ASRJC Current of Electricity

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 14-1 Additional Notes Topic 14: Current of Electricity Content A Electric Current B Potential Difference C Resistance and Resistivity D Power and Heating Effect E Sources of 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 =V I, 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 = l / 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

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 14-2 Additional Notes A Electric Current A.1 Electric Charge, Q • The electric charge is a property of some elementary particles (such as electrons and protons) that gives rise to an interaction between them / causing them to attract and repel each other. • The charge of an atom arises as a result of an excess or deficit of electrons with respect to protons in the atom. If the atom gains or loses an electron, it becomes charged and is called an “ion”. • The smallest known electric charge is the elementary charge e. proton charge, e = 1.60 × 10–19 C electron charge, −e = −1.60 × 10–19 C • Subsequently, any charge Q can be quantized as a multiple of the elementary charge, Q = Ne where N = number of charge particles • The SI unit of charge is coulomb (C). One coulomb is that charge flowing per second past a point in a circuit in which the current is 1 ampere. The electric charge ΔQ passing one point in a circuit in an interval of time Δt is given by ΔQ = I Δt for a constant electric current I in the circuit. charge in coulombs = (current in amperes) × (time in seconds) ➔ ∆Q = I ∆t • If the current is not constant in a circuit then the charge which flows can be found by using the area beneath the current-time graph. In calculus terms this can be written a