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Satic Electricity - Basic explanation

 

Lightning - photograph

 

Static electricity is the imbalance of positive and negative charges.

Everything we see is made up of tiny little parts called atoms. The atoms are made of even smaller parts. These are called protons, electrons and neutrons. They are very different from each other in many ways. One way they are different is their "charge." Protons have a positive (+) charge. Electrons have a negative (-) charge. Neutrons have no charge.
Usually, atoms have the same number of electrons and protons. Then the atom has no charge, it is "neutral." But if you rub things together, electrons can move from one atom to another. Some atoms get extra electrons. They have a negative charge. Other atoms lose electrons. They have a positive charge. When charges are separated like this, it is called static electricity.
If two things have different charges, they attract, or pull towards each other. If two things have the same charge, they repel, or push away from each other.

Electrical charges develop inside a storm cloud. Positively charged atoms go to the top of the cloud. Negatively charged atoms go to the bottom. If the negatively charged atoms become too crowded, they "jump" to another part of the cloud, to a different cloud, or to the ground. The sudden discharge of electricity that results produces heat and light. We see the light as lightning, and the heat makes the air expand with explosive force to produce the sound of thunder.

The same phenomena which create lightning and thunderstorms are around us every day, creating incredibly high voltages which cause sparks and shocks.

Walking across certain rugs or plastic flooring will cause your shoe soles to touch the dissimilar material of the rug. This is enough to separate the negatives from the positives, electrons move from the flooring to you. Now you have extra electrons. Touch a door knob which acts as a conductor, the electrons move from you to the knob. You get a shock as the very high voltage built up in your body discharges.

 

 

My sketch below shows what happens, when a positive charge is transferred to your hair, by quickly pulling off a woollen hat. As it rubs against your hair, electrons move from your hair to the hat. Now each of the hairs has the same positive charge. Remember, things with the same charge repel each other. So the hairs try to get as far from each other as possible. The farthest they can get is by standing up and away from the others.

 

 

Another example that most of us have experienced is when getting out of the car. You may get a shock on touching the door. The source is usually static charges which have built up between your body and the car seat, but remain neutralised until you get up. A considerable electric charge is with you as you get out of the car. If the charge has no discharge path, then several thousand Volts can build up very quickly. When you reach for the door, the high voltage causes a spark, giving you a shock, as it quickly discharges to the car. The spark is painful because it's extremely hot. It drills into your skin like a white-hot needle, creating a microscopic burned area. If you suffer from this phenomenon, try this method. Before you leave the car, hold onto the metal door frame as you get out of the seat, this allows the static to harmlessly discharge as you get up.

You will incidentally notice static electricity more in the winter when the air is very dry, than in the summer,when the air is more humid. The water in the air helps electrons dissipate more quickly, which reduces the build up of a big a charge.

Although we are constantly carrying static electricity in our bodies, most of the time we are unaware of the fact. Whenever two materials touch and then separate, one material charges positively, and the other negatively. The charge which is generated may not be noticed, as we are constantly carrying out tasks in our daily lives eg. turning on the tap, placing a saucepan on the oven, etc, that create an electrically conductive path which dissipates the static electricity harmlessly away. The build-up of static electricity depends on whether the rate of charge dissipation is greater than the rate of charge generation. If it is, there is no problem

 

Apart from the discomfort of receiving a static shock from a door knob, the discharge of static electricity can be a dangerous and costly event, For example:

If there is a flammable atmosphere present, such as a solvent vapour or dust cloud, then the risk of explosion becomes a serious hazard. Less dramatic, but also very important, is the possibility of ESD damage in electronic manufacture. Here, even a small amount of static can destroy sensitive electronic components.

Petrol and other liquid fuels are very good insulators. When an aircraft is being refuelled, so much liquid flows along the pipe at high speed into the fuel tanks, that a charge could build up, and any spark at this stage could have horrendeous consequences. To prevent this happening, an earthing lead is attached to the aircraft body, allowing any charge to run safely to earth preventing sparks.


Where static electricity is controlled to reduce these effects, it is usually by choice of materials specified as having certain electrical properties. For example on Oil Tankers, all maintenance tools are made of a non-magnetic material, and in the electronics industry, the work areas are well earthed, and the operators wear anti-static straps.

 

There are positive uses to which static electricity can be put. It is being used in various industries to achieve effects which may be difficult to achieve in other ways. For example:

Corona treatment of plastic surfaces. Corona discharges can be used to improve the ability of a plastic surface to receive print or adhesive treatments.

Dielectrophoresis. Particles in a non-uniform electric field can be made to be repelled from, or attracted to, regions of high field strength.

Electrostatic atomisation. Static electric forces are used to form a fine spray of liquid particles.

Electrostatic paint spraying. Static electricity is used to attract the paint to the target, reducing paint wastage and improving coverage of the target.

Electrostatic precipitation. Electrostatic forces are used to precipitate smoke particles and clean the gas emitted from a chimney. In large power stations, 20 tonnes of soot per hour can be removed by this method.

Electro-osmotic dewatering. Electrostatic forces speed up the settling of fine particles suspended in water, and can be used to dewater a sludge.

Photocopiers and laser printers. These use electrostatics in the printing process.

 

"Here is a simple experiment:

On a cool, dry day, scuff your feet along a carpet, then reach your hand into a friend's mouth and touch one of his dental fillings. Did you notice how your friend twitched violently and cried out in pain? This teaches us that electricity can be a very powerful force". (Only joking kids).

 

 

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