20 Replies Latest reply on Jan 3, 2015 10:58 AM by D_Hersey

    Current Flow

      What exactly is current, and how does it behave?

        • Re: Current Flow
          supper_slash

          Here is a video, that kinda covers the basics.



          But, current is the flow of electrons in a circuit.  If u looked at your electric meter, and watched it, it measures the flow of electricy (in wats, wich is a calculation of amps, and volts) or (electrons) through the circuits in your home.

           

          If all you are using is a light bulb, the flow would be low. If everything in your house was running at the same time, the flow would be high.

          It is measured in a unit called AMPERE (or amp's for short) and its electrical sign is "I".

           

          I hope this helps.

          • Re: Current Flow
            supper_slash

            Someone correct me if i am wrong, but i believe they flow from positive to ground, or negative.

            • Re: Current Flow
              D_Hersey

              Benjamin Franklin made a guess, and he guessed wrongly, a fifty-fifty deal. Current is charge carriers past a point.  They are usually electrons, but they can be other things like ion complexes, or holes.

              • Re: Current Flow
                D_Hersey

                Another case of current flow can be in the liquid state like in batteries, electrolysis, electrophoresis.  Anions and cations.  Current flow can occur for a finite period in electrostatic situations where stuff is modeled as rotating to a bulk low-entropy condition rather than flowing.  I mean, current is flowing in conductors where it can be measured, but what if happening is like a topping-up with the characteristic logarithmic curves.

                 

                Current is the stuff!  The kinetic.  Notice that the other entity we multiply with current to work out power is potential, a quantity we measure in volt units.  [I am a block south of Volta elementary school as I type we share an alley, I can hear the kids during recess and gym and when school starts/ends.  Alessandro was his prenom.]

                 

                Sidewalks have people currents!  If you are a quantum fan, you believe this quantity is ultimately an integer number.  I leave these considerations to physicists.  I am mostly a hacker of stuff and a coder.  Perpendicular to this current flow will be a magnetic field, and if we squint, a miniature undergraduate student contorting his hand in a way that looks painful, but obviously amuses him.

                 

                Feynman said that nobody had understanding of the fundamental physical forces to an ultimate degree.  Even the physicist has only come to know them as characters in formalisms.

                 

                Attach a battery to a resistor.  Current flows according to ohm's law through the Thevenin equivalent resistance of the battery, the connector drops and the load resistance.  Attach a battery to a capacitor.  Current flows initially then tapers off.  Attach a battery to an inductor.  Current doesn't flow immediately, but ramps up with time evermore so, using the same curve as a falling object.

                • Re: Current Flow
                  D_Hersey

                  A way to make Ohm's law a little more concrete is to ponder its economic analog:  Money equals work times resistance.  In a world where there are few workers, unions  and work-rules are strong, rents and taxes are low, recreation is cheap, private savings and capitalization high, and workers have access to cottage industry and gardens and public libraries and public theater, wages will be relatively high in real terms, otherwise. . .

                   

                  The potential term is called the effort parameter.

                  The current is called the flow parameter.

                   

                  Resistance is the tendency of most stuff to heat up (blackbody radiate IR) when current is forced through it.  Stuff that has a linear dose-response curve in this regard is called ohmic.  The guys who look deepest into these properties are called materials scientists who tell us that most stuff isn't ohmic.  The resistor fab community is ever on the prowl for materials that actually behave in accord with the 'law.'

                   

                  Remember that all passive loads (RLC) are invertible.  And that the community considers this fact so significant that they have accorded other names to all of the inverted terms and elements.

                   

                  Other classes of non-linear resistances would include thermistors and thermocouples, Peltier coolers and neon bulbs.  When the resistance is non-linear or has a weird tempco, we do have to turn to physicists for an explanation.  Getting a neon bulb to turn on takes a higher potential then does keeping it on once 'struck.'  This property was exploited to construct oscillators.

                   

                  If we were testing some material, we could put a potential across it and measure the current that ensued.  This would put potential on the ordinate axis and our working eq would be 'potential equals the current - resistance product.'  But we could, symmetrically, force a current through the material and measure the potential developed across it in an attempt to derive its resistance.  Then, our formalism would be 'current equals the potential - conductance product.'  So, ya gotta kinda mind which are the dependent and independent variables in our test at hand.

                   

                  Potential is measured across two distinct points.  Current is measured through a point.  An ideal voltmeter offers infinite input impedance.  An ideal ammeter offers zero input impedance.  The current test, in vivo, is usually more expensive to conduct than the  potential test, because we must break the circuit in order to insert the ammeter.  So when we can we measure potentials and infer currents.  Potential is a posteriori, current is a priori.

                   

                  I'm no expert on the topic, but my bet would be that most current flow occurs in the plasma phase, celestially.  Figuring out mass-charge ratios in the gas phase is an important form of spectroscopy.   Current can be denominated as moles / second.

                    • Re: Current Flow
                      D_Hersey

                      A consequence of the quantized nature of current is 'shot noise.'   Which isn't really noise, but behaves just like noise.  Consider an audio channel.  We are using an electrical analog of variations in air pressure.  Shot noise is sorta analogous to round-off error.  N puts a hard limit on resolution.  The error doesn't have a tendency, so it sounds like noise.  Obviously we can up our power level, but then we face the phenomenon of thermal noise.  This is proportional to how high we are above 0K, which will increase if we crank things up as a sole step.  This is why some audio folks still use big stuff such as discretes and thermionics in their preamps.  

                    • Re: Current Flow
                      D_Hersey

                      One has to bear in mind power at the same time when thinking about current.  Power is the current-potential product.  My elder son (HF fanatic) lives in Portland Or.  His brother visited him on his own birthday and was gifted with a mobile transmitter/receiver for the six-meter band.  Gets home to  Madison.  Goes to the park and sets up a horizontal dipole.  Calls me on his cell.  I got a current node at my feedpoints and a potential node at my endpoints.  What's up w/dat?

                       

                      Well sonny, neglecting the slightly slower SOL relative to a vacuum in the medium of (neglecting skin effect) your antenna, power is constant relative to time.  If power is constant we can model it as the area of the top of a rectangular board with constant area.  Real solns would have the length and width of the board (think of a common area for concreteness, like a linoleum tile or something) finite.  A long skinny board could require the same amount of paint as a more squarish one.  At zero degrees, all of the energy is loaded into the current and it gets hot there at the node and it is a hazard because you could be burned.  At ninety degrees all of the energy is loaded into potential.  It is a hazard at that node  because you could be shocked.  Think about the forty-five degree locus.  Relatively benign environment there, both edges of the conceptual board are equal in length.  Both params moderate.

                       

                      Thinking about Franklin's experiment, it takes work to separate charge, as it does to change the magnetic bias of a core.  Modern pedagogy has it to teach capacitors and charge before resistors.  It is helpful to think of the power implications of changes to potential and current.  A current has to flow to change the potential across a reactor.  Energy storage in a capacitor is ((V^2)*C)/2 IIRC. A coulomb of charge is an ampere-second is about 6.241×1018 electrons.  Energy storage in an inductor is ((I^2)*L)/2 by symmetry.  A one farad capacitor stores a Coulomb if there is a potential difference of one volt across its terminals.  A similar amount of energy would be stored in a one Henry inductor if a current of an ampere were flowing through it.

                      • Re: Current Flow
                        supper_slash

                        WOW. easy question, has gone DEEP!

                        very interesting tho.

                        • Re: Current Flow
                          D_Hersey

                          One thing a current does is establish a perpendicular (to the conduction) magnetic field.  If the current changes, the magnetic field changes and this can be 'picked up' by another conductor, so long as it isn't perpendicular to the original conductor.  Induction breaks the symmetry when we discuss Ls and Cs.

                           

                          Also, changes in the magnetic field in the proximity of a conductor induce current in said conductor.

                           

                          One type of coil spirals wire to concentrate the magnetic field.  A coil with a magnetic core would be more volumetrically efficient, but prone to saturation.  "Air doesn't saturate."  But even if the wire was straight, some inductance would associate with it.  This fact coupled with a reciprocal one about the charge field determines the speed of light in a vacuum, which pertains to a limit upon information flow as well as material transport.

                           

                          Magnetic fields diminish in intensity as the square of distance.

                          • Re: Current Flow
                            D_Hersey

                            This leads us to the consideration of a special device, the EMP bomb.  Suppose we made a special capacitor.  What's special is that the dielectric layer is made of UV sensitive HE.  We charge the capacitor.  Energy is V*V*C.  We detonate the dielectric.  If we get the geometry right, this forces the plates apart rapidly.  Parallel plate capacitor capacitance is proportional to plate area / distance.  Distance increases, C gets smaller.  But the electrical energy didn't evanesce!  V*V must increase proportionally by law!  So, we have a voltage spike capable of wrecking Hi-Z input stages all over the neighborhood!

                             

                            When you energize a reactor, it gets a little smaller.  Opposites attract.

                            • Re: Current Flow
                              D_Hersey

                              It all gets cleared up once you consider the space-time vectors:

                               

                              http://www.reciprocalsystem.com/bpm/bpm20.htm