Noadswood Science, 2012

Static Electricity  To understand what causes static electricity Saturday, May 09, 2015

Atom  Draw and label a sodium (Na) atom – what happens to the charge of the atom if it gains or loses an electron?  Sodium (Na): –  Mass number of 23 (total number of protons and neutrons)  Atomic number of 11 (number of protons (as well as informing you of the number of electrons is 11 too)) Na

Atom  Sodium (Na): –  Mass number of 23 (total number of protons and neutrons)  Atomic number of 11 (number of protons (as well as informing you of the number of electrons is 11 too)) Na Electrons (-ve) Protons (+ve) and neutrons (neutral) within nucleus

Atom  In an atom the number of electrons is the same as the number of protons  This means the atom has no net charge (the –ve electrons and +ve protons cancel each other out)  Atoms can gain or lose electrons, forming ions (groups 1, 2, 6 and 7 are the elements which most readily form ions): -  Group 1 and 2 elements lose electrons to form +ve ions (cations)  Group 6 and 7 elements gain electrons to form –ve ions (anions)

Balloon  Why does a balloon stick to our hair / the wall when it is rubbed?!

Static Electricity  Static electricity is caused by charges which are not free to move – this causes them to build up in one area, and often ends with a spark / shock when they are finally able to move…  When two insulating materials are rubbed together, electrons are scrapped off one, and dumped onto the other  This leads to a positive static charge on one, and a negative static charge on the other (only the electrons move) Polythene rod (e - move to rod)Acetate rod (e - move to duster)

Static Electricity  Like charges repel, opposite charges attract  As charge builds up, so does the voltage – causing sparks!  The greater the charge on an isolated object, the greater the voltage between it and the Earth  If the voltage gets big enough it can cause a spark which jumps across the gap  High voltage cables can be dangerous for this reason  A charged conductor can be discharged safely by connecting it to Earth with a metal strap

Static Electricity  When something charged comes near something which isn’t charged it is induced (electrons in the uncharged object move towards or away from the charged object)  The new arrangement of charge always makes the two objects pull together, because repelling charges are now further apart than the attracting charges attraction

Van de Graaff Generator  Observe the Van de Graaff generator  How does it work?  Why does it make your hair stand on end?  Why can it be dangerous?! *Do not go near if you have a pacemaker

Van de Graaff Generator  The Van de Graaff generator has a dome which charges up when the generator is switched on (massive sparks can occur if the charge on the dome builds up too much)  The belt rubs against a felt pad, becoming charged  The belt carries the charge onto an insulated metal dome  Sparks are produced when the dome can no longer hold any more charge

Van de Graaff Generator  Sparks - connect the discharge wand to Earth and switch generator on (wait a few minutes and then bring wand close to dome to see sparks)

Van de Graaff Generator  Hair - have individual on insulating box and hold generator  Why does your hair stand on end?  The charge transfers to the individual and hair follicles get charged  The hair follicles then try to repel one another (which is why it stands on end)

Van de Graaff Generator  Soap – blow some bubbles towards the generator  Why do they float towards it, but then float away?  The bubbles are initially attracted to the charged dome – a few bubbles will actually touch the sphere and pick up the charge on the sphere  Once charged by the sphere they will be repelled and burst, causing charged ions to spray onto the other on-coming bubbles – once sprayed by the charged ions the on-coming bubbles will repelled, suddenly stopping and then reversing direction

Van de Graaff Generator  Rice krispies – place (and stick) and small plastic bowl on the top of the generator and add some rice krispies  Why do they erupt out of the bowl?!  The rice krispies all pick up the charge from the generator, and try and repel one another

Van de Graaff Generator  Flying saucers – place 10 metal saucers the top of the generator  Why do fly off, one by one?  The top pan lifts off as it picks up the charge and is repelled, then next pan picks up charge and it is repelled etc…

Van de Graaff Generator  What will happen to the fluorescent tube when it is placed on the Van de Graaff generator which has been on (for a short time)!  Charge flows from the Van de Graaff generator through the tube and down to Earth, lighting it up (small charge and small enough voltage (will still give a slight shock)!

Lightning  What causes lightning?

Lightning  Rain droplets fall to Earth with a positive charge – as they do a huge voltage builds up, eventually leading to a huge spark!

Static Problems  Static electricity – clothing crackles  When synthetic clothes are dragged over each other (e.g. when they are in the tumble dryer / being pulled over your head) electrons get scrapped off, leaving a static charge on both parts  This leads to an attraction, as well as little sparks / shocks as the charges rearrange themselves

Static Problems  Static electricity – car shocks  Air rushing over the car can give it a positive charge – getting out and touching the door causes the electrons to flow from earth, through you and to the car (which causes a shock for you)  Some cars have conducting rubber strips which hang down behind the car, grounding it

Static Problems  Static electricity – chutes, rollers and fuel filling  As fuel flows out of a filler pipe, paper drags over rollers or grain shoots from grain pipes then static electricity can build up leading to a spark (which can be very dangerous around flammables)  This is why nozzles or rollers are made out of metal so the charge is conducted away instead of building up (as well as having earthing strips between the fuel tanks and fuel pipes)

Static Problems  Static electricity – surgery  In hospitals anaesthetic gases are used during surgery (if this were to escape into the air then a tiny spark could make it explode)  To eliminate static charge an antistatic material is used for the floor surface (the material is a poor electrical insulator, so it conducts charges to Earth)  The surgical clothes are also antistatic, again preventing any charges building up causing a spark

Static Electricity – Uses  There are a variety of uses for static electricity, including: -  Photocopies  Inkjet printers  Electrostatic precipitators

Photocopiers  Photocopies and laser printers work in a similar way: -

Electrostatic Precipitators  Many power stations burn fossil fuels such as coal and oil, producing smoke  Smoke comprises tiny solid particles, such as carbon which has not reacted, which can damage buildings and cause breathing difficulties so the smoke is removed from waste gases before they pass out of the chimneys using an electrostatic precipitator…

Electrostatic Precipitators  Smoke removal… 1. Smoke particles pick up a negative charge 2. Smoke particles are attracted to the collecting plates 3. Collecting plates are knocked to remove the smoke particles