2 Overview of the Chapter Charges in an AtomCharging materials by frictionForces between chargesForces between charged and uncharged objectGold leaf electroscopeUse of static electricityProblems of static electricity
3 8.1: Charges within an atom All atoms contain small particles called protons, neutrons and electrons.Protons and neutrons are found in the centre of nucleus of the atom.Protons have a relative charge of +1.Electrons have a relative charge of -1.Neutrons have no charge.Normally the number of protons and electrons in an atom are equal. So, the atom has no overall charge. It is neutral.If an atom gains extra electrons it becomes negatively charged.If an atom loses electrons, it becomes positively charged.
5 8.2: Conductor and Insulator Materials through which electricity or charges can flow are called electrical conductors.Metals are good conductor – such as copper, gold etc.Materials though which electricity or charges cannot flow are called electrical insulators.Plastic, rubber, glass, wood are insulators.
6 8.3: Charging by frictionIf an uncharged plastic rod is rubbed with an uncharged cloth, it is possible for both of them to become charged.During rubbing, electrons from the atoms of the rod may move onto the cloth. There is now less electrons in rod so rod is now positively charged.As there is excess electrons in cloth so cloth become negatively charged.
8 8.3.2: Points to RememberWhile explaining charging two points should be stated clearly.1. Due to friction, electron moves (from object A to B if stated in question)2. So there is imbalance of charge in the atoms of the object (object B has more/less electrons)So charged (object B is negatively/positively charged)
9 8.4: Laws of electric charges: Similar charges repel and opposite charges attract.
10 8.5: Explaining with static charges: A build-up of static electricity means that each hair on the girl’s head has the same charge. Same charges repel. This electrostatic repulsive force is strong enough to overcome weight of the hair. So the hair rises.
12 8.6: Forces between charged and uncharged object: It is possible for a charged object to attract something that is uncharged.If a negatively charged balloon is brought near to an uncharged wall, some of the electrons are repelled from the surface of the wall. This gives the surface of the wall a slight positive charge that attracts the negatively charged balloon.
13 8.6.1: Forces between charged and uncharged object:
14 8.7: Electroscope:The electroscope is an instrument for detecting electric charge.Example: Pith ball electroscope, gold leaf electroscope.By using electroscope we can:Indentify the presence of chargeIndentify the type of chargeCompare the amount of charges on two objects.(measure the amount of charges-not needed for your level)
15 8.7.1: Gold Leaf Electroscope: Picture shows a modern gold leaf electroscope.A very thin gold leaf is hinged with the metal rod which deflect due to charge.
18 8.8: Use of static electricity: Electrostatic Paint Spraying:When the paint comes out from the nozzle of the spray they get same charges.As alike charges repel they form a fine cloud of paint droplets.The metal body is given opposite charge.As unlike charges attract, the paint droplets are attracted by the metal objects which should be painted.In this way less paint is wasted and paint cal reach to awkward corners.
19 8.8: Use of static electricity: Electrostatic Paint Spraying: Check videoInkjet printed: Check videoPhotocopier: Check videoElectrostatic Precipitators: Check video