EJC Physics H218 Alternating Current - 1. Notes (2024) Full

Commemorative silver coin. In 2018, the Serbian Mint began a coin series to honor Nikola Tesla. It features a portrait of Tesla, and his induction generator on the other side. The generator generates Alternating Current and revolutionized industrial mechanics 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.) values 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 of the form 0 s tinxx = (d) distinguish between r.m.s. and peak values and recall and solve problems using the relationship rms 0 / 2=II for the sinusoidal case (e) show an understanding of the principle of operation of a simple iron-core transformer and recall and solve problems using / / / S P S P P SN N V V== II for an ideal transformer (f) explain the use of a single diode for the half-wave rectification of an alternating current. An alternating current (a.c.) is an electric current which has a flow direction that reverses periodically with time . At the microscopic level, an a.c. can be considered as having the charge carriers oscillate about fixed points. Compare that with a d.c. where the direction of flow of current is maintained in the same direction. A typical AC/DC adaptor. As seen in topic H215, this particular “handphone charger” outputs 5V D.C. at a maximum of 1 A. It accepts an input alternating voltage of 180 to 240 V rms, at a frequency of 50Hz or 60 Hz, taking up an current of 0.15 A.

Quantity Symbol Description period T time taken for a.c. to complete one cycle frequency f number of complete cycles of the a.c. per unit time angular frequency  (for a sinusoidal a.c.), product of 2π and the frequency of the a.c., 2 f= peak value (amplitude) 0I maximum value of the a.c. in either direction within a 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 average value of an a.c. over a given time interval root-mean- square (r.m.s.) value rmsI value of a steady direct current that will dissipate thermal energy at the same average rate as the a.c. in a given resistor A common representation of an a.c. is 0 si n t=II : I The mean (or average) current value is zero for an a.c. where 0 si n t=II . 0 for s n i0 t==I I I Within a complete cycle, there is a positive value of current and a corresponding negative value, so mean is zero. But a resistor heats up when an a.c. flows through it. The mean current value of an a.c. does not effectively characterise an a.c. t I 0 T peak va