What is Electric Current?
Really, when we use an ammeter to give us a value in Amperes, or we measure a voltage drop across a current sensing resistor and divide that by the resistance, what is the actual thing we are measuring?
At their base function, our measuring equipment is really giving us an after effect of the reality that occurs before hand. The d'Arsonval Movement common in analog ammeters and Galvanometers works on the principle of a motor. It converts electric current to a magnetic moment that exerts torque on the rotor as it works against an existing permanent magnet field present in the movement. So if the magnetic field produced around the inductor in the meter movement is an after effect, what causes it?
Most Digital Mulit-Meters (DMM's) do not use this magnetic conversion approach using permanent magnets and inductors. Instead, they use another approach which also is an after effect. They use the principle of Ohm's Law which states the voltage across a resistance is proportional to the current flowing in the resistance - E = I · R where E is electromotive force (aka voltage) I is intensity (aka current) and R is resistance. But again, if the voltage drop across the resistor is a secondary effect, what is the primary effect? What is the real physical thing that we are measuring as Amperes?
Well this is a question that was floating around some 149 years ago when James Clerk Maxwell wrote his paper "On Physical Lines Of Force" which introduced Displacement Current that was later incorporated into the equations presented in 1864 in the paper "A dynamical theory of the electromagnetic field" All together Maxwell had 20 equations with 20 variables. Oliver Heaviside really wanted to understand these equations. He did not have any training in Calculus at the time and set out on a self taught course to learn Calculus and comprehend what Maxwell had given the world.
In Heaviside's work found here: Electromagnetic effects of a moving charge - Wikisource
we find some interesting clues to what electrical current is.
(Bold Mine)
So what is a moving charge? This is a very important question because how it is answered impacts the very foundations of all physics connected with electricity.
A moving charge is literally anything with an electric charge that displaces space during a time interval.
As you may imagine, this begs a frame of reference. After all, the battery sitting motionless here on my desk is electrically charged, and is involved in very complex motion as the earth rotates and revolves and the solar system moves and the galaxy moves etc. Are we to conclude that this 'moving' charge is considered to be an electrical current? And if so, relative to what? The definition therefore implies an observation point. One Ampere of current is that quantity of charge that moves past a given point of observation in one second and this would be equivalent to around 6.242 × 10^18 positrons or protons for a positive conventional electric current or electrons for a negative conventional electric current. This is true because positrons and protons have a +1 electric charge while electrons have a -1 electric charge.
Let us consider the electric current flow in a Cathode Ray Tube, well known these days as the older CRT monitors or CRT television screens. This device uses hot cathode emission from a heater element to produce an extremely high quantity of free electrons that are easily attracted to the anode some tens of centimeters away. My LG High Definition CRT anode is over 40 centimeters from the 'guns' (there are 3, one for each color) in the neck of the tube. Now, there are no wires connecting the gun to the anode inside the tube. Those electrons must travel through that emptiness in between with no conductor. Special electric rings in the gun and magnetic coils around the outside of the glass tube focus the stream of electrons into a tight single file beam that is moved across and down the screen drawing the image just one dot at a time. Those moving electrons inside that emptiness constitute an electric current.
It should be noted that this current is part of a closed loop and therefore subject to Kirchhoff's Current Law. Thus, the electric current moving through that emptiness must match the electric current flowing through the HV anode wire and can be measured with an ammeter.
Now, does this mean that all electric current is somehow tied to electrons in motion? No. The electrons themselves need not move for electric current to flow. Remember, current is a movement of charge and this could be positive. Therefore, Ion's can also serve as charge carriers for electric current. And they do in every lightning bolt and air-borne spark known to man. When it comes to electric current it is all about electric charge distribution and equalization.
Perhaps our next question should be "What is Electric Charge?"
Really, when we use an ammeter to give us a value in Amperes, or we measure a voltage drop across a current sensing resistor and divide that by the resistance, what is the actual thing we are measuring?
At their base function, our measuring equipment is really giving us an after effect of the reality that occurs before hand. The d'Arsonval Movement common in analog ammeters and Galvanometers works on the principle of a motor. It converts electric current to a magnetic moment that exerts torque on the rotor as it works against an existing permanent magnet field present in the movement. So if the magnetic field produced around the inductor in the meter movement is an after effect, what causes it?
Most Digital Mulit-Meters (DMM's) do not use this magnetic conversion approach using permanent magnets and inductors. Instead, they use another approach which also is an after effect. They use the principle of Ohm's Law which states the voltage across a resistance is proportional to the current flowing in the resistance - E = I · R where E is electromotive force (aka voltage) I is intensity (aka current) and R is resistance. But again, if the voltage drop across the resistor is a secondary effect, what is the primary effect? What is the real physical thing that we are measuring as Amperes?
Well this is a question that was floating around some 149 years ago when James Clerk Maxwell wrote his paper "On Physical Lines Of Force" which introduced Displacement Current that was later incorporated into the equations presented in 1864 in the paper "A dynamical theory of the electromagnetic field" All together Maxwell had 20 equations with 20 variables. Oliver Heaviside really wanted to understand these equations. He did not have any training in Calculus at the time and set out on a self taught course to learn Calculus and comprehend what Maxwell had given the world.
In Heaviside's work found here: Electromagnetic effects of a moving charge - Wikisource
we find some interesting clues to what electrical current is.
Originally posted by Oliver Heaviside - Electromagnetic effects of a moving charge Part II
So what is a moving charge? This is a very important question because how it is answered impacts the very foundations of all physics connected with electricity.
A moving charge is literally anything with an electric charge that displaces space during a time interval.
As you may imagine, this begs a frame of reference. After all, the battery sitting motionless here on my desk is electrically charged, and is involved in very complex motion as the earth rotates and revolves and the solar system moves and the galaxy moves etc. Are we to conclude that this 'moving' charge is considered to be an electrical current? And if so, relative to what? The definition therefore implies an observation point. One Ampere of current is that quantity of charge that moves past a given point of observation in one second and this would be equivalent to around 6.242 × 10^18 positrons or protons for a positive conventional electric current or electrons for a negative conventional electric current. This is true because positrons and protons have a +1 electric charge while electrons have a -1 electric charge.
Let us consider the electric current flow in a Cathode Ray Tube, well known these days as the older CRT monitors or CRT television screens. This device uses hot cathode emission from a heater element to produce an extremely high quantity of free electrons that are easily attracted to the anode some tens of centimeters away. My LG High Definition CRT anode is over 40 centimeters from the 'guns' (there are 3, one for each color) in the neck of the tube. Now, there are no wires connecting the gun to the anode inside the tube. Those electrons must travel through that emptiness in between with no conductor. Special electric rings in the gun and magnetic coils around the outside of the glass tube focus the stream of electrons into a tight single file beam that is moved across and down the screen drawing the image just one dot at a time. Those moving electrons inside that emptiness constitute an electric current.
It should be noted that this current is part of a closed loop and therefore subject to Kirchhoff's Current Law. Thus, the electric current moving through that emptiness must match the electric current flowing through the HV anode wire and can be measured with an ammeter.
Now, does this mean that all electric current is somehow tied to electrons in motion? No. The electrons themselves need not move for electric current to flow. Remember, current is a movement of charge and this could be positive. Therefore, Ion's can also serve as charge carriers for electric current. And they do in every lightning bolt and air-borne spark known to man. When it comes to electric current it is all about electric charge distribution and equalization.
Perhaps our next question should be "What is Electric Charge?"
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