Electric charges Electrostatics at work Introduction Two kinds of charges Electric force between charges Where do charges come from? Unit of charges E

Electric charges Insulators and conductors Attraction of uncharged objects Check-point 1 Earthing Fun with electric charges Check-point 2 ? ?

Electric charges Electrostatics at work A lot of everyday life phenomena are related to electrostatics: Dangerous Lightning Useful Photocopying, Laser printing Unpleasant Slight electric shock in a cold dry day

Electric charges Introduction After being rubbed, a plastic ruler can attract paper scraps. The ruler carries electric charge. The interaction between static electric charges is called electrostatics. This charging method is called charging by friction. It exerts electric force on paper.

Electric charges Experiment 14a Observe what happens when 2 rubbed plastic strips are brought close. Charging by friction Video

Electric charges 1Two kinds of charge In Expt 14a, Rubbed acetate/polythene strips repel.  The two strips have the same type of charges on them. Rubbed acetate strip and rubbed polythene strip attract.  The two strips have different types of charges on them.

Electric charges 2 Electric force between charges Like charges repel; unlike charges attract. Charges: positive (+) or negative (–)

Electric charges 2Electric force between charges Electric force increases as the separation between the charges decreases. Electric force increases as the amount of charge on the object increases.

Electric charges 3Where do charges come from? Matters are made up of atoms. E – – – + – proton (+1.6  10 –19 C) neutron (neutral) electron (–1.6  10 –19 C) atom nucleus

Electric charges If electrons no. = proton no.  neutral 3Where do charges come from? E If electrons no. > proton no.  gaining electrons, –ve charged If electrons no. < proton no.  losing electrons, +ve charged

Electric charges When a duster rubs an acetate strip... 3Where do charges come from? E – electrons are pulled from strip to duster. Duster has more electrons than usual – – – – – Duster: –ve charged Strip: +ve charged acetate strip

Electric charges Similarly, when a duster rubs polythene strip... 3Where do charges come from? E – electrons are pulled from duster to strip. Duster has less electrons than usual – – – – – Duster: +ve charged Strip: –ve charged polythene strip

Electric charges Rubbing materials does not create electric charges. It just transfers electrons from one material to the other. 3Where do charges come from? E Charge cannot be created or destroyed.

Electric charges 4Unit of charge Unit of charge: coulombs (C) The charge of an electron is -1.6  10 –19 C. The charge of a proton is +1.6  10 –19 C.

Electric charges 5Insulators and conductors Insulators: materials that do not allow electrons to flow through them easily. Insulators can be easily charged by friction as the extra electrons gained CANNOT easily escape.

Electric charges 5Insulators and conductors Conductors: materials that allow electrons to flow through them easily. Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape. Outermost electrons of atoms of metals are called free electrons.

Electric charges 6Attraction of uncharged objects When charged rod is put near a metal foil... electrons of the foil are pulled towards the rod.  top of the foil: –ve attraction > repulsion – – – – – metal foil buttom of the foil: +ve induced charges attraction repulsion

Electric charges 6Attraction of uncharged objects Simulation

Electric charges 6Attraction of uncharged objects Similarly, when charged rod is close to paper scrap molecules of paper align.  attraction between the rod and –ve charge > repulsion between the rod and +ve charge. paper + – + – + – + – + – + – + – + – attraction repulsion

Electric charges 6Attraction of uncharged objects

Electric charges Check-point 1 1Use arrows to show…Use arrows to show… 2When we charge an…When we charge an… 3When we charge an…When we charge an… 4How does a positively charged rod...How does a positively charged rod...

Electric charges Check-point 1 – Q1 Use arrows to show the electric forces acting on the balls. (‘N’ denotes that the ball is neutral.) (a) (b) (c) (d)

Electric charges Check-point 1 – Q2 If the rod can attract some paper scraps, which of the following cannot be the charge of paper scraps? ANeutralBPositive CNegative When we charge an acetate rod by rubbing it with a piece of woollen cloth, the acetate rod becomes positively charged.

Electric charges Which of the following have been transferred between the acetate rod and the woollen cloth in rubbing? AElectrons BProtons CNeutrons E Check-point 1 – Q3 When we charge an acetate rod by rubbing it with a piece of woollen cloth, the acetate rod becomes positively charged.

Electric charges When a +ve charged rod is put near neutral object, ______________ is induced on the side of the object near the rod and _____________ is induced on the side away from the rod. Check-point 1 – Q4 How does a positively charged rod attract a neutral object? negative charge positive charge

Electric charges The rod can attract the netural object because _________ between rod and –ve induced charge > the ________ between rod and +ve induced charge. Therefore, the net force on the object is attraction. Check-point 1 – Q4 How does a positively charged rod attract a neutral object? attraction repulsion

Electric charges 7Earthing An object is earthed when it is connected to the earth through a connecting wire. aWhat is earthing? If a charged conductor is earthed, it will become neutral.

Electric charges bHow does earthing occur? When we touch a metal ball of positive charge... electrons flow from the earth to the metal ball to neutralize the metal ball. – Metal ball becomes neutral. E

Electric charges Similarly, if the metal ball is of negative charge... bHow does earthing occur? – – – – – – extra electrons flow from the metal ball to the earth and the ball becomes neutral. E

Electric charges 8Fun with electric charges Van de Graaff generator can produce a large amount of negative charge at its dome.

Electric charges Experiment 14b Fun with electric charges What can you observe?

Electric charges Experiment 14b Fun with electric charges Video

Electric charges The metal-coated ball is first attracted to the dome and it is repelled after touching the dome. 8Fun with electric charges  ball gains the same type of charge of the dome on touching it.

Electric charges 8Fun with electric charges Hairs stand on-ends.  the same type of charge is given to the hairs and they repel each other.

Electric charges 8Fun with electric charges Sparks can be seen.  charges pass from the dome through the air to the metal sphere and then the earth.

Electric charges 8Fun with electric charges The galvanometer shows a reading.  charges flow to the earth from the dome and this forms a current.

Electric charges 8Fun with electric charges

Electric charges Check-point 2 1At which of the…At which of the… 2Why do petrol...Why do petrol...

Electric charges Check-point 2 – Q1 At which of the following positions can touching earth the charged metal sphere? AP BQ CR DAll of the above P Q R insulating stand metal sphere

Electric charges Check-point 2 – Q2 Why do petrol tankers usually have metal chains at the back? When cars run, their tyres and bodies are usually charged by _______. For petrol tankers, if the accumulated charge is large enough, _______can be produced and an _________ will occur if petrol vapour is ignited. Metal chains conduct the charge on the bodies of the tankers to the _______ in order to avoid the danger friction sparks explosion ground

Electric charges The End

Electric charges Example 1 A and B are identical metal- coated balloons. Charges on balloons (a)A and B attract when A is rubbed (–ve charge). Why? What is the net charge on B? +ve and –ve charges are induced on B. A B – – – – – – Attraction between A and +ve induced charge > repulsion between A and –ve induced charge  they attract. The net charge on balloon B is zero. nylon thread

Electric charges Example 1 Charges on balloons A B – – – – – – A B – – – – – – A and B are identical metal-coated balloons. (b)If B is also rubbed in the same way as A, what happens? Why? When B is rubbed, it has the same type of charge as A. Objects of the same type of charge repel  A and B repel each other.

Electric charges Return

Electric charges Example 2 The hairs of a girl stand on-ends when she touches the dome of Van de Graaff generator. Using Van de Graaff generator (a)Why do the girl’s hairs stand on-ends? The ends of the girl’s hair are given the same type of charge. They repel each other and stand on-ends.

Electric charges Example 2 Using Van de Graaff generator The hairs of a girl stand on-ends when she touches the dome of Van de Graaff generator. (b)Why does she stand on plastic stool? What if she stands on ground?

Electric charges Example 2 Using Van de Graaff generator If she stands on ground, the dome will be discharged through her body.  electric shock and her hairs will not stand on-ends. She is insulated from the ground when standing on a plastic stool.

Electric charges Example 2 Using Van de Graaff generator (c)What if she stands on a plastic stool and touches a boy on ground? Both the girl and the boy will get an electric shock. Her hairs will not stand on-ends.

Electric charges Return