H2_Current_of_Electricity_Lecture_Notes

9749 H2 PHYSICS; 8867 H1 PHYSICS Lecture Notes Nanyang Junior College 1 Chapter 13 CURRENT OF ELECTRICITY Content  Electric current  Potential difference (p.d.)  Resistance and resistivity  Sources of electromotive force (e.m.f.) Learning Outcomes Candidates should be able to: (a) show an understanding that electric current is the rate of flow of charge. □ □ □ (b) derive and use the equation I = nAvq 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 Qt I □ □ □ (d) recall and solve problems using WV Q □ □ □ (e) recall and solve problems using PV I , 2PR I and 2VP R □ □ □ (f) recall and solve problems using VR I □ □ □ (g) sketch and explain the I−V characteristics of various electrical components such as an ohmic resistor, a semiconductor diode, a filament lamp and a negative temperature coefficient (NTC) thermistor. □ □ □ (h) sketch the resistance-temperature characteristics of an NTC thermistor □ □ □ (i) recall and solve problems using R A  □ □ □ (j) distinguish between electromotive force (e.m.f.) and potential difference (p.d.) using energy considerations. □ □ □ (k) show an understanding of the effects of the internal resistance of a source of e.m.f. on the terminal potential difference and output power. □ □ □

9749 H2 PHYSICS; 8867 H1 PHYSICS Lecture Notes Nanyang Junior College 2 The term “e lectricity” describes the set of physical phenomena associated with the presence and flow of electric charge . These phenomena include but are not limited to lightning, static electricity, electric current , and electromagnetic induction. Electricity is involved in powering household appliances, transportation devices, and industrial machineries; it is also used to transfer energy over large distances; and to send information via telecommunications. This chapter introduces concepts required for the following chapters: D.C. Circuits, Electric Fields, Electromagnetism, Electromagnetic Induction and Alternating Currents. 1 ELECTRIC CURRENT AND CHARGE Charge carriers (or charged particles) such as electrons, protons and ions carry either a positive or negative charge. An electric current is said to be present when there is a net flow of charge. 1.1 ELECTRIC CURRENT Symbol: I SI unit: Ampere (A) Definition of Electric Current Electric current is the rate of flow of charged particles. Calculating Current dQ dtI Where I is the current in amperes, Q is the charge in coulombs and t is the time elapsed in seconds. If the current is constant, Q tI Conventionally, the direction of current is the direction in which there is a net flow of positive charge carriers; it is opposite to the direction of net flow of negative charge carriers. The diagram below shows net movement of charge and the corresponding direction of current. Net +ve charge