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TOPIC 2: ELECTRICITY

2.8 Static electricity

As the name implies, static electricity does not flow through substances - it stays in one place. It is made when charges build up on insulators.
Before we go any further with this section, we need to understand the difference between conductors and insulators.

Conductors and insulators

An electrical conductor is a substance with charges that can flow relatively freely, and when a voltage is applied, a significant current flows. As you should know, the most common conductors are metals, of which copper is probably the most widely used in the electricity industry. Other conductors include tap water and graphite (a form of carbon found in pencils).
Metals have a large number of free electrons that can move through the substance, creating the current.

Insulators do not have free electrons or any other charges that can move easily and produce a current. The are poor conductors with a very high resistance. Common insulators are plastics, rubber and air.

How is static electricity produced?

The most simple way to produce static electricity is through friction. You should know how to perform a simple practical to produce static electricity and also to test that it is present, as shown in this video.

 

 

Youtube 1. Producing and demonstrating static electricity

 

The video shows how rubbing the plastic with the cloth produces friction, and it is this friction that produces a build-up of static electricity.

Simply rub the cloth on the plastic (or other insulator like Styrofoam or PVC).
To show that a static charge has been produced, try picking up small pieces of tissue paper as shown in the video. This demonstrates that a force has been produced between the plastic and the paper. You can then try other substances and see which ones work and which do not. Not all insulators work with this type of cloth duster, and the experiment needs to be in a dry, warm place. Static leaks away quickly in the presence of water, so if you are in a humid country, this experiment rarely works!

In the next video, we can see a wide range of demonstrations and some great tricks!

Youtube 2- Awesome tricks with static electricity (brusspup)

 

How static charges are produced

Insulators do not have freely moving electrons like metals. However, when we rub a cloth on an insulator we can 'dislodge' a few of the electrons on one surface, and they pass from one surface to the other, as shown in figure 1 here:

Rod before charging rod after charging
Before the experiment After rubbing with the cloth

Figure 1 - Charging a rod with a cloth

This diagram simplifies the picture by only showing a few of the charges present - four of each type instead of billions! As can be seen in the first image, the cloth and rod have equal positive and negative charges, so both of them are neutral. (No net charge present).

However friction dislodges some of the charges. In this example, the negative electrons move from the rod to the cloth. This makes the rod have an overall positive charge and the cloth has a negative charge.

Some substances hold on to their electrons tighter than others, and so the direction of travel of the electrons depends on the substance used. In figure 2, a different plastic is used, resulting in the electrons moving in the opposite direction:

A rod being charged with a negative charge through friction

Figure 2 - Producing a negative static charge on the rod

 

In this example, the negative electrons now move from cloth to rod, leaving the rod negative and the cloth positive.

Note that it is ONLY negative electrons that move - the positive charges (protons in the nucleus) are not able to move through friction.
You do not need to learn which plastics turn positive or negative, only how the process works.

 

Attraction and repulsion

Just like magnets, static charges produces forces of attraction and repulsion, and the rules are very similar:

attraction and repulsion of charges

Figure 3 - Static repulsion and attraction

 

Both of the YouTube videos at the start of this section show uncharged objects being attracted to a charged one. This is the standard demonstration of static electricity, although why it happens is quite tricky, as explained here:

Picking up paper

An uncharged piece of paper has billions of positive and negative charges, in equal numbers. In our simplified example, we will look at a few of them. The paper has a neutral charge overall. However, when a charged rod is held nearby, the charges separate a little. In this example, a positive rod causes the positive charges in the paper to be repelled to the bottom, and the negative charges to be attracted upwards. The movement can be microscopic, but again it is exaggerated here.

The negatives are pulled upwards, and the positives are pushed downwards. However the negative charges are now slightly closer to the rod, so this has a stronger upwards force than the repulsive downward force.
The end result is that the paper is pulled upwards.

Figure 4 - picking up paper with static electricity

 

Here are some common examples of attraction and repulsion:

 

Questions:

1. Describe a simple experiment to produce static electricity. Include an explanation of how you can demonstrate the presence of static electricity.

The answer should include a description of:

To demonstrate the presence of static, the insulator must be placed next to another insulator such as hair, small tissue paper pieces (or another named insulator that is free to move) and a force of attraction (or repulsion if similar charge) stated.

 

Uses and dangers of static electricity

Uses

As well as producing some interesting tricks and demonstrations as shown earlier, the attraction and repulsion caused by static charges can be very useful.

1). Inkjet printers. If a rod is charged up, it can pull on water droplets flowing from a tap, and change the direction of the water flow. (Have a look again at the youTube video 2 above, 3 mins 34 secs). In the same way, a jet of ink droplets can be deflected, and this is the trick used in some inkjet printers. A stream of very small, charged ink drops is directed between charged plates. These plates can be charged to the correct voltage to push or pull the drop to the desired location on the paper.

One-line summary: Charged ink droplets are directed at the correct spot on the paper by forces produced from charged plates either side of the jet.

inkjet printer diagram

Figure 5 - how an inkjet printer works

 

2). Photocopiers. Inside a photocopier, a cylindrical surface called the 'drum' builds up a static charge. An image of the document to be photocopied is directed at the drum, and this causes some areas of the drum to lose the static charge. (The details here are not needed, but bright light causes the charge to be neutralised).
Only areas of the drum that are still charged will attract charged black ink (called toner), and so an ink picture of the document is created. Finally when a blank piece of paper is loaded on the drum, a copy of the original document is imprinted in ink.

One-line summary: Static electricity is used to attract ink to the correct location on a copy of the image, and this is then printed on the paper.

3). Dust in chimneys. Dust and soot is produced by fires, for example when burning coal. Fine soot and dust particles can cause asthma, and produce many other adverse health problems. However as the dust/soot is uncharged, it will be attracted to any charged surface. If large, charged metal plates are added to tall chimneys in power stations, the dust can be attracted to the sides and then safely removed.

One-line summary: Charged plates in power station chimneys attract uncharged soot and dust (caused by combustion) for safe removal.

Dangers

The main danger from static electricity is when their is a very high build up of charge and a spark is produced as this flows away to ground.

1. Petrol stations . When refueling a car or aircraft, a spark could ignite the fuel. Friction between the fuel flowing through the pipes can -in certain circumstances- produce large sparks. To prevent any build up of charge, the metal nozzle of the fuel pump is connected to an earth wire to discharge any static build-up.

The video below shows that static electricity near fuel can be extremely dangerous!

YouTube 3 - The dangers of static electricity (Brainiac Store)

2. Electronics. Many modern microchips and other miniature devices are very sensitive, and a static spark could destroy them. To prevent this, technicians often wear an earth wire on their wrists to discharge any static build up and prevent damage to expensive circuit boards.

3. Lightning. This is produced by friction in storm clouds - believed to be between water and ice particles. This build up of charge can be enormous, and eventually the voltage is so high that a giant spark jumps from cloud to the ground, or cloud to cloud. If a lightning bolt hits a building, damage can occur. To prevent this, tall buildings are fitted with a metal spike on top and a thick wire connected to the ground. The lightning strike will flow down this cable rather than through the building, preventing damage.

Questions

2. Explain how a positive charge can be produced on a plastic rod.

Friction between the rod and a cloth / duster results in electrons being transferred, from the rod to the cloth. This leaves more positive charges than negative on the rod.

 

Now test your understanding using these quick, 10 minute questions on this topic from Grade Gorilla:


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Topic 2: Electricity
Topic 3: Waves
Topic 4: Energy
Topic 5: Matter
Topic 6: Magnetism
Topic 7: Radioactivity
Topic 8: Astrophysics