Re: A question about current....
 

An arc between two charged regions across an insulating region is extremely difficult to model. The ability of an arc to cross a region is dependent upon the difference in Voltage across the region and several other factors, including gas density and distance. Also temperature and humidity are important factors to consider.

The reason for breakdown is explainable. When tremendous voltage differences appear between surfaces, the Townshend (sp?) effect occurs in gases. In this effect, outer electrons disassociate (or lose some of their bond strength) in gas molecules. These newly-freed electrons can now carry charge, and allow for conduction.

Imagine two electrodes a distance r apart from one another, with a gas whose density is p. There is a law, Paschen's law, which states that the required breakdown voltage, the voltage at which electric current can flow across the insulator, is a function of p and r. This function is not linear and is
dependent upon the type of gas, geometry of the setup, etc. And furthermore, it is only truly useful at pressures near 1 atmosphere and distances of roughly 1 centimeter.

In air, an approximation exists that is often used on short distances.

Breakdown_Voltage = 366 (V/(cm*torr)) * gas density * (distance/(1.18 + ln(distance * gas density)))

where distance is measured in centimeters.

There is also another effect at work, wherein air will not break down, regardless of distance, unless a certain threshold voltage has been crossed. In air, this voltage is approximately 330 volts. This threshold voltage is affected by the chemical composition of the involved electrodes; for example Aluminium electrodes in pure air result in a breakdown voltage of 352 volts.

There are a few fun demos possible with Paschen's law. If you have ever seen a Windhurst machine, that is one of them. This machine uses two spheres mounted such that the distance between them can be controlled. Also, a human spins a crank, which has two brushes attached, which build up
very massive voltages on the two spheres. Eventually the breakdown voltage is reached and crossed, and "lightning" appears. (Side note: Try placing a plastic ruler with a hole in it inbetween the two spheres, and watch the lightning jump through).

One other note: When the Townshend effect results in the breakdown of air in a large area, radio communications become extremely difficult to sustain. I have done some experiments trying to sustain FHSS walkie-takies through a row of Jacob's ladders, trying to see how well radio works with that much ionization occuring. Maybe because I was using terrible units or maybe because ionization is that good at stopping radio, but very little communication made it through.

Re: A question about current....
 

Venkatesh,
True, arcs are extremely hard to model because there are so many variables. What practice dictates is the safety margins involved to protect persons and property. Power line height is dictated by the known breakdown characteristics under the worst rain ever encountered with the most conductive particles contained in the water plus a safety margin to make up for those variables that can all combine to cause an arc and/or personal injury.
As Doug has stated above, there are fields surrounding power lines. Since they are AC fields, placing a wire in such a field will cause current to flow in the wire. The field will also induce current flow in the ground under the wires and in the towers that the wires hang on.
As to interference, the arc is a current flow. Current flowing in a conductor (in this case ionized gas) produces a field about the conductor. An arc is not a steady state or DC current (even it is generated by a DC source) but wildly alternating in nature. This field is the same as takes place in an antenna. The arc produces a very large and wideband electromagnetic field and as such overpowers the radios in your experiment. Spark gap transmitters were the source of Marconi's radio waves at the beginning of the last century. The energy produced by the gap was also wideband and Marconi and others attempted to narrow the output to a smaller range of frequencies using what are now known as tuned circuits. As broadcasting progressed, the spark gap transmitter fell out of favor and was eventually outlawed as using too much spectrum. To overpower your little handhelds (radios) takes a field strength of just a few millivolts per meter at the distances you suggest.

Re: A question about current....
 

I've "heard" lightning on my radio many of times! You can also take 9 Volt battery and connect one end to a metal file and the other end to a wire. Run the free end of the wire across the file and watch all the little sparks and listen to your radio too (try the AM band).

Re: A question about current....
 

Doug,
You are quite right. I will try to stay away from ambigous terminology in the future. (It's something that drives everyone around me nuts!!)

Sparks

Re: A question about current....
 

I have read with interest the discussion on this thread. However, it appears to have gotten a bit off topic… At issue is “why does current flow from positive to negative and electrons flow from negative to positive?” As I am letting my Thanksgiving dinner digest, let me take a stab at this by using an analogy to make a complex subject more (I hope) understandable.

Imagine a telephone pole lying on the ground. As an observer, you are standing off to the side so that you can see the whole pole. Now we strike the end of the pole to the left of the observer with a sledge hammer. The pole moves one centimeter to the right of the observer when hit.

In the fraction of a second before the hammer impacts the pole there is a 1 cm “hole” at the far right end of the pole where the wood ends. A fraction of a second after impact, that hole is replaced by wood and the 1 cm “hole” has appeared at the end of the pole to the left of the observer.

The wood flow is from left to right and the hole flow is from right to left. You can not have one without the other.

The observer can define positive flow as wood flow (and therefore negative flow is the hole flow) OR he can define positive flow as hole flow (and negative flow as wood flow). It makes no difference as long as he or she is consistent.

To continue this concept, let’s assume that the hammer is invisible and that most of the pole is hidden from view by a pipe which encloses the pole. Now he or she sees wood being injected into the tube from the left and exiting to the right. He or she also sees a hole being injected from the right and exiting from the left.

The wood is, of course, analogous to the electrons in a wire (the pipe) and the hammer is analogous to a (negative) voltage.

Realize that a positive voltage attracts electrons and repels holes. A negative voltage attracts holes and repels electrons.

Holes are actually the absence of an electron in the outer shell of an atom of a conductor or semiconductor. Holes are every bit as important as electrons to the function of electricity.

A single electron exists in the outermost electron shell of an atom of copper, the most common conductor we use. We tend to call this single valence electron a “free” electron. While a copper atom with 29 electrons is electrically neutral, its outer shell wants to either give up an electron (the shell would be empty) or gain an electron (the shell would have two electrons). Thus we can inject a hole (take away an electron) to get a balanced outer shell (empty) but negatively charged atom or inject an electron to get an equally balanced outer shell (two electrons) but positively charged atom.

This delicate balance of electrons and holes makes the magic of electronics work. When you complete the circuit of wire from positive to negative of a voltage source, the number of positively and negatively charged atoms is constrained by the fact that the wire is a short circuit. The atoms want to be at a neutral charge so a heavy current flows as the electrons migrate from negative to positive and the holes migrate from positive to negative.

A convention was adopted long ago where a positive current was defined as the current which flows from a positive voltage to a negative voltage. Positive conventional current is hole flow.

This convention was, in all fairness, arbitrary but convenient for 95% of all electrical engineering. An engineer who is designing electron guns for cathode ray tubes, for example, may want to use non-conventional current (electron flow), however, this is more of a rarity as we become more and more dependent on semiconductor technology.

Last notes:

In type P semiconductors, the material used requires less electrons be lost (more holes be added) to make a stable outer shell than the number of electrons added (holes lost). N type material is the opposite. This difference in materials and valence electrons is why you may hear of majority carriers and minority carriers. In conductors, the holes and electrons have equal billing.

Lastly, the telephone pole analogy points out another curiosity. If you were to lay down and put your head against the far right end of the pole and I were to hit the left end with the sledge, the wood will only move 1 cm but you will get a headache instantaneously. The actual speed of migration of electrons in a 2 mm diameter copper wire passing 1 ampere of current is just under 9 cm per hour yet the signal travels at about nine tenths the speed of light… Cool eh?

Re: A question about current....
 

Mike,
I go back a bit further in electronics and for the most part "hole" theory didn't really become part of my education until semiconductor discussions. It was simple enough to know that a conductor when connected to a voltage source (i.e. battery) would draw electrons from the outer shell of the atoms at the positive terminal of the battery. Those atoms now wanting an electron to stabilize the outer shell in turn stripped them from neighboring atoms and so on until the battery was required to give up electrons at the negative terminal. Since I had a healthy background in vacuum tubes, (electron flow from cathode to plate is essential to their operation) I always have that mode stuck in the back of my head. The cathode generates excess electrons through the act of heating a material. When you introduce a positive voltage to the plate these electrons then migrate through the tube elements causing current flow.
Conventional current flow is a way for users to easily measure and predict other phenom. An ammeter will deflect "up" when the positive lead is connected to the more positive part of the circuit. The "right hand" rule for magnetic fields works with conventional current. Most electronic circuits consider the positive portion of the circuit at a potential above ground and so therefore current flows into the ground. Interfering signals are thought of as moving into a shielding material at ground potential, etc. Most of which are conventions used in the US but are different in other parts of the world.
Whatever method you use of analyzing a particular behavior is just a tool in your toolbox that helps you get the job done. Multiple tools makes the work easier. My dad was fond of telling me to use the "right tool for the right job."

Re: A question about current....
 

Al,

I also started with vacuum tubes (electronics technician school in 1971) and understand well... I thought that introducing the concept of hole flow would be helpful to today's generation.

BTW, my 1959 Austin-Healey is a positive ground system...

Re: A question about current....
 

Mike,
Austin dual 6 volts, one under each seat? Makes it hard to put a CD player in doesn't it. (Of course what am I saying, there is no room!)

Re: A question about current....
 

Al,

Mine is a 100-6 two-seater. The batteries are indeed dual sixes located behind the seats. Did I mention how much I love Lucas regulators? :D

Re: A question about current....
 

Are you having problems? I have never personally worked on one but if they are anything like GM regulators, I know the contact spacing and alignment were crucial. I worked in a GM parts store in high school back before Delphi when GM ignition and electrical parts were Delco Remy. The store had this panel that helped in alignment. As I remember it had about three round meters and several switches. I had a friend who finally found an interesting problem with an MG and the two batteries. The springs in the seat were broken or worn and they would occasionally short a cell or two on top of one of the batteries. Until it started a small fire there was no indication that the seat was the problem.

Re: A question about current....
 

Al,

The truth be told, I have not had it running for over 10 years. Before I started dismembering it, I used to carry a small tack hammer and bang on the the top of the regulator when the battery started discharging...

Mike

Re: A question about current....
 

Back on topic... I've heard via PM from a few engineers about my telephone pole analogy. I'd like to hear from students...

There are no more fundamental concepts in EE than current flow, Ohm's law, grounding, et cetera. These can also be some of the most confusing... You guys and gals must have more questions...

Bring 'em on now... In about 6 weeks we mentors will be to busy to type up lengthy answers.