of nine base quantities.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    tahir has asked a very reasonable (and quite important) question that no one has yet answered and we are moving away from.

    There are historical reasons for the modernday choice of current rather than charge as the basic electrical unit, which I was leading up to in my post #5. It was not always so.

    tahir, if you are still interested, please have a go at my question - it is relevant.
     
    tahir51214 likes this.
  13. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    Huh? I specifically addressed it in Post #8. We define charge in terms of current because it was more accurate to use a current balance to establish a standard current. It was much harder to accurately measure and maintain a standard charge.
     
  14. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    I see no reason given in post#8, just a straighforward statement of the format "that is the way we do it".

    I see no problem whatsoever with an accurate absolute determination of the coulomb or the ampere by electrochemical means. Both this method and the MKS method require a force balance.
    I agree the esu method was fine in principle but more difficult in practice.

    The full history is more complicated.
     
  15. tahir51214

    Thread Starter New Member

    Nov 11, 2011
    29
    1
    So we should change our definition of fundamental quantities or we should not define current in terms of charge. Because it is derived quantity that has to be defined in terms of base quantity. Here the case seems to be different. Another thing Vectors must be defined in a way to prove current scalar.
    So sorry to ask a Physics question in Electronics forum.. Not knew much about all the forums present on site..
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Hello tahir, did you attempt the my question in post#5?
     
  17. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    Don't confuse lay definitions with international standards. For instance, the definition of the base unit for length is not the distance between two scratched on some chunk of metal in Paris (which is used to be), but rather so many wavelengths of the light produced by a particular transition in a particular atom. Yet, for all practical purposes, we use the meter as the base unit.

    Standards are always changing as the ability to make more accurate and precise measurements evolve. The official standards can shift radically if we learn how to measure something that is presently a derived quantity to a more accurate and precise level than we can measure the quantity it is currently derived from.
     
  18. tahir51214

    Thread Starter New Member

    Nov 11, 2011
    29
    1
    Because of moving charge an electromagnetic field is formed. Which is basis of electrical machinery.
     
  19. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,910
    2,172
    Current defined as the slow directional flow of charge works very well in metals where the charge carrier is the sea of 'free' electrons because the relationship of flowing Charge to moving energy is simple math at DC, but when you try to use 'current' to understand the flow of energy in a vacuum, plasmas, ion beams or batteries things get complicated because Charge and energy are two different things and Charge can move in any direction back and forth but energy can still move forward.

    Electrical energy always flows
    in the space outside of the wires. Electrical energy is coupled to the electron-waves traveling in the conductor but the fields surrounding the conductor actually move the energy.

    For me the key to the physics of electrical energy was
    knowing the misconceptions of current flow and understanding electric charge-flow, and of electric energy flow.
     
Loading...