Wednesday, June 11, 2008


June 11, 2008 Educational Radio Net, PSRG 4th session

Much like any radio talk show I will "set up" the topic and then allow time at the end for questions or comments. Truth be known this subject is a mathematical adventure but, given that we do not have a "white" board for graphic illustration, I will attempt to convey simple ideas verbally.

This session is the 3rd in the impedance series. Given that impedance is the combination of reactance and resistance and, further, that reactance is an alternating current phenomenon it is clear that we must have some fundamental definitions under our belts to fully appreciate the subject. This multi-session series is an attempt to elevate participants, in narrative fashion, to an intuitive level of electrical understanding without using any serious mathematics.

In part 1 we developed the idea of electrical current consisting of moving charge and defined the ampere as 1 coulomb of charge moving past a fixed point in 1 second. One coulomb was defined as a collection of charge numbering 6.24 x 10^18 electrons.

In part 2 we continued with the notion of mechanical "work" and considered objects at different "potential" levels in a gravitational field. The concept of "voltage", also known as electrical potential difference, and the relationship of voltage to current follows closely with the idea of a mechanical weight being moved between different levels. In both cases work is being done and energy is being manipulated in various ways.

In part 3, tonight's session, I had intended to continue with the voltage idea however the discussion of lightning phenomenon last week has presented a fine opportunity for reviewing moving charge so I will seize the moment and rehash moving charge in the context of lightning. Never fear... voltage will be dealt with next week.

So... consider a cloud. The average cloud consisting of water vapor is electrically neutral, more or less, so sticking probes here and there throughout the cloud would show no significant "potential difference" or voltage. Now consider the Earth under the cloud. Pretty much electrical neutral or tending toward a slight negative charge.

There are times, however, when the cloud is experiencing severe temperature differences and turbulence which can lead to a process of charge separation. The process of charge separation is actually electrical "work" being done in the cloud. Negative charge, or electrons, migrate to the cloud bottom known as the N-region and positive charge, that left over after the separation process, migrates to the cloud top per some rule of nature where it resides as the P-region. The cloud has been transformed by mother nature from a neutral blob of water vapor to a polarized, or charge separated blob. Texas A&M meteorology experts believe that the separated charge in the cloud can amount to around 40 coulombs in both the negative and positive regions.

Now let us consider the Earth below that polarized cloud. We have not talked about it but everyone knows that opposites attract and that likes repel. The same holds true for charge... charge of opposite sign, + and -, experience some force of attraction whereas charge of like sign, ++ or --, repel one another. Since the Earth is nearest the bottom of the cloud and the cloud bottom is negatively charged, any free negative charge moves away from the immediate vicinity of the cloud. As a result the Earth surface becomes polarized below the polarized cloud. The cloud is said to have induced the opposite charge on the ground.

There are other examples of charge separation. Consider what happens when you shuffle your feet on the carpet on a dry day and then approach a door knob. Ouch for sure. Shuffling your feet over certain carpets causes a separation of charge on your body and the discharge spark between finger and door knob is just a miniature lightning strike. Excess charge on your body and clothing is looking for a lower potential object with which to neutralize itself.

Another example is laboratory demonstrations of charge separation. Rubbing fur across a rubber rod will cause the rubber rod to become negatively charged whereas rubbing silk on a glass rod will cause the glass rod to become positively charged. Separating charge by rubbing or friction is called the Triboelectric effect.

One more bit of information figures into our discussion. Charge distribution. Let me assert without proof that charge distribution goes inversely as the radius of the charged object. Take an egg for example... it has a pointy end and a not so pointy end. Since the pointy end radius of curvature is smaller than the not so pointy end radius, the pointy end will accumulate more charge than the blunt end. If the pointy end of the egg were similar to the point of a needle then the bulk of the charge on the egg would be found at the needle point. Such large concentrations of charge are associated with large potentials and tend to produce corona discharge which is sometimes called St. Elmo's Fire. Many times you can even hear the sizzle during a thunder storm. Given that your finger is more pointy than the rest of your body there will be a charge concentration at your finger tip and get your attention when discharged to a door knob.

Back to lightning. The surface of the earth under storm clouds certainly has pointy irregularities such as transmission towers or church steeples or some such so the induced charge is not distributed uniformly. Even the ocean may have large waves with pointy tips. In like manner the polarized cloud bottom may have features which tend to concentrate charge at points. Since pointed objects have large charge accumulations and large charge accumulations go hand in hand with high potentials there is the strong possibility of corona discharge from both the Earth and cloud. High speed cameras have shown that corona discharges called "leaders" are produced from the Earth upwards and from the cloud downwards. When two leaders touch then a low impedance... that Z word again... path or circuit is established which provides an easy path for charge transfer.

Now we can finish the review of charge and the transfer thereof.

Texas A&M folks also believe that the average lightning strike involves about 25 coulombs of charge transfer. Recall that 1 coulomb per second equals 1 ampere. Consider that a 120 volt, 120 watt incandescent house lamp requires 1 ampere to operate. So, the average lightning strike charge of 25 coulombs, if transferred in 1 second would operate 25 of the 120 watt lamps for 1 second. Suppose that the 25 coulombs were transferred in 1/10 second. Then the current climbs to 10 times 25 amperes or 250 amperes. Suppose that the lightning discharge lasted only 1 millisecond... then the current would be 1000 times 25 amperes or 25,000 amperes. Since the average strike lasts a bit less than 1 microsecond... 1 millionth of a second... we can say that the current or moving charge is at least 25 mega amperes. The point here is that small amounts of charge, if moved rapidly, represent huge amounts of current flow which can instantly and explosively boil the sap in trees or blow roots right out of the ground.

In summary, we can see that neutral objects... clouds or people or laboratory rods... can suffer charge separation and become polarized which may lead to lightning strikes or mild shocks when reaching for a door knob. Additionally, small amounts of charge can produce enormous electrical currents if moved very quickly and cause serious damage to people and things including sensitive electronic equipment. Finally, the distribution of charge on any object depends on surface irregularities.

This concludes the set up discussion of charge in motion as in lightning strikes. Are there any questions related to the concept of moving charge?

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