| Electrical Charge |  |
We have always seen the effects of the electrical charge around us. Every one of us has the experience of rather unpleasant electrical discharge spikes when we touched a metal or another person. This happens because of the storage of electrical charge in our body due to the friction of our cloth to other material in our environment. In general, when two nonconductor material rub against each other, some electrical charge (electrons) is transferred from one to the other. The material carrying the charge (for example a plastic ruler) can absorb other material (like hair) by absorbing their electrical charge and therefore moving the other material around.
Electrical charge is an entity that produces electrical field around it. Electrical field generates some force on any other electrical charge in the field. The unit of Coulomb shown as ‘C’ is used to show the value of the electrical charge. There are two forms of electrical charge, positive and negative, where charges with the same sign repel and charges with different signs absorb each other as shown in the figure below. In this figure Q and q are two different charges and r is the distance between them. The electrical field is always perpendicular to the charge surface, no mater what the size or shape of the charge carrying material is.
In every atom there are different particles. Two famous particles are Electron and Proton, which in normal conditions are the only charge carrying particles in an atom.. Electrons are extremely small and always carry negative charge. They are the moving parts of an atom, which can transfer electricity around. Protons are much larger in size and carry positive charge. They are normally fixed in place relative to the atom core shaping the core along with other particles, unless under some special conditions. The amount of charge in one electron and one proton is the same, equal to 1.60217646 × 10-19 Coulomb with negative sign for electron and positive sign for proton.
When two electrical charges are close, each one receives a force from the other one, which is equal in value but reversed in direction as shown in the figure above. The value of electrical force 'F' in Newton (N) is calculated using the following formula:
Where r is the distance in meter (m), Q and q are the amount of charges in Coulomb (C) and K is a constant almost equal to 9x109 N.m2/ C2. If the value of the force is positive, charges are repelling each other and vise-versa.
There is another parameter called Electrical Field Intensity or Strength shown as 'E', which represents the electrical field intensity of for example the charge Q in the location of the charge q. E is actually the force of the charge Q on a unity charge of one Coulomb in the distance r (force per unit charge) in N/C unit, calculated from formula below:
Electro Static Charge
Electro static charge, or simply static charge is called to some charge stored in a substance, which is unable to move out. it is unable to move out because either there is no way out, or the substance storing the charge is not a conductor of electricity and this makes it hard for the charge to get away. Please refer to Electrical Material to get familiar with different categories for material based on their electrical properties.
Where does the static charge come from? As mentioned above in the first paragraph of the page, the most common way is rubbing two substances against each other. This results in transferring some charge from one matter to the other as one grabs and holds charge from another. In this case at least one of them has to be a nonconductor or insulator. This doesn't happen in the case of two conductors as the charge is extremely free in conductors and can't be imprisoned by one of the conductors. Other ways of generating electro static charge is through electrical circuits, such as storing charge in a capacitor.
Electro Static Discharge (ESD)
One famous example of electro static charge is the charge resulted from rubbing clouds and air against each other. Clouds hold an extremely high quantity of charge due to this friction. The result is the roaring thunder jumping from one cloud to another or to the ground. This is called Electro Static Discharge or ESD.
Assume there is a lot of positive or negative charge in a matter. It is like a balloon filled with air, which the air intends to get out of the shell as the air inside has more energy and wants to diffuse it in the lower energy air. Similar to the air in the balloon that is under a higher pressure, the charges in the charged substance are repelling each other and forcing each other to jump out. Therefore the charged substance intends to give out its charge to the surrounding environment to achieve the lowest energy level, as all the substances in the universe intend to do. Making a contact between this substance and another substance results in a trade between their charges until they both hold the same energy. In case of insulator material it doesn't happen very easily as the charge can't move in them freely, yet it moves with a very slow rate. No matter how good an insulator is, eventually it gives out its energy to its environment. In contrast, conductors such as metals can't hold charge for a long time as they can't capture the charge as hard as insulators.
Electro Static Charge or ESD is the source of many problems for electrical equipment. It generates electro magnetic interference and can burn the device if strong enough.
Break Down
If the difference between the electrical energy stored in two substances is high enough, they trade their charge if they are close enough, even though they are not touching, similar to the discharge of clouds resulting in a lightening. That's because even the best insulator will break down under a high electrical potential energy. Air, as an insulator is no exclusion. Air will break down under about 26 kilo volts of potential energy per centimeter resulting in a spike. The more the potential energy difference is, the longer distance spikes can happen, such as the lightening from clouds to the earth. You can imagine how huge the energy must be in such thunders.
For a hundred meter thunder, which is much smaller than many thunders, a potential energy difference of about 260 million or mega volts is required. This huge potential breaks down the air molecules into ions. It creates a suitable path for electricity allowing a huge electrical current to pass through the ionized channel. Resulting in an excessive heat, the electrical current quickly expands the air which ends in a great show of light and sound. This is how insulators pass electricity. For other more conductive material, they already are similar to a pool of floating charge that allows electricity to pass, the ability that the insulators are lacking. But still every atom has electron and proton, which get absorbed to the external charge. As their structure is more resistant to the electricity, they won't let their particles to escape. But who can tolerate a great energy? Eventually a high energy divides their powerful bond and breaks them into charged ions. Every insulator substance has its own break down potential, some stronger than the other. When they break, electrical charge passes through this channel, which is not always a straight line as electricity always seeks for the easiest way to flow. Because this process generates great heat, this can result in burning the substance or changing its molecular structure. In the case of air, it has nothing to burn. Therefore after the discharge usually it returns to its former structure. Yet some oxygen in the air changes from O2 to O3 which is called Ozone, protecting the earth from harmful rays.
Charge Distribution
In an insulator substance, the charge can be stored anywhere on or inside the insulator as it can't move. But in a conducting substance, the charge is always distributed on the surface, creating a surface charge. This is because the charge particles stored in the substance, such as electrons, repel each other and want to get as far as possible from one another and therefore move to the furthest distance, which is the surface of the substance. It is possible for an insulator to hold both positive and negative energy as an insulator keeps them apart, but not in conductors. Because opposite charges quickly move and neutralize each other and only the charge with a higher volume remains.
Not to forget that the charge on a conducting substance surface is accumulated on sharper or pointer areas as shown in the figure below. The sharper the pointy head is, the more charge is accumulated on the point end, resulting in a much higher potential energy at that end. Therefore a substance with an opposite charge can be discharged to a pointy object from a much further distance compared to a substance with an even surface.
Lightening Rod
The fact above is why they say you shouldn't be standing on a hill in a thunder storm, because not only you get closer to the charged clouds, as they absorb the ground charge to its surface, a huge charge is accumulated in you as the pointy object on the surface. Clouds will love to trade their charge with you! That's why in thunder storms when the thunder strikes earth, it usually gets penetrated to trees. This fact is actually used to protect very tall buildings. They mount a tall rod at the highest end of the building, which is wired all the way to the ground. This is called a lightening rod. Using the lightening rod causes the clouds to discharge to the rod instead of the building itself. Then the prepared path guides the current safely to the ground. This is because as the charged cloud flies over the earth, it absorbs the opposite earth charge to its surface, especially accumulating a huge charge in the rod as a pointy object, and the rod is the closest to the cloud. As I said before, electricity always find the shortest and least resistive path and therefore strikes the lightening rod.
Safety Notes
Hear are some safety notes for you, if you are interested to play with electricity:
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Don't play with it! It's always dangerous, so always proceed with caution. |
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It is never a good idea to be on high grounds in thunder storms, so avoid it. |
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Never think that standing under a tree will save you from lightening, actually it is worse. If you are far from the tree, it is more probable that the lightening hits the tree. But when you are under the tree, although the lightening will hit the tree, as the discharge current flowing in the body of the tree gets close to your head, it jumps from the tree to your head to continue its path. It is because your body provides a better conductor. |
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It is even more dangerous if you hold a rod in you hand and point it to sky! Isn't that obvious?! But if you connect this rod to a good conductive ground, it protects you much better than not having the rod. Because the lightening will choose the best path which is the rod to the ground instead of your body. Even if you touch the rod the electricity won't be transferred to you as your body is an extremely bad conductor compared to a metallic rod. |
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When you want to change light bulbs or touch devices with electrical shock hazard, always make sure that your body is not in contact with ground. This cuts possible paths for the electricity to pass through your body. |
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Never touch the house electricity wires, or if you are in a position that it may occur, make sure you are not touching the ground and never let both phase and null wires to touch you at the same time. |
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If you need to touch hazardous conductors, always wear special insulator gloves to eliminate electrical shock. |
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Always know how big the potential on a device is and where it is, so that you can prepare for possible hazards. |
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Remember that the electro static discharge can always happen even if you are not touching the ground. Because although there is no path for the current to flow, your body acts like a massive storage of charge that a charged body will trade charges with, so that both get the same energy. Therefore although the shock is not as hard, you will feel it. |
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Even if there is a very small electro static discharge, it creates electro magnetic waves around it that can interfere with sensitive devices and affect their performance. This is not usually a big problem. But where it involves with health care devices such as pace makers, you have to be very careful. |
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If you want to touch sensitive low voltage electrical devices that can be easily damaged due to electro static discharge, ground yourself and the device to free your body from charges and become the same potential with the device. |
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Always read safety rules and regulations when using any device. |
| Written by: Mehdi Sadaghdar | |