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

Conductors and insulators: All the matter and materials on earth are made up of atoms. All atoms are electrically neutral i.e. they have the same amounts of negative and positive charge inside them. Some materials allow electrons to move relatively freely through them (e.g. most metals, the human body). These materials are called conductors. Other materials do not allow the charge carriers, the electrons, to move through them (e.g. plastic, glass). The electrons are bound to the atoms in the material. These materials are called non-conductors or insulators. There can be a force of attraction between a charged and an uncharged neutral insulator due to a phenomenon called polarisation. The latter is explained in terms of the movement of polarised molecules in insulators. Learners are also introduced to the electroscope, a very sensitive instrument which can be used to detect electric charge.

Electric circuits

This section starts by revising concepts learners have dealt with in earlier grades such as: uses of electricity, closed circuits, representing electric circuits using symbols, how to connect resistors in series and in parallel, series and parallel circuits and alternative energy.

The text guides learners to gain a better understanding of potential difference. They need to know that charges will not move unless there is a force provided by the battery in the circuit. A parallel is drawn between the change in potential energy of an object in a gravitational field and electric potential difference. The amount of work done to move a charge from one point to another point equals the change in electric potential energy. Note: it is a difference between the value of potential energy at two points, therefore potential difference is measured between or across two points. Potential difference is defined as: the difference in electrical potential energy per unit charge between two points. The unit of potential difference is volt (V). 1 volt = 1 joule (energy)/1 coulomb (charge). Electrical potential difference is also called voltage.

The concepts of potential difference across resistors connected in parallel and in series in electric circuits are explored in depth. Diagrams show the points between which the potential difference is measured, how the voltmeter (an instrument that measures potential difference) is connected and the voltmeter readings obtained. The concept emf as the voltage measured across the terminals of a battery is developed in a similar way.

Current is defined as the amount of charges that move past a fixed point in a circuit in one second. Use the picture in the learners' book and description to explain to learners that the charges in the wires can only be pushed around the circuit by a battery. When one charge moves, the charges next to it also move. The current flowing can be calculated with the equation: $I=\frac{Q}{t}$ , where I is the symbol for current measured in amperes (A) and Q the symbol for charge measures in coulomb (C). One ampere is one coulomb of charge moving in one second.

The current in series and parallel circuits are investigated first using the brightness of a light bulb as indication of the amount of current flowing, and then an ammeter is connected to measure the amount of current through a given circuit element.

Resistance slows down the flow of current in a circuit. On a microscopic level, resistance is caused when electrons moving through the conductor collide with the particles of which the conductor (metal) is made. When they collide, they transfer kinetic energy. The electrons therefore lose kinetic energy and slow down. This leads to resistance. The transferred energy causes the resistor to heat up. We use the symbol R to show resistance and it is measured in units called ohms with the symbol Ω. $\mathrm{Ohm}=V\cdot A^{-1}$

An important effect of a resistor is that it converts electrical energy into other forms of heat energy. Light energy is a by-product of the heat that is produced.

Learners need to see the bigger picture and be able to explain why batteries go flat. In the battery, chemical potential energy (chemical reactions) is converted to electrical energy (which powers the electrons to move through the circuit). Because of the resistance of circuit elements, electrical energy is converted to heat and light. The battery goes flat when all its chemical potential energy has been converted into other forms of energy.

Safety in the laboratory

It is very important for learners to know the general safety rules and guidelines for working in a laboratory, as a laboratory can be a dangerous place. The learners must also know the common hazard signs, and be able to identify them and know what they mean.