Attraction and repulsion 3.1 The sticky balloon If you rub a rubber balloon on a woollen jumper, they may both become charged. You can use the charged balloon to attract small pieces of paper, or your hair. � � Rubber, wool, paper and hair are all electrical insulators. Two insulators may become charged when they are rubbed together. The electric charge on an insulator is called its electrostatic charge. � � Any two charged objects exert a force on one another. The electrostatic force can either pull the objects together or push them apart. For example, the charged balloon and jumper attract each other. But two charged balloons repel each other. � Rubbing charges on and off � � � � � � � � � � � � � � � � � � � � � � � � � � Here is an explanation of the observations described above. There are two types of electrostatic charge, called positive charge and negative charge. After it has been rubbed, the jumper has a positive charge. The balloon has a negative charge. They will attract each other because opposite charges attract. On the other hand, two charged balloons repel each other. Each has a negative charge, and charges that are the same repel one another. First object Second object negatively charged balloon negatively charged balloon negatively charged balloon positively charged nylon thread positively charged piece of wool positively charged nylon thread Attract/repel/ no effect? Questions a Copy and complete the table, to show how the pairs of objects will interact. b If you comb your hair, your hair is attracted to the comb. After combing, it is light and fluffy – your hairs are repelling each other. What do these observations tell you about the electrostatic charges on hair and comb? Here is a deeper explanation of the observations. You have to think about the atoms of which matter is made. An atom has a positively charged nucleus with negatively charged electrons around it. When you rub one object against another, charged particles are transferred from one to the other. Because the electrons are on the outside of the atoms, it is these particles which are transferred. 194 An uncharged material has equal amounts of positive and negative charge. A material which gains electrons becomes negatively charged. A material which loses electrons becomes positively charged. P2 The Van de Graaff generator in the photograph can be charged up to two and a half million volts. Electric charges build up on the metal domes; when the charge is great enough, sparks jump through the air. In the picture, the balloon has gained a negative charge. The jumper has an equal amount of positive charge. If we put the jumper and balloon back together, the two charges would cancel each other out. Questions c Look at the picture of the jumper and balloon. Which has gained electrons, and which has lost them? d Use the idea of electrons to explain why, when two objects are rubbed together, their charges are equal in size (although one is positive and the other negative). e Explain why your explanation of electrostatic charge in Question d is ‘deeper’ than your explanation in Question b. Dangerous sparks The more charged up an object becomes, the greater is the potential difference (p.d.) between the object and the earth. This is the same as saying that there is a large voltage between the object and the earth. If the p.d. is great enough, a spark may jump from the object to earth, or to any nearby conductor which is earthed (connected to earth). This is why lightning occurs – a cloud becomes highly charged, so that the p.d. between it and the ground is many millions of volts. This p.d. is big enough to make a current flow through the air, and we see a flash of lightning. The tip of a lightning conductor is high up, closer to the cloud, and it is connected to earth. This gives the lightning a shorter distance to travel to reach the earth, and so it strikes the conductor and the electrostatic charge flows through the conductor and is discharged safely to earth. ���������� � � � � � � ����������� ����� ���������� � � � � � � � � ����� �� ��� ���������� ���� ����� If the charge on the big sphere is large enough, a spark will jump across to the small sphere. The charge then flows down the conducting wire to earth. Static electricity can be dangerous. Sparks can give you a shock, or ignite a fire. This petrol tanker is delivering fuel to an aeroplane. The fuel may become charged by friction as it flows along the pipe, like a balloon rubbed on a jumper. A spark could then jump through the air, and this might ignite the petrol vapour. To avoid this danger, the pipe is connected to the earth using a metal wire. Question f How does connecting the pipe to the earth with a wire avoid the danger of sparks? Key points When insulating materials are rubbed against each other, electrons are rubbed off one material onto the other and both become charged. Opposite charges attract one another; like charges repel one another. If an object has a high enough p.d. compared to the earth, a spark can jump from the object to the earth. Electric charge 195 Using static electricity 3.2 Friend or foe? We have seen that static electricity can be dangerous. But it also has many uses. For example, liquid crystal displays are found on many computer monitors, mobile phones, calculators and TV sets. Each dot on the screen can be changed from black to white by applying a static voltage. (Coloured filters give the different colours.) The picture shows another use of static electricity – in painting metal components. The spray gun is connected to one terminal of a high-voltage supply, so that the droplets of paint are charged. The metal component is connected to the other terminal. The paint is attracted to both sides of the door panel. ������������� ������ ��������� ����� � � � ������ ��� � � � �� � � � � � � � � � � � � � � � � Static electricity helps in spray-painting this car door panel. Question a Explain why the paint is attracted to the car door panel. Smoke precipitator Many power station chimneys send large amounts of dust up into the atmosphere. This is a major source of pollution but, in the UK, it has now been greatly reduced, thanks to static electricity. In a power station, coal is burned, producing hot, dirty exhaust gases. These are passed through a precipitator where they are ‘scrubbed’ to remove as much of the dust as possible. The precipitator chamber has several wires running down the middle, connected to the negative terminal of a high voltage supply (approximately 40 kV). �������������� ������� The large blue structure at the centre of this photo is a pair of dust precipitators fitted to a power station. Waste gases travel through these before passing up the chimney stacks at the back. As the exhaust gases pass through this high voltage region, the dust particles gain electrons. The particles are then repelled by the negative wires so that they move towards the walls of the chamber. The walls become covered in dust, which is periodically scraped off and disposed of. Questions b What force causes the dust particles to move towards the walls of the chamber? c Why might a small-scale smoke precipitator be useful for an asthma sufferer? Where else might smoke precipitators be useful? 196 P2 ������ ������ �� ��������� �������� ���������� �������� ���� ������ ������ A smoke precipitator uses static electricity to remove particles of dust which would otherwise pollute the atmosphere. What’s the idea? Photocopier ���������� ������ You put down the lid and press the ‘Copy’ button. A bright light scans the page and a copy emerges – all thanks to static electricity. White paper reflects light, black ink absorbs it. Inside the photocopier, the light reflected from the white areas of the page is focused onto a charged drum which turns as the light scans the page. When the light hits the charged drum the electric charge gains energy and can flow to earth, leaving these parts of the drum uncharged. The only parts of the drum that stay charged are the parts that have not been hit by light. These correspond to the dark areas of the page. They pick up a special dry ink called toner, which is then transferred onto the paper. ������ ����� ������ ����� ������ �� �� � � � � ���� � ������������ ������� ������ ����� ����� Inside a photocopier. Finally, the paper with toner is heated. This melts the toner and sticks it to the paper. Question d The drum, toner and paper are all charged. For each, say what charge it must have, positive or negative. Explain your answers. It is easier to understand the stages in making a copy if we picture the drum as a flat metal plate with a special coating. It is also easier to think in terms of positive charges moving, even though it is really electrons which move in the opposite direction. 1. Positive charge is sprayed onto the plate (drum). 2. Where light falls, it gives the charges enough energy to flow to earth. This happens wherever the original page was white. �������������� � ������� ����� ������ � � � � � �� � ������ � �� � � ����������� �������� ����� �� � ������ 4. A positively charged sheet of paper picks up the toner powder, which is then melted on. e After the image has been focused on the base plate, what charge do the black areas have? What charge do the white areas have? f Why is the drum not made of metal? � � � � � � � 3. The rest of the plate remains positive. These areas attract negatively charged toner powder. Questions � ���� � ����� ������������ �������� � �� � � �� � � Key points Static charges can be useful. They are used in devices such as the smoke precipitator and the photocopier. Electric charge 197