DHS 09 Current of Electricity (Lecture Notes & Tutorial)

Dunman High School (Senior High Physics) 1 Topic 9 Current of Electricity Guiding Questions: • How does the macroscopic phenomenon of current flow relate to the movement of microscopic charges? • How are current, voltage, and resistance related in an electrical circuit? • What happens to energy in an electrical circuit? 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) define the resistance of a circuit component as the ratio of the potential difference across the component to the current passing through it 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 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.

Dunman High School (Senior High Physics) 2 Introduction Charge is a property possessed by some (but not all) elementary particles (e.g. electrons, protons). Charge is found as two types, positive and negati ve, which ‘neutralise’ one another if brought together in equal quantities. Objects that have an electrical charge exert electrostatic forces on each other. The SI unit used to measure electrical charge is the coulomb (C). Physicists used to think that electrical charge could only be found in whole number multiples of a fundamental amount of charge, e = 1.602192 × 10−19 C. Charge on the proton = +e Charge on the electron = −e However, in the late 1960s, small particles called quarks were discovered with electric charge of either 2 3 e+ or 1 3 e− . These, together with –e, seem to be the basic quantities of electric charge. (a) show an understanding that electric current is the rate of flow of charge (c) recall and solve problems using the equation Q = It Electric current (SI base unit: ampere ), consists of charges in motion from one region to another. When this motion takes place within a conducting path that forms a closed loop, the path is called an electric circuit. In metals, current is due to the f