## Overview This topic covers series and parallel electrical circuits including resistor combinations. The use of a potential divider in circuits is investigated. The terms electromotive force and the internal resistance of a source are introduced. ## Working Scientifically The specified practical work in this topic gives learners the opportunity to evaluate results and draw conclusions with reference to measurement uncertainties and errors; to present data in appropriate ways; to make and record observations; to keep appropriate records of experimental activities; to design, construct and check circuits using D.C. power supplies, cells and a range of circuit components. ## Mathematical Skills There are a number of opportunities for the development of mathematical skills in this unit. These include recognising and making use of appropriate units in calculations; using ratios, fractions and percentages; finding arithmetic means; identifying uncertainties in measurements and using simple techniques to determine uncertainty; solving algebraic equations, including quadratic equations; translating information between graphical, numerical and algebraic forms; understanding that y = mx + c represents a linear relationship. ## How Science Works There are opportunities within this topic for learners to carry out experimental and investigative activities, including appropriate risk management: to analyse and interpret data to provide evidence, recognise correlations and causal relationships. Learners can investigate electrical circuits and use this experience to learn about the risk management issues involved when handling sources of power. The opportunity to design circuits and carry out fault finding techniques will enable them to evaluate their methodology and to resolve conflicting evidence. ### Learners should be able to demonstrate and apply their knowledge and understanding of: (a) the idea that the current from a source is equal to the sum of the currents in the separate branches of a parallel circuit, and that this is a consequence of conservation of charge (b) the sum of the potential differences across components in a series circuit is equal to the potential difference across the supply, and that this is a consequence of conservation of energy (c) potential differences across components in parallel are equal (d) the application of equations for the combined resistance of resistors in series and parallel (e) the use of a potential divider in circuits (including circuits which contain LDRs and thermistors) (f) what is meant by the emf of a source A LEVEL PHYSICS 24 © WJEC CBAC Ltd. (g) the unit of emf is the volt (V), which is the same as that of potential difference (h) the idea that sources have internal resistance and to use the equation V E Ir = − (i) how to calculate current and potential difference in a circuit containing one cell or cells in series SPECIFIED PRACTICAL WORK • Determination of the internal resistance of a cell