2018_Alternating_currents_Lecture_notes_(tutors)

9749 H2 Physics Lecture Notes Nanyang Junior College 1 Chapter 18 ALTERNATING CURRENT Content • Characteristics of alternating currents • The transformer • Rectification with a diode Learning Outcomes Candidates should be able to (a) show an understanding of and use the terms period, frequency, peak value and root-mean-square (r.m.s.) value as applied to an alternating current or voltage. (b) deduce that the mean power in a resistive load is half the maximum (peak) power for a sinusoidal alternating current. (c) represent an alternating current or an alternating voltage by an equation in the from sinoxx t ω= (d) distinguish between r.m.s. and peak values and recall and solve problems using the relationship o rms II= 2 for the sinusoidal case. (e) show an understanding of the principle of operation of a simple iron - cored transformer and recall and solve problems using ss P PPS NV I==NVI for an ideal transformer. (f) explain the use of a single diode for the half -wave rectification of an alternating current.

9749 H2 Physics Lecture Notes Nanyang Junior College 2 18.1 Introduction An alternating current (a.c.) is an electric current which has a flow direction that reverses periodically with time. An a.c. at the microscopic level can be considered as having the charge carriers oscillate about a fixed point. Examples of Direct Current (d.c.) Examples of Alternating Current (a.c.) It is important to understand a.c. because they are so much a part of our everyday life. Each time a television set, computer or any other electric appliances which are connected t o the wall socket are turned on, an a.c. provides the power to operate them . The principles of direct current in resistors learned in previous topics can be applied to resistors in a.c. circuits. However, major differences in circuit analysis arise when inductors and capacitors are to be considered. 18.2 Quantities Characterising an Alternating Current Quantity Symbol Description Period T The time taken for the a.c. to complete one cycle. Frequency f The number of complete cycles undergone by the a.c. in one second. Angular Frequency ω A way of expressing the frequency of the a.c. in terms of radians per second instead of cycles per second. Peak Value (Amplitude) Io The maximum value of the a.c. in either direction within a cycle. Peak to Peak Value The difference between the positive peak value and the negative peak value of the a.c. within a cycle. Mean Value <I> The average value of an a.c. over a given time interval. Root Mean Square Value Irms The value of a steady d.c. that will dissipate energy at the same rate as the a.c. in a given resistor. t I 0 t I 0 t I 0 t I 0 t I 0

9749 H2 Physics Lecture Notes Nanyang Junior College 3 18.3 Sinusoidal Alternating Current The most commonly encountered a.c. takes a sinusoidal form as shown and the wa