If you rub a piece of amber (fossilised tree resin) it becomes electrically charged and can be made to attract small objects. The Ancient Greek name for amber is ἤλεκτρον (ēlektron) which is where we get the word electricity. There is a force of attraction between positively charged protons and negatively charged electrons called the electrostatic force. It is one of the four fundamental forces of nature. When two surfaces touch, it is possible for electrons to be rubbed off one and onto the other. This results in one surface having a positive charge (because it has lost electrons) and the other surface having a negative charge (because it has gained electrons). If one of the surfaces is an insulating material (doesn’t conduct electricity) then the charged particles do not spread out but remain static (fixed in place). This is known as a static charge; or static electricity.
The ease with which materials either give up or grab electrons is compared in the triboelectric series. Materials near the top tend to lose electrons and become positively charged whilst those near the bottom gain electrons and become negatively charged. The further away two materials are on the table, the greater will be the charge produced when they are brought together and then separated.
The Van de Graaff generator is a device for making and storing very large electrostatic charges. The one we have at school has two rollers connected by a belt. The top roller is made from aluminium and the bottom roller is made from Teflon The belt is made from rubber so as it rubs against the bottom roller it acquires a positive charge (it is higher in the triboelectric series than Teflon). When the belt reaches the dome it attracts electrons away making the dome positively charged too. The belt is now neutrally charged as it heads back down to the bottom roller.
The dome is made of metal so the charge is evenly distributed. The charge builds up until it can discharge to earth. A certain amount of charge leaks into the atmosphere (depending on how humid the air is) so there is a limit to how much charge the dome can hold at one time.
When you put your hands on the metal dome, some of the charge is transferred to you. Each individual hair on your head gets the same charge, and since like charges repel one another, your hair tries to get as far away from every other hair as possible. That is why it sprays out into a spherical arrangement.
Van de Graaff generators are capable of producing huge voltages; tens to hundreds of thousands of volts. That sounds more dangerous than it is because they do not produce very much current and the current they do produce only lasts for fractions of a second. Whilst about 1 A is enough current to stop a human heart, it takes quite a significant amount of electricity to force 1 A to flow through a human body. We do not make great conductors. Mains electricity, being alternating current at 240 V, is enough to kill you though so please don’t be tempted to dismantle or otherwise play with mains powered devices. The Mythbusters tested a claim that a plastic pipe on a building site became so statically charged that it killed a construction worker. As part of the program they built a large Van de Graaff generator trying to produce some really high voltages.
Questions…
- What device is used to measure current?
- What is different about the way you connect ammeters and voltmeters in a circuit?
- Give three examples of insulating materials.
- Apart from being conductors, suggest what other physical properties all metals share.
- What is the largest current that standard UK domestic appliances can draw?
- How does alternating current differ from direct current?
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