AC

Dunman High School (Senior High Physics) 9646 Physics (2014) Topic 17: Alternating Currents 17-1 H2 Topic 17 Alternating Currents A typical electrical transformer found at power sub-stations A typical power adaptor for an electrical device

Dunman High School (Senior High Physics) 9646 Physics (2014) Topic 17: Alternating Currents 17-2 Learning Objectives Content • Characteristics of alternating currents • The transformer • Rectification with a diode Learning Outcomes Candidates should be able to: (a) show an understanding and use the terms period, frequency, peak value and root -mean-square value as applied to an alternating current or voltage. (b) deduce that the mean power in a resistive load is half the maximum power for a sinusoidal alternating current. (c) represent an alternating current or an alternating voltage by an equation of the form x = x 0 sin ωt. (d) distinguish between r.m.s. and peak values and recall and solve problems using the relationship Irms = I0 / 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 Ns /Np = Vs /Vp = Ip /Is for an ideal transformer. (f) explain the use of a single diode for the half -wave rectification of an alternating current.

Dunman High School (Senior High Physics) 9646 Physics (2014) Topic 17: Alternating Currents 17-3 17.0 Introduction It is important to understand alternating currents (a.c.) because they are so much a part of our everyday life. Every time we turn on a television set, computer or any of a multitude of other electric appliances, we are using a.c. to provide the power to operate them. At the microscopic level, a.c. can be considered as having the charge carriers oscillate about a fixed point. An alternating current (a.c.) is a current that varies periodically with time in magnitude and direction (Polarity of the voltage source constantly changes). Below are some examples of a.c. 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. 17.1 Quantities Characterising an Alternating Current Quantity Symbol Description Period T Time taken by alternating current to make one complete alternation. Frequency f The number of times the current passes through its zero value in the same direction in unit time. Angular Frequency ω A way of expressing the frequency of the alternating current in terms of radians per second instead of cycles per second. Peak Value (Amplitude) Io Maximum value of the alternating current in either direction of zero value in a periodic cycle. Peak to Peak Value Difference between the positive peak value and the negative peak value of the a.c. within a cycle. Mean Value <I