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It's shocking

Like your movement over carpet, other forms of travel also cause the build up of static electricity. The friction from air resistance on a moving car builds up as static electricity, which discharges when the car stops. The passengers in the car might feel a slight shock if they touch the metal body of the car before the electricity discharges. Some cars have an earthing strap that discharges the static electricity when the car stops to prevent passengers from getting a shock.

The same thing occurs in air travel. A plane builds static electricity from air resistance during a flight. A spark from a discharge could start a fire when the plane refuels, so it is important to discharge the static in another way. An aeroplane usually has static discharge wicks on its wings to release the static into the atmosphere and an earthing strap to discharge the electricity to the ground. These discharge methods ensure that the electricity does not leap from the plane to the fuel pipe and cause a fire. Refer Image 1

Does your computer screen need a clean? Dust tends to accumulate on electrical equipment because electricity has a charge that attracts particles of dust. This might also occur when you polish surfaces with a cloth - the rubbing motion of the cloth gives the surface a charge, which then attracts dust. No wonder things seem dirty even though you have just cleaned them!

Cleaning companies have developed microfibre cleaning cloths to counteract this problem. The chemical process used to manufacture microfibre leaves the fibre with a charge, so when you run the cloth over a dusty surface, you use static electricity to attract and remove the dust.

The Van de Graaff generator

In 1929, American physicist Robert Van de Graaff developed an electrostatic generator capable of producing 80 000 volts in a hollow metal sphere. Electricity comes from the frictional movement of a belt within the generator, transferring electrons to the sphere. If you touch the sphere, the negative charge from the excess electrons enters your body, giving you a negative charge. Negative charge repels negative charge, so your hair stands on end due to repulsion. Refer Image 2

Another component of the generator is a small sphere, which is earthed. When the voltage reaches a certain level, the charge becomes visible through the air between the large sphere and the small sphere. The Van de Graaff generator can be used in a number of experiments involving static electricity.

Static electricity in nature

Lightning is a form of electricity that occurs in nature. The movement of air and water droplets in a cloud generates a charge where the positive particles sit near the top of the cloud and the negative particles sit at the bottom. When the cloud meets another cloud, the positive particles from one flow to the negative particles in the other, which is expressed as light (lightning) and sound (thunder).

Eighteenth century scientist Benjamin Franklin was the first to hypothesise that static electricity caused lightning. Franklin famously wrote about an experiment that involved flying a kite in a storm with the kite string attached to a metal key. If he conducted the experiment in this manner, he was certainly insulated (protected from electrocution) because the power from a lightning bolt would have killed him. It is more likely, however, that he flew a kite on a cloudy day and observed small sparks on the key, which proved that static electricity existed in the atmosphere, leading to his conclusions about static electricity causing lightning.

Using static electricity

In addition to microfibre cleaning cloths, we also use static electricity in other aspects of technology. Black and white photocopiers use the contrast between light and dark to reproduce an image. The photocopier scans the image to find the light and dark parts of the text or image. The dark regions are given a positive charge, and then negatively charged toner, a type of powder, becomes attracted to the positive areas. Paper rolls over the toner and is then subject to heat to fix the toner to its surface. The image remains on the paper.

Some spray nozzles use static electricity to charge the particles of liquid, such as paint or pesticide, emerging from the bottle. If all the particles are the same charge, then they will repel each other when they leave the nozzle, so they spread evenly when the liquid reaches the object.

Other items that use static electricity include air filters, which have charged wire meshes to attract particles in the air, and audio speakers, where an electrostatic field emits enough force to move a diaphragm in a speaker, influencing the movement of air and thus sound.


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Question 1/5

1. Which force makes your hair stand on end if you touch a Van de Graaff generator?

earthing

discharge

repulsion

attraction

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