## Overview This unit introduces the concept of capacitance and the factors affecting it. Capacitors connected in series and parallel are studied together with the factors affecting the energy stored in them. The mechanism by which a capacitor charges and discharges through a resistor is examined. ## Working Scientifically The specified practical work in this topic gives learners the opportunity to apply investigative approaches and methods to practical work; to follow written instructions; to make and record observations; to present information and data in a scientific way; to use electrical multimeters to obtain a range of measurements; to correctly construct circuits from circuit diagrams using D.C. power supplies, cells and a range of circuit components, including those where polarity is important. ## Mathematical Skills There are a number of opportunities for the development of mathematical skills in this unit. These include recognising and using expressions in decimal and standard form; using calculators to find and use power, exponential and logarithmic functions; using an appropriate number of significant figures; finding arithmetic means; making order of magnitude calculations; changing the subject of an equation, including non-linear equations; using logarithms in relation to quantities that range over several orders of magnitude; plotting two variables from experimental data; determining the slope and the intercept of a linear graph; understanding the possible physical significance of the area between a curve and the x-axis and being able to calculate or estimate it by graphical methods; interpreting logarithmic plots. ## How Science Works There are opportunities within this topic for learners to use appropriate methodology, including ICT to answer scientific questions and to solve scientific problems; to carry out experimental and investigative activities, including appropriate risk management, in a range of contexts. Learners can investigate the use of capacitors to store charge, in time delay circuits and use risk management appropriately when dealing with power supplies. Opportunities exist to study electrostatic precipitation and devices such as photocopiers. ### Learners should be able to demonstrate and apply their knowledge and understanding of: (a) the idea that a simple parallel plate capacitor consists of a pair of equal parallel metal plates separated by a vacuum or air (b) a capacitor storing energy by transferring charge from one plate to the other, so that the plates carry equal but opposite charges (the net charge being zero) (c) the definition of capacitance as Q C V = (d) the use of o A C d  = for a parallel plate capacitor, with no dielectric (e) the idea that a dielectric increases the capacitance of a vacuum-spaced capacitor (f) the E field within a parallel plate capacitor being uniform and the use of the equation V E d = (g) the equation 1 U QV 2 = for the energy stored in a capacitor (h) the equations for capacitors in series and in parallel (i) the process by which a capacitor charges and discharges through a resistor (j) the equations: 0 1 t Q Q e RC   − = −     and 0 t Q Q e RC − = where RC is the time constant [[Specified Practical Work]] - Investigation of the charging and discharging of a capacitor to determine the time constant - Investigation of the energy stored in a capacitor