Static Electricity 4 th form IGCSE Textbook: Chapter 8, p. 66.

Static Electricity

4th form IGCSETextbook: Chapter 8, p. 66

Static Electricity?Until it discharges...

So what is Lightning?

• Slow motion view• A flow of electric

charge - current• But where did the

charge come from?•

Reminder: Conductors and Insulators

• Good conductors allow charges to flow through them easily– Usually metals

• If a conductor becomes charged there tends to be a flow of electricity to earth to neutralise it

• Good insulators do not allow charges to flow easily– Plastic, rubber, glass, wood, ceramic etc

• If a charge builds up on an insulator, it will stay there

How do you charge something up?

• To create a static charge we need to add or remove some electrons– Remember the protons are stuck in the atomic nuclei

and can’t move• We can do this in a variety of ways:– Connecting to an electrical supply– Introducing another charged object– Squashing some materials– Heating some materials– Or just by rubbing electrons on or off!

Triboelectricity

• From greek τριβή: friction• First recorded by Thales of Miletus (c 600BC)• Amber (greek name: elektron) charges up

when rubbed with wool• In general, when two different materials come

in contact, some charge is transferred from one to the other– Effect is increased by rubbing

Triboelectric Series

Charging with friction

• Rubbing one material with another transfers some electrons.

• Which material gains electrons and which loses them depends on their positions in the series.

Why does this need to be insulated from its surroundings?

• Friction charges water and ice particles• Charge builds up in the cloud• It is insulated from its surroundings – static electricity

Lightning animation

Lightning facts

• There are 44 lightning flashes/second on Earth

• Only 20% of these are to the ground

• Kikufa, Congo, receives the most strikes per year at 158/km2

• Lightning travels at 130,000 mph and can reach temperatures of 30,000°C

• Annual world deaths due to lightning strike are estimated at 24,000

Experiment

• Charge a rod by rubbing it.• Hang it in a stirrup, making sure it doesn’t

touch anything.• Charge another rod of the same material

and bring it close to the charged end of the suspended rod. What happens?

• Repeat with different materials, record what you find.

• Can you draw any conclusions?

Forces between charges

• Objects with the same charge repel each other• Objects with opposite charges attract each

Action at a distance

What’s happened to this boy?

Van der Graaf Generator

• A convenient way to build up a store of static electricity

• Charges are transferred up the belt to the metal dome

Force between charges and uncharged objects

• A charged object always attracts an uncharged object.

• Can you explain why?

Force between charges and uncharged objects

• The uncharged object stays uncharged, but the electrons are attracted to or repelled from the charged object

• This leaves the opposite charge near the surface. So the two objects attract one another

• The same process explains why balloons stick to the wall.

• Water bends because it is a polarised moleculeapplet

The gold leaf electroscope

• A simple device to detect electric charge

• Metal plate, rod and leaf are insulated from surroundings

• Initially electrons are spread evenly throughout neutral electroscope

• If a charged object comes close the electrons move and the leaf rises

Charging an electroscope

• The leaf rises if a charged object comes near, but falls when it moves away.

• If the charged object touches the metal plate electrons can flow on or off the electroscope, leaving it charged– The leaf remains up

• How could you discharge it?

Charging by induction

• You can use a charged object to separate the charge and then add or remove electrons from earth to charge the electroscope.

Static dangers• Lightning is a static discharge.

Lightning conductors are used to protect buildings

• Static on hands can damage sensitive electronics. An earthing strap must be worn.

• Static builds up on aeroplanes as they fly. Planes must be properly earthed before refueling.

• Also applies to petrol stations

Static dangers

• When powders are pumped through pipes they can build up a static charge by friction.

• If the pipe is not earthed this can produce a spark and cause an explosion.

Applications of static electricity

• By creating opposite charges on paint particles and the target, spray paint is attracted to the target, improving even coverage and reducing wastage.

Demo film

Applications of static electricity

• Ink in an inkjet printer is given a charge. It can then be directed onto the right spot on the paper by varying the charge on metal plates as it flies through the air.

Applications of static electricity

• Static is used to selectively stick toner where it is required to the photocopier drum

• Animation• Secret life video

Applications of static electricity• An electrostatic precipitator can remove ash particles

from smoke in a chimney, reducing pollution.

Students will be assessed on their ability to:2.20 identify common materials which are electrical conductors or insulators, includingmetals and plastics2.21 recall that insulating materials can be charged by friction2.22 explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons2.23 recall that there are forces of attraction between unlike charges and forces of repulsion between like charges2.24 explain electrostatic phenomena in terms of the movement of electrons2.25 recall the potential dangers of electrostatic charges, eg when fuelling aircraft and tankers2.26 recall some uses of electrostatic charges, eg in photocopiers and inkjet printers.