DHS 18 Alternating Current (Notes & Tutorial)

DUNMAN HIGH SCHOOL (SENIOR HIGH PHYSICS) YEAR 6 (2025) H2 Physics (2025) 18-1 Topic 18: Alternating Currents TOPIC 1 8: Alternating Current ___________________________________________________________________________________ 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? 18.0 Introduction During the 1880s in the US there was a heated and acrimonious debate between two inventors over the best method of electric-power distribution. Thomas Edison (1847-1931) favoured direct current (d.c.) – i.e., steady current that does not vary with time. George Westinghouse (1846 - 1914) favoured alternating current (a.c.), with sinusoidally varying voltages and currents. He argued that transformers (which we will study in this chapter) can be used to step the voltage up and down with a.c. but not with d.c.; low voltages are safer for consumer us e, but high voltages and correspondingly low currents are best for long -distance power transmission to minimize I2R losses in the cables. Eventually, Westinghouse prevailed, and most present -day household and industrial power - distribution systems operate with a.c.. Every time you turn on a light, a television set, a stereo, or any of a multitude of other electrical appliances in a home, you are using alternating currents to provide the power to operate them.

DUNMAN HIGH SCHOOL (SENIOR HIGH PHYSICS) YEAR 6 (2025) H2 Physics (2025) 18-2 Topic 18: Alternating Currents In this chapter we will learn how resistors behave in circuits with sinusoidally varying voltages and currents, why transformers are useful and how they work, and how a single diode converts the a.c. to d.c. An alternating current is an electric current that periodically reverses its direction in a circuit with a frequency. 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. 18.1 Quantities characterising an alternating current or voltage Quantity Symbol Description period T 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 f 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. Frequency is related to period by the equation 1f T . For a sinusoidal A .C. waveform, the concept of angular frequency 2 f  is useful. peak value I0 Maximum absolute value of the alternating current or voltage in either direction of zero value in a periodic cycle. For a sinusoidal waveform, this corresponds to the amplitud