Static Electricity And Lightning

Let's go back to the charged sphere of Figure 2.5. If the second metal ball is placed closer to the charged ball so that they actually touch, the

Figure 2.11 A) Water molecule. B) Water molecules in random orientation with no field applied. C) Water molecules line up when an electric field is applied.

Figure 2.11 A) Water molecule. B) Water molecules in random orientation with no field applied. C) Water molecules line up when an electric field is applied.

Static Electricity Free Energy

negative charge will now redistribute itself evenly over the two balls. If the balls are moved apart again, each will now have half of the original negative charge. If balls of unequal size were used, then more charge would end up on the larger ball, because it has a larger surface area. If the charge is large enough, very high voltages will be produced as the balls are brought close together. When the electric field exceeds the breakdown strength of air (typically about 5000 volts per centimeter), a spark will occur between the two balls, allowing charge to transfer without physical contact. In this phenomenon, air molecules become ionized, forming a jagged conducting path between the two conductors. In the process, some of the original charge is lost to the air ions. What you have just learned is the process of static discharge (illustrated in Figure 2.12). The same effect occurs when you walk across the carpet and then get shocked by a doorknob. Your body becomes charged while walking across the carpet. Called the triboelectric effect, it consists of charge separation when certain materials are placed and/or rubbed together and then pulled apart. The outcome is that your body takes up a charge and the carpet stores an equal but opposite charge. Typical static discharges you encounter will be in the range of 5 kV to 15 kV and will produce a peak current of about 1 amp! Quite a large jolt.

You may wonder why such high voltage and current hurts but is not dangerous. There are several reasons for this. First of all, the entire discharge only lasts about 1 msec. For this same reason, shocks from spark plugs usually cause no harm. Second, the current is mostly concentrated near the discharge (i.e., your finger) and then quickly spreads and dissipates. Negligible current will traverse your heart, which is the danger zone for current in the human body. Typically, a person can sense currents of 1mA or more, with currents over 40 mA being possibly lethal as they pass through the heart. Therefore, it is how much current flows through the body and where it flows that is of concern. When a person becomes part of a circuit, there are four parameters that determine the current: 1) source voltage, 2) source resistance, 3) contact resistance, and 4) internal body resistance for the current path. The second two parameters are a function of body physiology. The contact resistance is mainly caused by the dead skin layer where contact is made. Contact resistance is typically on the order of 100 ohms for sweaty skin to 100 kohms for very dry skin. Beware that if the skin is cut, the contact resistance becomes negligible. The internal body resistance is fairly low due to the fact that nerves and blood vessels make good conductors. Any limb-to-limb internal resistance can be approximated as about 500ohms. The effects of electricity are felt differently, depending on frequency. The most dangerous frequency range is from about 5 Hz to about 500 Hz and peaks in danger right about 60 Hz, the frequency of power lines. The frequency sensitivity has to do with the physiology of the human nervous system, which typically communicates via pulse trains in this range of frequencies. A 60 Hz current has approximately two to three times the danger as the same current at DC. As opposed to DC signals, these AC signals can cause muscles to lock up, leaving a person unable to let go of the voltage source.

Figure 2.12 A) A neutral sphere (right) is brought close to the negative sphere (left). B) The spheres become very close and a path of ionized molecules forms in the air, causing a spark. C) After the spark has dissipated, some of the charge has been carried away by the ions in the air. The remaining charge is split equally between the spheres.

Figure 2.12 A) A neutral sphere (right) is brought close to the negative sphere (left). B) The spheres become very close and a path of ionized molecules forms in the air, causing a spark. C) After the spark has dissipated, some of the charge has been carried away by the ions in the air. The remaining charge is split equally between the spheres.

Static Electricity Poster Ks1

Lightning is static electricity on a grand scale. In the large circulating winds of a thunderstorm, raindrops and ice crystals collide, causing charge separation. The process of charge separation is not fully understood, but it is known that positive charge collects at the top of the cloud and negative charge collects at the bottom of the cloud. The negative charge at the bottom of the cloud induces an equal but opposite region of charge on the ground below. You can think of it as a localized charge shadow. When the charge builds up to a high enough voltage (typically 10MV to 100MV), the air starts to ionize in the form of a jagged "leader" which migrates from the cloud to the ground in discrete jumps of (typically) a few hundred feet. When the ionized leader gets one jump away from the ground, its path is affected by the objects within the immediate vicinity, often connecting to the highest projecting object. Upon contact, a conducting path then connects the cloud with the ground, allowing the cloud to discharge. Peak currents can range from a few kAmps to 150kAmps, and the event typically lasts about several hundred microseconds. With such high currents, lightning is always dangerous to people.

So what happens to all this negative charge that accumulates on the ground? It gradually migrates back to the atmosphere through the small concentration of ions always present in air. During fair weather, the upper atmosphere has a positive charge and the earth has a negative charge, forming a giant capacitor (about 5000 Farad) that is discharging an average of 1800amps at any given time. This "fair weather current" is needed to balance out the currents from thunderstorms. We are standing in the middle of it! Consequently, the air of a typical day has a DC electric field in it of about 100 volts/meter. Even though we are in the middle of this high field, we don't experience much of a problem because, being very good conductors when compared to air, we locally short out the field. Approximating head-to-toe human body internal resistance as 700ohms and the air resistance of a 6 foot long, 3 foot by 3 foot columnar region as about 1014ohms, an equivalent circuit can be constructed. We end up being a very small resistance in series with a very large resistance (the miles of air between a person and upper atmosphere). Using these assumptions, the voltage across the body is approximated as 3 ||V.

Large electric fields can cause other interesting effects. As mentioned earlier, large electric fields can cause gases to ionize; that is, electrons are freed from the gas atoms, leaving charged ions behind. Ionized gases conduct electricity and produce visible light in the process. Electrical corona is the term used to describe the glowing region of ionized gas that can occur around conductors. The glow is visible radiation produced when an atom gives up or accepts an electron. Coronas are created near conductors that have high electric fields emanating from them. Highvoltage power lines must be placed far enough apart to avoid creating coronas since a corona consumes energy. Fluorescent light bulbs work via the same phenomena. A high voltage causes the gas inside the bulb to ionize. In fact, an unconnected fluorescent light bulb will glow if held in air near high-voltage wires! The large electric field causes the gas inside the bulb to ionize, which in turn causes the visible light.*

*To be exact, the gas emits UV radiation, which is absorbed and then reradiated by the fluorescent powder on the inside surface of the bulb. The reradiated light is in the visible region of the spectrum.

Figure 2.13 Electric field surrounding a capacitor with DC voltage applied. This figure was created using Ansoft Corporation's Maxwell 2D field solver software (http://www.ansoft.com).

Figure 2.13 Electric field surrounding a capacitor with DC voltage applied. This figure was created using Ansoft Corporation's Maxwell 2D field solver software (http://www.ansoft.com).

Pointed objects such as flagpoles and ship masts tend to concentrate electric charge and can produce a corona during a strong thunderstorm. Before this effect was understood, sailors thought that it was the sign of a ghost or spirit and called it St. Elmo's fire. If you see a corona during a storm, this is a bad sign, since whatever object is producing the corona is acting as a lightning rod and will attract a lightning bolt if one approaches the area. A lightning rod is a metal rod that is connected to earth and protects a house or structure by attracting nearby lightning strikes. The lightning rod conducts the lightning current safely into the earth, preventing it from finding alternative paths to ground (like through your roof). It is a common myth believed by many engineers and scientists that a lightning rod prevents lightning by slowly discharging the cloud immediately above. Let me emphasize that this myth has been proven false many times over by lightning researchers. In fact, it would take over 800 hours for a lightning rod to discharge the typical

Figure 2.14 Electric field surrounding another capacitor with DC voltage applied. This figure was created using Ansoft Corporation's Maxwell 2D field solver software (http://www.ansoft.com).

Figure 2.14 Electric field surrounding another capacitor with DC voltage applied. This figure was created using Ansoft Corporation's Maxwell 2D field solver software (http://www.ansoft.com).

Pic Cloud And Earth The Capacitor

cloud, and most of the ions the lightning rod releases get dispersed by the high storm winds.

The triboelectric effect can cause other, less dramatic problems. Moving cables can cause noise via the triboelectric effect. The movement of the cable causes friction between the insulation and the metal, rubbing charge from the insulation. A similar effect can happen with outdoor cables or antennas that are blown by the wind, especially if the weather is stormy and the air is well ionized.

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