DHS 18 Alternating Current (Lecture Slides)

DHS Y6 Physics H2 [Alternating Currents] 02/04/2025 [For Internal Use Only] lim.boonsiong@dhs.edu.sg 1 ALTERNATING CURRENTS SECTION V ELECTRICITY AND MAGNETISM T opic 18 ALTERNATING CURRENTS • Characteristics of alternating currents • The transformer • Rectification with a diode GUIDING QUESTIONS •How is alternating current different from direct current? •How can we describe alternating current, e.g., mathematically and graphically? •Why is alternating current used in the generation and transmission of electricity? Content •Characteristics of alternating currents •The transformer •Rectification with a diode 1 2 3 4

DHS Y6 Physics H2 [Alternating Currents] 02/04/2025 [For Internal Use Only] lim.boonsiong@dhs.edu.sg 2 Learning Outcomes • understand and use • period (T) • frequency (f) • peak value (I0, V0) • root-mean-square value (Irms, Vrms) • deduce <P> = P0 / 2 for sinusoidal I • represent a.c. by • solve problems using for sinusoidal case 0 sin tI I ω rms 0 2I I Learning Outcomes (continue) • understand principle of operation of a simple iron - cored transformer • for an ideal transformer, recall and solve problems using • explain the use of a single diode for the half-wave rectification of an a.c. s p s p p sN N V V  I I Recall: A simple alternator θ cos cos sin B B BA BA t dN NBA tdt      Φ θ Φ ω ω ωε sinusoidal e.m.f. is induced in a coil rotating with constant speed in a uniform B field Recall: A d.c. generator the ring halves are attached to the loop and rotate with it; the commutator reverses the connections to the external circuit at regular positions where the e.m.f. reverses (arrangement of split rings) θ 5 6 7 8

DHS Y6 Physics H2 [Alternating Currents] 02/04/2025 [For Internal Use Only] lim.boonsiong@dhs.edu.sg 3 Difference between AC & DC Alternating current (A.C.) : an electric current which periodically reverses direction Direct current (D.C.) : an electric current which only flows in a single direction around a circuit Characteristics of A.C. period The period of an A.C. source refers to the time taken to complete one cycle. The SI units for time is seconds (s). frequency The frequency of an A.C. source refers to the number of complete cycles per unit time. The SI units for frequency is Hertz (Hz) or s −1. peak value Maximum absolute value of the alternating current or voltage 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. Characteristics of A.C. T = 0.5 s f = 2 Hz ω = 2πf = 13 rad s−1 peak value = V0 peak to peak value = 2V0 Characteristics of A.C. T = 2 s f = 0.5 Hz positive peak value = V1 negative peak value = V4 9 10 11 12

DHS Y6 Physics H2 [Alternating Currents] 02/04/2025 [For Internal Use Only] lim.boonsiong@dhs.edu.sg 4 Voltage across a sinusoidal source V t 0 Equations for sinusoidal A.C. 0 sin tI I ω 0 si