Static Electricity d:-l }\.l- (! ) I Facts to Know :lr-'' - Q: Explain how electrostatic charge is produced by rubbing.. Electrostatic charge is produced by rubbing two insulated materials.. Electrons are being dragged off from one material to another by friction between the two materials.. The material which gains electrons is negatively charged and the material that loses electrons is ly charged.. The electrostatic charges stay at the area where rubbing occurs because the charges are not able to spread as both materials used are insulators. Q: What are the two types of charges and the unit of charge?. They are and negative charges. The unit is coulomb. Q: What is the basic law of electrostatics?. Unlike charges attract and like charges repel. Q: Describe an electric field.. An electric field is a region in which an electric charge experiences a force. Q: Distinguish between electrical conductor and insulator and give typical examples of each. Materials that allow electrons to flow through them easily are called electrical conductors. In metals, the outermost electrons in each atom are so loosely held that they are able to move freely between atoms. These free electrons make metals very good electrical conductors. Materials, which do not conduct charges, are called non-conductors or insulators. Their electrons are all tightly held to atoms and are not normally free to move. Examples of insulators are rubber, glass, most plastics, dry wood and dry air. Q: Describe examples in which electrostatic charging is a potential hazard. Lightning In thunderstorm, the clouds are charged by friction with air currents. Electrons are deposited at the bottom of the cloud and induced a strong charge at the Earth's surface. The strong attractive force on the cloud molecules pulls the electrons from them and causes an avalanche of electrons rushing at high speed towards the Earth. The lightning flash produced is so powerful that it can burn forests, damage houses and kill people. Fire and Explosion A spark may be created when a charged object is placed near another object. This happens because an electric field is built up between the two objects causing electrons to travel at high speed from one object to another object. In the presence of inflammable substances, the spark can cause fire or even explosions. 126 Chapter 16 Static ElectricitY
( i Q: Describe examples of the use of electrostatic charging. Photocopier A light sensitive drum is first coated with charges. The shadow of the printing material is being cast onto the drum by strong light. The area exposed to the light is being discharged, whereas the shaded area retained the charges. Negatively charged powder (toner) is sprayed onto the drum. The light powder is attracted to the ly charged region only. The toner on the drum is then rolled onto a piece of paper. The paper is heated to melt the toner to fuse it onto the paper. Spray painting In spray painting, particles of paint are given charges as they leave the nozzle of a spray gun. The object to be painted is earthed so that there is an electric field between the nozzle and the object. The charged paint droplets follow the field lines and are forced to spread evenly over the surface of the object. Electrostatic prectpit41g1 1. Precipitator is used to remove dust particles from the smoke in a factory chimney. 2. Convection current causes the smoke to rise. Smoke first passes through a negatively charged grid making the dust particles negatively charged. 3. Positively charged plates are placed above the grid. Negatively charged smoke particles are attracted to them and the air that is released to the environment is less polluted. negatively charged grids - - - Q: Sketch the electric field of (i) a single point charge, (ii) a single negative point charge, (iii) between two oppositely charged plates, (iv) two point charges of the opposite charge, (v) two point charges of the same charge. ly charged plates smoke 121 Chapter 16 Static Electricity
Single point charge Single negative point charge Between two oppositely charged plates I Two point charges of the opposite charge Q: Describe experiments to show electrostatic charging by induction. With two conducting spheres 1. Place two uncharged conducting spheres in contact. 2. Place a negatively charged object near one sphere. Many electrons in the conducting sphere nearer to the charged object are repelled into the other conducting sphere. 3. Separate the two spheres without removing the negatively charged object. Two point charges of the same charge I \ lr \ 128 Chapter 16 Static Electricity
4. The sphere previously nearer to the negatively charged object is ly charged and the other is negatively charged. With one conducting sphere 1. Place a negatively charged object near an uncharged conducting sphere. The electrons in the conducting sphere will be repelled to the further end of the sphere. 2. Earth the sphere momentarily (touch and remove). The electrons will flow out of the sphere to the Earth. 3. Remove the negatively charged object. 4. The sphere is now ly charged. /.F Problems to Solve Electrons flow to the Earth 1 A ly charged conducting sphere and a negatively charged conducting sphere are placed close bui not touching each other. A small metallic dust particle drops on the ly charged conducting sphere and starts to jump to-and-fro between the two spheres. a. Explain why the small metallic dust particle jumps to-and-fro between the two spheres. ( 7 marks ) t29 Chapter 16 Static Electricity
b. State when the sequence will stop. (lmark) 2. Object X sticks to a surface due to electrostatic attraction. Is object X an insulator or conductor? Discuss all possibilities. ( 6 marks ) I I 130 Chapter 16 Static Electricity
Misconceptions to Clarifr { A negatively charged rod is moved near a conducting sphere as shown. What will be the movements of the electrons and protons in the conducting sphere? p negatively ctreea roa r// electrons A move to left B move to right C does not move D move to right conducting sphere protons move to right move to left move to left does not move ( 2. Two tiny neutral water droplets are suspended in the space between the two charged plates as shown. What will happen to the charge distribution when both water droplets touch each other? ly charged plate droplet 2 droplet I A neutral B neutral C D negative (\\/ /\ / \./ droplet 1 negatively charged plate droplet 2 neutral negative () 3 A neutral conducting rod is moving towards a ly charged conducting sphere as shown below. What are the charges on the rod and the sphere when the rod is placed near the sphere? ly charged conducting sphere charge on rod A neutral B negative C negative D neutral conducting rod charge on sphere neutral 4. Which of the following is/are true? () (1) When an object is earthed, the electrons flow from the body to the Earth. (2) When a solid body becomes ly charged after rubbing, it implies that protons have rubbed onto the body. (3) The direction of the electric field is the direction of the force acting on a point charge. A (3) only B (1) and (2) only C (1) and (3) only D (r),(2) and (3) ( ) 131 Chapter l6 Static Electricity