0.2 Tg physical science - chapter contexts  (Page 9/10)

The pattern of the Earth’s magnetic field is as if there is an imaginary bar magnet inside the Earth. Since a magnetic compass needle (a north pole) is attracted to the south pole of a magnet, and magnetic field lines always point out from north to south, the earth’s pole which is geographically North is magnetically actually a south pole. The Earth has two north poles and two south poles: geographic poles and magnetic poles. The geographic North Pole, which is the point through which the earth's rotation axis goes, is about ${\mathrm{11,5}}^{°}$ away from the direction of the Magnetic North Pole (which is where a compass will point). Learners are made aware of the importance of the earth's magnetic field acting as a shield to stop electrically charged particles emitted by the sun from hitting the earth and us. Charged particles can damage and cause interference with telecommunications (such as cell phones).

Solar wind is the stream of charged particles (mainly protons and electrons) coming from the sun. These particles spiral in the earth's magnetic field towards the poles. If they collide with particles in the earth's atmosphere they sometimes cause red or green lights, or a glow in the sky which is called the aurora, seen at the north and south pole.

Electrostatics

Electrostatics is the study of electric charge which is static (not moving). Remind learners that all objects surrounding us (including people!) contain large amounts of electric charge. There are two types of electric charge: positive charge and negative charge. If the same amounts of negative and positive charge are brought together, they neutralise each other and there is no net charge; the object is neutral. However, if there is a little bit more of one type of charge than the other on the object, then the object is electrically charged. The concepts: positively charged (an electron deficient) and negatively charged (an excess of electrons) are explained mathematically and with illustrations

The unit in which charge is measured is coulomb (C). A coulomb is a very large charge. In electrostatics we often work with charge in microcoulombs ( $1\mathrm{\mu C}=1\times {10}^{-6}C$ ) and nanocoulombs ( $1\mathrm{nC}=1\times {10}^{-9}C$ ).

Objects may become charged by contact or when rubbed by other objects. Charge, like energy, cannot be created or destroyed - charge is conserved. When a ruler is rubbed with a cotton cloth, negative charge is transferred from the cloth to the ruler. The ruler is now negatively charged and the cloth is positively charged. If you count up all the positive and negative charges at the beginning and the end, there is still the same amount, i.e. total charge has been conserved!

An electrostatic force is exerted by charges on each other. The electrostatic force between:

• like charges are repulsive
• opposite (unlike) charges are attractive.

The closer together the charges are, the stronger the electrostatic force between them.

Perform the suggested experiment to test that like charges repel and unlike charges attract each other. The electrostatic force also determines the arrangement of charge on the surface of conductors, because charges can move inside a conductive material. On a spherical conductor the repulsive forces between the individual like charges cause them to spread uniformly over the surface of the sphere, however, for conductors with non-regular shapes, there is a concentration of charge near the point or points of the object.