Additional Physics Topic 8 - Static Electricity
Static Electricity can be described as "electricity at rest". You will have discovered static electricity for yourself when taking off your school jumper on a very cold, dry day. When I was at school (more years ago than I care to remember), there was a nylon carpet in one of the study rooms, and you could get quite a charge off it by shuffling up and down it.
Static electricity occurs in insulating materials (materials that do not conduct electricity), and arises due to the separation of charge.
Current electricity flows in conducting materials (e.g. metals).
How is static electricity made?
You will remember how in atoms there is a positively charged nucleus with protons and neutrons in it. Surrounding the nucleus there are electrons that are negatively charged. If an electron is removed from the atom, the atom is positively charged. If an electron is added to the atom it gets negatively charged.
What happens to the protons?
Let's suppose we rub a polythene rod with a cloth:
Electrons from the cloth are rubbed onto the polythene rod.
What charge does this give the rod?
Now suppose we charge an acetate rod with the cloth:
This time we rub electrons off the rod onto the cloth. The rod becomes positively charged, and the cloth becomes negatively charged.
If we hang the rods so that they can swing freely, we observe the following:
A charged polythene rod brought close to another charged polythene rod repels;
A charged acetate rod brought close to another charged acetate rod repels;
A charged polythene rod brought close to another acetate polythene rod attracts;
We can conclude from this simple experiment that:
like charges repel; unlike charges attract.
Investigating Charged Objects
The Gold Leaf Electroscope is a common instrument to investigate statically charged objects. Here is a picture of one:
Let's think what happens if we bring a charged rod up to the cap.
The negative charge on the polythene repels the electrons from the cap. The electrons go down the rod some to the bottom and some to the gold leaf. They repel and make the leaf rise. This is an example of producing static electricity by induction.
It is important to understand that only the electrons move. The protons are in the nucleus and NEVER move.
Now look at this animation and study it closely. Then answer Question 3.
A positively charged rod is brought up to the cap of the gold leaf electroscope. What happens and why?
Charge and Potential Difference
Charge is a measure of how many electrons are crowded onto a surface. Charge is measured in Coulombs (C). 1 C = 6.3 × 1018 electrons.
A 1 C positive charges arises when there is a deficiency of 6.3 × 1018 electrons.
A 1 C negative charges arises when there is an excess of 6.3 × 1018 electrons.
If we crowd more electrons onto a surface, we increase the charge and we also increase the potential difference (p.d.). The potential difference is also called the voltage, and is measured in volts (V). In static electricity, we have high voltages. It is easy to charge up an object to 30 000 V. If you touch that, you will only get a small shock, because the current is so tiny.
The most spectacular static electricity machine you will come across is the Van der Graaf generator.
The machine pumps electrons upwards to the top sphere. The electrons crowd onto the top sphere and make the potential difference very large indeed. As in all metal objects, the electrons crowd onto the outside surface.
If the breakdown voltage of air, 3000 V/mm is exceeded, a spark will jump to the discharge sphere. The charge will be conducted back to the base through the wire. This is shown in the animation.
When a big charge is being made on the top sphere, you can hear the motor slowing down as it has to work harder.
A spark is 10 cm long. What voltage is this?
Physics students love the Van der Graaf. Big sparks fly about the place, and students love to give each other shocks. A student (or teacher) can stand on the bench and its hair stands on end as in the picture below. It works better if you stand in a plastic tray.
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Once when I was doing this a spark jumped from my foot, across the plastic tray and the bench, to the base of the machine. The distance must have been about 40 cm, indicating a voltage of 1.2 million volts. I certainly felt it, and my class remembered it for a long time!
When you use the Van der Graaf generator, you must keep delicate electronic components well away. They can be destroyed by the high value electric fields produced by the machine. I always get my students to put their mobiles and laptops away in a metal cupboard which will shield them from large electric fields. I would also turn off the computer on my desk and the overhead projector. A colleague of mine ignored this advice. The resulting surge ruined the computer and caused the electricity supply to be cut off for the whole campus of our college.
Why would a shock of this sort of voltage not kill you?
There are other tricks you can play, e.g. the electron windmill. If you attach a spike to the top sphere, electrons stream off the machine. If you place an arrangement like this on the spike, it will turn.
The stream of electrons from a spike can be felt, and can blow out a candle flame.
If you place a plastic canister like this onto the machine:
You will see the following happen:
The balls pick up electrons and a negative charge is applied. They are attracted to the top plate (positive) and lose their electrons there. They then drop back to the bottom and the process is repeated.
If you blow a bubble towards the machine, the electrons in the bubble are repelled around the back, leaving the face nearest the top sphere positively charged. The positive charge is induced. The bubble is attracted. As soon as it touches the top sphere, electrons rush onto the bubble and it's immediately repelled.
Static Electricity can be Dangerous
Aeroplanes are expensive machines to run. They use huge amounts of fuel. A Jumbo Jet needs about 80 tonnes of fuel. They only earn money when they are flying, so it's important to get the fuel on board as quickly as possible. However rapid pumping of liquid fuel can result in a massive static charge building up that will lead to a spark. It doesn't take a genius to see how dangerous that could be.
Therefore a bonding line is placed between the aeroplane and the pump to conduct the charge to earth. In the picture below you can see the wire connecting the plane to the ground connected to the body just above the nose-wheel.
The same danger can occur when powders are pumped at high speed.
Lightning is the ultimate display of static electricity. The heat of the ground causes moist air to rise. As it gets colder the higher up you go, ice forms, and as ice particles bump into each other, a separation of charge occurs as shown in the picture below.
The charges separate. The positive charges go to the top of the cloud. The negative charges are at the bottom of the cloud. They induce positive charges in the ground by repelling electrons into the ground, leaving the positives where they are.
If the potential difference gets big enough, a spark will jump. The voltage is about 20 billion volts and the current about 20 000 amps. Fortunately the spark only lasts for about 1/1000th of a second.
In the thunderstorm, the repulsion of like charges results in the characteristic anvil cloud. The animation above shows an Earth strike (forked lightning) but it is just as likely for there to be a cloud discharge (sheet lighting) within the cloud, or between clouds
The energy can set fire to things, boil the sap in trees to make them explode, or kill a person who is struck (many people do survive though). The intense heating of the air results in a loud bang, the echoes of which give the distinctive rumbling sound of thunder.
In a flour mill, there are many tiny particles of flour (starch). These are jostling about all the time, exchanging electrons. Some flour particles gain too many electrons, while other flour particles lose electrons. A charge separation occurs, leading to a discharge. This can ignite the flour leading to a major explosion.
Static Electricity can be Useful
Your teacher will often give you photocopied sheets. The photocopier on which they were produced uses static electricity.
If you want to know how the photocopier works, click on the link below.
Another use is the electrostatic dust precipitator. The idea is shown in the picture below:
When the dust particle passes between the plates, electrons on the dust particle are repelled leaving the positives where they are. This is a process called induction. The left side of the particle is more negative, so the particle is attracted to the positive plate. It sticks there.
This device can remove over 95 % of dust particles.
Coal fired power stations produce huge amounts of ash, much of which goes up the chimney. Dust precipitators remove the vast majority of this. The plates are periodically struck by heavy mechanical hammers and the dust falls off into bins.
Sometimes it's useful to add a static charge. For example sprayed paint is given a positive charge. It is then attracted to the object which has negative charge. The paint is attracted, not just to the side being painted, but also round the back.
A very even coat of paint gets applied. A similar system can be used to apply herbicides to weeds.
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Electrostatic loudspeakers have a very thin electrostatically charged diaphragm (less than the width of a human hair) between two thin plates one of which has a very high negative voltage, and one that has a very high positive voltage. The alternating voltages set up by the musical signals attract the diaphragm forwards and backwards.
The advantage of this system is that the diaphragm is very light compared to a normal speaker cone. Therefore the speaker responds much better to the subtle signals in the music, which makes the music much more "alive".
The disadvantage is that they are hard to make and quite easy to damage. Therefore they are very expensive. So most hi-fi manufacturers continue to put cones into boxes. The picture below shows an electrostatic loudspeaker in front of an ordinary cone loudspeaker.
A company in Huntingdon makes electrostatic loudspeakers. The QUAD ESL-63 was manufactured for many years from the early 1960s. The panels are mounted in what was going to be a Belling electric heater. The heater sold very badly and was taken off the market; QUAD bought all the cases!
Now that they have run out of old Belling cases, they have designed something a bit more modern.
Photos by Quad Hi-Fi
Complete the interactive space-fill exercise.