Static electricity is the accumulation (building up) of charged particles in one place. For charged particles to accumulate in this way they must be ‘trapped’ in some way. Otherwise the potential difference that builds up would result in the movement of the particles. The charge will try to neutralize itself by allowing the particles to flow to earth if possible.
Electrons can be transferred from one material to another and held there creating a static charge, plastics and other insulators do not allow the electrons to flow easily and this helps to hold the charge. Metal objects can also be charged providing the metal is insulated from the ground. Car bodies can build up a charge in this way as they are insulated from the ground by the rubber tyres.
When two material are rubbed against each other, there is friction between the two materials, this friction moves electrons from one material to the other. The material gaining electrons becomes negatively charged, the other material has lost some electrons and has gained a positive charge. The size of the charge on each object is equal but opposite.
Once charged the excess electrons will repel each other, making them spread out evenly across the material. If there is any conducting material connected to the earth even via another object the electrons will flow along this route to earth and once the excess electrons have been removed the material will be neutral again.
Interaction of Charges
Charged particles experience an electrostatic force that make them try to move. Like charges (+ & + or - & -) will repel each other, opposite charges (+& -) will attract each other, in a similar way to the behaviour of magnets.
This electrostatic force effect is responsible for the flow of electrons away from a charged surface or object towards the earth.
Example 1: Electrostatic Shocks
Some everyday materials are made of plastics and other insulators, if these rub past each other a charge can build up. Walking on a nylon carpet in nylon socks can cause a charge to build up on the person. This will not escape until they touch a conductive material like a door
The charge can not escape the car as it is insulated from the earth by the rubber tyres. If you then touch the car the electrons with flow via you to the earth, causing a mild electric shock.
Example 2: Lightning
Probably the most spectacular example of static electrical discharge is a lighting strike. The lightning that is seen is the result of the electrical discharge heating up the air to over 800 degrees. This makes the air expand very rapidly causing a sound wave which we hear as the thunder.
Within a thundercloud hail and ice crystals are being formed by the rapid movement of the air inside the cloud. The lighter ice crystals flow upwards and the heavier hail flows downwards. As they pass it each they rub against each other and cause an exchange of electrons from the ice to the hail. Gradually the top of the cloud with more ice becomes positive in charge and the bottom of the cloud with more hail becomes negative.
Once the charge on the base of the cloud is large enough it is discharged to other clouds, the ground or any object sticking up from the ground like a tree, a build or a person. The excess electrons in the cloud jump to earth causing the lightning strike.
Example 3: Induced Charges
In the examples about the charge has been created by the frictional removal of electrons from one place to another. There has been physical movement to force the electrons to move. It is possible for the electrons to be moved by the presence of another charged object. This process is known as induction and results in the electrons in the upper layer of an object being ‘pushed’ deeper in or to one side, leaving the affected area with an induced positive charge, from the presence of the positive nuclei that remain in place.
This can be demonstrated by rubbing a balloon on a cotton or nylon shirt and then placing it on a dry wall. The balloon will stick for a while, until the charge gradually disappears.
The same effect can be seen by rubbing a plastic comb through dry hair or on a nylon cloth and then using it to pick up small pieces of paper. The balloon’s negative charge displaces the surface layer of electrons in the wall and creates a small area of positive charge, the balloon is negative so they attract each other. The comb is negatively charged and induced a positive charge on the paper which are then attracted to the comb.
Static electricity can be applied in several situations. The applications make use of the attraction of opposite charges and the repulsion of like charges.
Car Spraying: To ensure the maximum amount of paint is applied to the car body, the car is given a charge and the paint the opposite charge. When the paint is sprayed, the opposite charges attract and produces a even coating of paint over the car.
__Photocopiers: __The drum inside the copier applies a static charge to the paper where the printing is required, this then interacts with the black ink powder called toner which has the opposite charge via induction. The opposites attract and the toner coats the paper to form the required image.
Insecticide Sprays: As the spray leaves the nozzle from the spray container on the tractor or plane it is given a charge. As all the droplets of insecticide have the same charge they will repel each other making them spread out. This provides a even, wide area of coverage, and the charge is also attracted to the earth so ensure that it is targeted at the crops and not blow away to other fields etc.
__AED: __ Automatic External Defibrillators are devices that can be used by anyone to help someone suffering from a heart attack. They have two pads that are placed on either side of the chest, the AED then analysis the heart beat and if needed provides an electric shock to try and correct the heart rhythm.
The unwanted build up of a static charge can be dangerous in some situations. Although getting a shock from a car handle is an unpleasant surprise the current that flows in very small and only lasts a fraction of a second. Such a low current will not cause any harm, the potential for an accident can come from the small spark that occurs when the static is discharged.
A spark near any source of flammable materials, like fuels could be very dangerous. When fuel is being pumped into any vehicle the fueling system or the vehicle have to be earthed to allow any buildup of charge to escape to earth via the earthing wire and thus prevent a spark.
When an airplane is being refueled the flow of fuel in the pipe is enough to generate a static charge from the friction of the moving fuel. To prevent any risk of a spark the plane is earthed via a cable to allow for the safe discharge of any static.
During flight a plane has the potential to build up a static charge due to the friction of the plane with the air. This could cause sparks creating a fire risk, the static can also affect the planes radio equipment. To allow this charge to discharge back into the air the trailing edges of the wings have small metal strips called discharge wicks. These allow the excess electrons from the friction to escape harmlessly back to the atmosphere.
- If a strip of plastic is rubbed with a nylon cloth, the strip can build up a static charge, explain why this happens.
- Your answer should include: friction / materials / electrons / strip / cloth
Explanation: The friction between the two materials moves electrons from the strip to the cloth. This gives the cloth a negative charge as it accumulates electrons. The strip is left with a positive charge as it is losing electrons.
- Explain how static electricity can help with the efficient spraying of insecticides.
- Your answer should include: repel / spray / even / evenly
Explanation: The insecticide spray is given a small charge as it is sprayed from the container. The like charges on all the small droplets means that they repel each other to help the spray spread out evenly. The charge also produces an attraction force with the earth pulling the sprayed droplets downwards towards the crop being treated.