EXTENSION 6 Chapter 3 Encounters with Electricity: Electrical Energy in the Home Unit 3.2 Electric Circuits and Electric Charge A view of the Atom and Electric Charge Figure 1 The size of the nucleus is very small compared to the size of the atom. It is only within the past century that we have been able to explain the large-scale (macroscopic) effects of electric charge in terms of the charge associated with individual atoms and atomic particles. Although atomic structure is discussed in later chapters, a brief discussion here will help in developing an understanding of electrostatics. electron (ve) charge cloud protons (ve) neutrons nucleus If we break materials down into their smallest parts using a combination of physical and chemical means, we can identify our world as being composed of about 92 naturally occurring basic units called elements. Each element has its own unique physical and chemical characteristics such as colour, melting point, density, chemical combination with oxygen etc. If we continue splitting a material made up of a single element into smaller parts, we end up with the atom: the smallest part of an element which retains its unique chemical identity. A kilogram of copper contains around 10 25 atoms. In the twentieth century, it became possible to study the composition of the atom. Atoms have sizes about 10 10 m and are mainly empty space. They are also electrically neutral; that is, their total charge is zero. We picture an atom as being similar to a mini solar system. At the centre we find a collection of particles called the nucleus. The nucleus contains most of the mass of the atom and is made up of two kinds of particle: the neutron, which has no charge, and the proton, which is positively charged. The neutron and the proton have similar masses, about 1.67 10 21 kg. The nucleus is very small compared to the overall size of the atom. In this diagram the size of the nucleus is exaggerated to show its parts. The atom also contains negatively charged electrons. These have a much lower mass than the proton and neutron, and make little contribution to the mass of the atom. The negative charge on each electron is the same size as the positive charge on the proton. The positive charge on the nucleus is exactly matched by the negative charge on the surrounding electrons, making the whole atom electrically neutral in normal circumstances. The electrons move rapidly forming a charge cloud around the nucleus.
Conductors and insulators Atoms can be arranged to form solids, liquids and gases. Gases and liquids are different from solids in that the atoms are able to move around relatively freely. In a solid the atoms are located in fixed positions, held in a lattice by the forces of the neighbouring atoms. The atoms vibrate around these fixed centres. The higher the temperature the more energetic the vibration. In some materials the negatively charged electrons can move freely through the material. These materials are good conductors. The electrons follow random paths in the conductor unless the conductor is connected to a battery or other source of voltage. When a source of voltage is connected, the electrons migrate from one end of the conductor to the other. Metals are generally good conductors of electricity. In other materials the electrons, as well as the nuclei, remain fixed and are not free to move. These materials are called insulators. Materials such as wood, rubber and plastics are insulators. Charging by friction How does our view of solids explain how a rod becomes charged through friction against a piece of fabric or fur? First, notice that the solid rods which we have been usingperspex, ebonite, glassare all insulators! Figure 2 a Some electrons can move freely in a conductor. b Enlarged view showing the atoms vibrating about fixed lattice positions. a atoms electron b free electron nucleus vibration about lattice position Initially the rod and fabric are electrically neutral. When we rub our glass rod against silk, some of the atoms at the surface of the glass have their electrons ripped off and transferred to the silk. The silk therefore has an excess of negative charges, the electrons. It is negatively charged. The glass has a deficit of negative charges. It is positively charged. Since the process is just a transfer of negative charge from one object to the other, the negative charge lost by the glass must equal the negative charge gained by the silk. Since in historical terms a loss of negative charge is the same as a gain of positive charge this means that the gain in positive charge by the glass must exactly equal the gain in negative charge by the silk. This leads to the law of conservation of electric charge: The net amount of charge produced in any process is zero. Over a period of time the objects will lose their charge. Loosely held electrons from water molecules in the air, or positive and negative charges in the air, can attach themselves to the glass rod. Charging by induction Positive charges and negative charges are attracted to each other. Charges of a similar sign repel each other. This effect allows us to manipulate charges on the surfaces of metals on which
charges can move freely. If you place a positively charged glass rod close to a sphere made from a conducting material the freely moving electrons are attracted to the surface closest to the positively charged glass rod. The sphere still has the same number of positive and negative charges, but the distribution of the negatively charged electrons has been distorted by the glass rod. If the conducting sphere is now connected by a conducting wire to a large store of negative charge, extra negative charges flow onto the sphere, attracted by the positive charges in the glass rod (Figure 3). Since the biggest store of charge is the Earth, we call this earthing the conductor. If we remove the earthing wire while we keep the positively charged glass rod in position, we isolate the negative charges on the conducting sphere. When we finally take away the glass rod, the conducting sphere remains with an excess of electrons (negatively charged). The charges relocate to space themselves evenly across the surface of the sphere. This method of charging is known as charging by induction. Measuring and detecting charge We could use the motion of a positively charged glass rod hung on thread to detect the electric charge on other objects. However, this is somewhat inconvenient. Detection of charges is normally done using an electroscope. neutral conductor charge is polarised positive charges are fixed negative charges move to right positive insulator negative charges are attracted by positive charges on glass rod negative charges flow onto the sphere from the earth more negative than positive charges on sphere positively charged insulating rod remove positively charged insulating rod negative charges and positive charges uniformly pread across surface net negative charge Figure 3 Charging by induction. awhen a positively charged rod is brought close to a neutral conductor the charges on the conductor become polarised. b Earthing the conductor allows negative charges to flow into the conductor. cthe earth and then d the charged rod are removed and the conductor is left with an overall negative charge.
One form of electroscope consists of a conducting cap attached to a vertical rod, to which is hinged a thin metal foil. The assembly is held in a protective insulating bottle. There are two ways in which we can use the electroscope to test for and measure charge on other objects: By contact. When a charged object touches the cap at the top of the electroscope, some charge is transferred to the metal foil and the vertical rod. The charges on the foil and rod are the same and they repel each other. The foil rises away from the vertical. The angle to vertical is proportional to the amount of charge on the electroscope s cap. Figure 4 The electroscope. excess of ve charges electrons flow onto cap thin foil uncharged equal numbers of ve and ve charges the excess of negatively charged electrons cause leaves to move apart By first charging the electroscope and bringing the object close to it. If we contact the electroscope cap with a positively charged glass rod, some of the negative charge on the electroscope will flow onto the rod to neutralise the positive charge. The electroscope now holds an excess of positive charge. The positively charged electroscope is then placed close to a second charged object to determine if the charge is positive or negative. Figure 5 A charged electroscope can be used to determine whether the charge on an object is a positive or b negative. a b c Electroscope charged by contact with glass rod. Electrons flow to rod resulting in excess ve charge on electroscope. ve charges attracted to cap, less ve charge on foil. Foil has increased positive charge. ve charges repelled from cap move to foil. Positive excess charge on foil is reduced.
Exercises 1 Explain how a negatively charged ebonite rod can be used to charge an electroscope positively. 2 A charged rod is brought near a positively charged electroscope. The leaves diverge more strongly. Is the rod positively charged or negatively charged? 3 A thin stream of water is attracted to a charged rod whether the rod is charged positively or negatively. Explain this using the idea of electrostatic induction. 4 Why are metals good conductors of electricity? a Name a non-metal that is a good conductor. b Name some common electrical insulators. 5 Why doesn t a metal rod become charged when it is held in the hand and rubbed with a silk cloth? 6 What are the two particles in the nucleus of the atom? 7 An ebonite rod is charged negatively by rubbing with a cloth. What is the resulting charge on the cloth? What has moved from the cloth to the rod? 8 What is the type of charge: a on a proton b on an electron c on the atomic nucleus. 9 A plastic rod is rubbed with a piece of wool. It acquires a charge of 0.06 µc. How many electrons were transferred from the wool to the plastic rod? 10 Two metal spheres are attached to a support by an insulating string. One has a charge of 2.0 µc, the second is uncharged. If the spheres are brought together, what will be the charge on each? X Y 2.0 µc 11 Three conducting spheres are suspended vertically by light insulating threads. The spheres are initially charged as shown in the following diagram. The following steps are carried out: Step 1 Spheres Y and Z are brought into contact and then separated. Step 2 Spheres X and Y are brought into contact and then separated. What charge is now on Y? X Y Z 2.0 µc 4.0 µc Activity 1 Research materials that are commonly used as conductors to provide household electricity. In which context, device or appliance is each used?