H214 Current of Electricity - 1.0 Lecture Notes (1718)

Topic 14: Current of Electricity Page 1 of 32 9749 H2 Physics Lecture Notes Topic 14 Current of Electricity Content  Electric current  Potential difference  Resistance and resistivity  Electromotive force 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 Q = It (d) recall and solve problems using the equation V = W /Q (e) recall and solve problems using the equations P = VI, P = I2R and P = V2 / R (f) recall and solve problems using the equation V = IR (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 characteristic of an NTC thermistor (i) recall and solve problems using the equation R =ρl/A (j) distinguish between electromotive force (e.m.f.) and poten tial 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 termina l potential difference and output power.

Topic 14: Current of Electricity Page 2 of 32 9749 H2 Physics Lecture Notes Introduction Imagine a world without electricity. There would be no microwaves, no television, no hospitals or li fe- support systems, and worse for some, no wi -fi. In the classic science fiction movie, The Day the Earth Stood Still, an alien spacecraft neutralizes all e lectricity on Earth. As a result, society collapses, and technology is reduced to that of the 16th century. Electricity has become so vital in our daily life that we no longer notice its existence until we experience a power outage. In the future this inf luence can only grow. An understanding of the basic properties of electric current is important for its safe and effective use. 14.1 Electric Current & Charge When an electrical conductor is conducting electricity, an electric current is said to flow through them. These electric currents are made up of a net flow of tiny charge carriers (or charged particles) such as electrons (–ve), protons (+ve) or ions (+ve or –ve) through the conductor. In this section, we will set the foundation right by defining electric current and electric charge formally for future discussions. 14.1.1 Electric Current Electric current is one of the 7 base quantities. It is being introduced to quantify the rat e of flow of charged particles. The following table gives a simple summary of what you need to know about electric current. Physical Quantity Electric Current Definition Electric current is the rate of flow of charge. Type Scalar Symbol ܫ SI Unit Ampere (A) Formula In general