RI Chap 15 Currents Notes

15 CURRENTS H2 Physics 9478 Content Page 15.1 Current and drift velocity 2 15.2 Potential difference, electromotive force and power 5 15.3 Power supplies: d.c. and a.c. 7 15.4 Appendix 17 Learning Outcomes Candidates should be able to: (a) show an understanding that electric current is the rate of flow of charge and solve problems using Q t=I . (b) derive and use the equation nAvq=I for a current -carrying conductor, where n is the number density of charge carriers and v is the drift velocity. (c) recall and solve problems using the equation for potential difference in terms of electrical work done per unit charge, WV Q= . (d) recall and solve problems using the equations for electrical power PV= I , 2PR= I and 2VP R= . (e) distinguish between electromotive force (e.m.f.) and potential difference (p.d.) using energy considerations. (f) 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. [not in H1 syllabus] (g) represent a sinusoidal alternating current or voltage by an equation of the form 0 sinxx t ω= . [not in H1 syllabus] (h) deduce that the mean power in a resistive load is half the maximum (peak) power for a sinusoidal alternating current. [not in H1 syllabus] (i) distinguish between r.m.s. and peak values, and recall and use 0 r.m.s. 2 = II and 0 r.m.s. 2 VV = for the sinusoidal case. [not in H1 syllabus] (j) explain the use of a single diode for the half-wave rectification of an alternating current. [not in H1 syllabus]

Page | 2 Introduction Direct current (d.c.) is the unidirectional flow of electric charge. This flow of electric charge in a constant direction distinguishes it from the alternating current (a.c.). Such direct currents are produced by sources such as batteries and solar cells. Direct current may also be obtained from an alternating current supply by the use of a rectifier, which allows current in one direction only. Similarly, direct currents can also be converted into alternating currents with the use of an inverter. The first commercial electric power transmission that was developed by Thomas Edison in the late 19th century used direct current. Today, direct current is used in rail transport, electric vehicles, general electrical appliances, as well as to charge batteries. 15.1 Current and drift velocity  Charge and electric current The SI unit of charge is the coulomb (C), named after the French physicist Charles-Augustine de Coulomb (1736−1806). Charge is related to the electric current I by the equation: Q t=I where Q is the amount of charge that passes through the cross section at any point of a conductor in time t. Direction of current is the direction of flow of positive charges. Hence, electrons moving to the left gives rise to current to the right. Shock and Awe: The Story of Electricity Electric current is me