Скачать презентацию KS 4 Physics Static Electricity Boardworks Ltd

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KS 4 Physics Static Electricity © Boardworks Ltd 2005

Contents Static Electricity Static charge Forces between charges Uses of static electricity Electrolysis Summary activities © Boardworks Ltd 2005

What is static electricity? What do these things have in common? l Crackles when combing hair. l Cling film sticking to your hands. l Clothes clinging to each other in a dryer. l Getting a shock when rubbing your feet on a carpet. l Lightning. They are all caused by static electricity. Static electricity is due to electric charge that builds up on the surface of an insulator. The charge that has built up cannot easily flow away from the insulator, which is why it is called static electricity. © Boardworks Ltd 2005

Benjamin Franklin and static electricity Meet the amazing American scientist Benjamin Franklin (1706 -1790), who was also a printer, librarian, journalist, inventor, and statesman – he even helped to write the American constitution. Franklin was fascinated by electricity and introduced the terms ‘positive’ and ‘negative’. He noticed the similarity between lightning and the sparks made with a Leyden jar and wanted to prove that they were the same. © Boardworks Ltd 2005

Benjamin Franklin and static electricity One stormy night in 1752, Franklin carried out his famous kite experiment to prove that lightning is a form of electricity. He used a silk kite with a metal wire at the tip and a metal key attached to the string. When the kite was in a thunder cloud, Franklin noticed parts of the string standing up like hair does due to static electricity. Franklin touched the key and there was a spark! It was an extremely dangerous thing to do and he was lucky to survive! © Boardworks Ltd 2005

Benjamin Franklin and static electricity Franklin also attached a Leyden jar to his kite. The storm clouds charged the jar in the same way as static electricity. We now know that lightning is due to the build up of static charge in storm clouds. Franklin had proved his theory that lightning is a form of electricity. He used his experiments to invent the lightning conductor – a metal rod to protect buildings and ships from lightning damage. © Boardworks Ltd 2005

Where does static charge come from? All materials are made of atoms, which contain electric charges. electron (negative charge) Around the outside of an atom are electrons, which have a negative charge. proton (positive charge) The nucleus at the centre of an atom contains protons which have a positive charge. An atom has equal amounts of negative and positive charges which cancel each other out. This means an atom has no overall charge. Electrons do not always stay attached to atoms and can sometimes be removed by rubbing. © Boardworks Ltd 2005

Where does static charge come from? Static charge can build up when two materials are rubbed together, such as a plastic comb moving through hair. When this happens electrons are transferred from one material to the other: l One material ends up with more electrons, so it now has an overall negative charge. l One material ends up with fewer electrons, so it now has an overall positive charge. © Boardworks Ltd 2005

Creating static charge What is the name of the force between two materials that are rubbed together? friction An insulating material can be charged by friction. For example, if an insulator is rubbed with a cloth, it can become charged in one of two ways: B. Electrons move from A. Electrons move from the insulator to the cloth to the insulator. © Boardworks Ltd 2005

Charging a polythene rod © Boardworks Ltd 2005

Charging polythene If an insulator made of polythene is rubbed with a cloth, electrons move from the cloth to the insulator. What charge does the cloth now have? The cloth is positively charged. What charge does the polythene insulator now have? The insulator is negatively charged. © Boardworks Ltd 2005

Charging an acetate rod © Boardworks Ltd 2005

Charging acetate If an insulator made of acetate is rubbed with a cloth, electrons move from the insulator to the cloth. What charge does the cloth now have? The cloth is negatively charged. What charge does the polythene insulator now have? The insulator is positively charged. © Boardworks Ltd 2005

Static charge – true or false? © Boardworks Ltd 2005

Contents Static Electricity Static charge Forces between charges Uses of static electricity Electrolysis Summary activities © Boardworks Ltd 2005

How do pairs of charges behave? © Boardworks Ltd 2005

Pairs of charges – attract or repel? © Boardworks Ltd 2005

Identifying an unknown charge If a rod has an unknown charge, how can the unknown charge be identified using a positively charged rod? ? l If the unknown charge is brought near to a positively charged rod and it is attracted to this rod, then the negative unknown charge must be ____. l If the unknown charge is brought near to a positively charged rod and it is repelled by this rod, then the positive unknown charge must be ____. © Boardworks Ltd 2005

Identifying an unknown charge If a rod has an unknown charge, how can the unknown charge be identified using a negatively charged rod? ? l If the unknown charge is brought near to a negatively charged rod and it is attracted to this rod, then the positive unknown charge must be ____. l If the unknown charge is brought near to a negatively charged rod and it is repelled by this rod, then the negative unknown charge must be ____. © Boardworks Ltd 2005

Inducing a temporary charge If a negatively charged rod is brought near to a piece of paper, the paper sticks to the rod. The paper is uncharged (equal amounts of + and -), so why does it stick to the rod? +-+-++-+-+- As the negatively charged rod approaches the paper, the electrons in the paper are repelled away from the rod. This makes one side of the paper positive and one side negative. A charge has been induced on the paper and the positive side of the paper is attracted to the negative rod. © Boardworks Ltd 2005

Inducing a temporary charge If a positively charged rod is brought near to a piece of paper, the paper sticks to the rod. The paper is uncharged (equal amounts of + and -), so why does it stick to the rod? +-+-++-+-+- As the positively charged rod approaches the paper, the electrons in the paper are attracted towards the rod. This makes one side of the paper negative and one side positive. A charge has been induced on the paper and the negative side of the paper is attracted to the positive rod. © Boardworks Ltd 2005

Contents Static Electricity Static charge Forces between charges Uses of static electricity Electrolysis Summary activities © Boardworks Ltd 2005

Uses of static electricity Static electricity can be dangerous but it can also be useful, as long as it is used carefully. Examples of uses of static electricity are: Photocopiers 1. __________ Printers 2. __________ Spray painting 3. __________ Pollutant removers 4. __________ © Boardworks Ltd 2005

How does a photocopier work? © Boardworks Ltd 2005

Photocopier activity © Boardworks Ltd 2005

An electrostatic paint spray Static electricity can be used to spray a car with paint: - + - - - - Paint gun nozzle has a positive charge. electrostatic generator The nozzle of the paint gun is connected to one terminal of an electrostatic generator. - - Car is negatively charged. The other terminal is connected to the metal panel, which is earthed. © Boardworks Ltd 2005

An electrostatic paint spray The spray gun is designed to produce tiny charged droplets of paint. + + +- - - - Paint gun nozzle has a positive charge. electrostatic generator - - Car is negatively charged. As a result the charged droplets are attracted to the car body panel. This gives a uniform coating of paint. Also, the droplets travel along the lines of force of the electrostatic field to reach hidden parts of the panel. © Boardworks Ltd 2005

Smoke precipitator to reduce pollution Static electricity can be used to reduce pollution in the chimney of a power station. chimney wall A metal grid at a very high voltage runs down the very high middle of the chimney. voltage on metal grid earthed Earthed metal plates run metal down the inside of the plate dirty chimney. smoke Dirty smoke particles become charged in the electric field. These charged particles are attracted to the earthed metal plates where they lose their charge and then fall back down the chimney. The result is clean smoke out of the top of the chimney. © Boardworks Ltd 2005

Dangers of static charge Filling fuel, rollers for paper and grain shoots are situations where charge can be a problem. Static can build up as the fuel flows along the pipe or paper rolls over rollers or grain shoots out of tubes. This can easily lead to a spark and then an explosion. To prevent this happening, the nozzles or rollers are made out of metal so any charge build up is conducted away. Large petrol tankers always have earthing straps between the tanker and the storage tank to prevent the risk of sparks. © Boardworks Ltd 2005

Transferring flammable fuels What prevents the build up of large static charges that could cause an explosion? nozzle from tanker underground tank or aeroplane tank electrical link © Boardworks Ltd 2005

Contents Static Electricity Static charge Forces between charges Uses of static electricity Electrolysis Summary activities © Boardworks Ltd 2005

How is electrolysis used? Anode is made of impure copper and this dissolves. The cathode starts as a thin piece of pure copper. More copper is deposited. c C a a Cu 2+ 2+ Cu Cu Cu tt copper sulfate solution h h electrolyte o o d d Cu 2+ e e a A n n o o d d e sludge (impurities) © Boardworks Ltd 2005

Changing the rate of electrolysis © Boardworks Ltd 2005

Charge, current and time The relationship between charge, current and time is shown by the equation: charge = current x time Q = Ixt What are the units of charge, current and time? l Charge is measured in coulombs (C). l Current is measured in amps (A). l Time is measured in seconds (s). © Boardworks Ltd 2005

Q = It formula triangle A formula triangle helps you to rearrange a formula. The formula triangle for Q = It is shown below. Whatever quantity you are trying to find, cover it up and this will leave the formula required. So if you are trying to find current (I). . . …cover up I… …which gives the formula… Q I t Q I = t x © Boardworks Ltd 2005

Using Q = It A current of 6 A flows for 3 minutes. How much charge flows in that time? charge = current x time = 6 A x (3 x 60) s = 1080 C TOP TIP: Always remember to convert time into seconds! © Boardworks Ltd 2005

Energy, charge and voltage The relationship between energy, charge and potential difference (or voltage) is shown by the equation: energy = charge x voltage E = Qx. V What are the units of energy, charge and voltage? l Energy is measured in joules (J). l Charge is measured in coulombs (C). l Voltage measured in volts (V). © Boardworks Ltd 2005

E = QV formula triangle A formula triangle helps you to rearrange a formula. The formula triangle for E = QV is shown below. Whatever quantity you are trying to find, cover it up and this will leave the formula required. So if you are trying to find charge (Q). . . …cover up Q… …which gives the formula… E Q V E Q = V x © Boardworks Ltd 2005

Using the E = QV formula triangle © Boardworks Ltd 2005

Using E = QV A charge of 100 C is delivered at a potential difference of 5 V. How much energy is delivered? energy = charge x voltage = 100 C x 5 V = 500 J What is 500 J in k. J? 0. 5 k. J TOP TIP: In exams, you must always give the units. If you do not, you will lose a mark! © Boardworks Ltd 2005

Charge calculations © Boardworks Ltd 2005

Contents Static Electricity Static charge Forces between charges Uses of static electricity Electrolysis Summary activities © Boardworks Ltd 2005

Glossary lattraction – The force between two oppositely charged objects which pulls them together. learthing – Electrically connecting an object to the Earth to prevent charge building up. linduced charge – A temporary charge that forms on a neutral object because it is near a charged object. lrepulsion – The force between two objects with the same charge which pushes them apart. lstatic charge – A charge that builds up on an object but is not moving. © Boardworks Ltd 2005