P2aL 16 Uses and Dangers of Static Electricity
Key Words
Earthed conductor - wire connected to the ground, which carries a potential of zero volts.
High Voltage - potential difference of several thousand volts.
Induce - electrons repelled to the far side of a charged object by a negative charge.
Insulating - does not allow the flow of electric charge.
Ionised - Electrons knocked off (or added to) an atom to make it charged
Negatively charged - material having more electrons than protons.
Photoconductive - material that conducts electricity when exposed to light, but not when it's dark.
Positively charged - material that has fewer electrons than protons.
Static electrical charge - charge on a charged insulated object.
Grade E
If non conducting materials are moved quickly, a static electrical charge can build up, making a high voltage, which can cause a spark. While you may get a small shock when you get out of a car, you can understand that if there were flammable materials around, even a tiny spark could ignite an explosion. This can happen:
- in operating theatres that use flammable gases for anaesthetics;
- in flour mills where large amounts of dry powder are moved at high speed;
- when oil tanks in ships are cleaned out;
- when large amounts of fuel are pumped at a high rate onto an aeroplane.
To prevent this an earthed conductor is included so that charge can flow away to the ground, preventing charge from building up.
In this picture you can see a wire hanging down by the nose-wheel of the aeroplane. It is called a bonding line and conducts the charge away to earth.
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.
Grade C
When smoke from a power station boiler goes up the chimney, huge amounts of soot, ash, and dust could make a mess of the surrounding area. The dust can be removed by an electrostatic dust precipitator, which applies a negative charge to the dust particles. They are attracted to positively metal plates as shown in this animation:
The photocopier uses a photoconductive drum to apply toner powder to a piece of paper. Where there are light regions, the static electricity applied by a corona wire (a wire charged to a high voltage) is conducted away. Where there are dark regions, the static electricity makes the toner powder stick to the paper. The paper is then heated to melt the toner powder so that it sticks. Click on the button to find out more.
Grade A
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. Click on the picture to go to the QUAD website.
The price for these is £6500 (7000 euros). If you can afford to live in a house like that...