TJC Unit 15 Currents

Student’s Copy Unit 15: Currents 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 the equation I = Q/t (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 for potential difference in terms if electrical work done per unit charge, WV Q= (d) recall and solve problems using the equations for electrical power P = VI, P = I2R 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 (g) represent a sinusoidal alternating current or voltage by an equation of the form x = xo sin t (h) deduce that the mean power in a resistive load is half the maximum (peak) power for a sinusoidal alternating current (i) distinguish between r.m.s. and peak values, and recall and use Irms = 2 0I and 2 VV o rms= for the sinusoidal case (j) explain the use of a single diode for the half-wave rectification of an alternating current

2025 Temasek Junior College 2 1 Charge and electric current LO (a)(b) 1.1 Charge The property of matter that causes the electrical force is charge. All bodies are either positively charged, negatively charged, or neutral (zero charge). Bodies with the same charge repel each other and bodies with opposite charge attract. Charge cannot be created or destroyed so therefore charge is conserved. The law of conservation of charge states that in a closed system the amount of charge is constant. Charge is a scalar quantity. The SI unit of charge is the coulomb (C). The charge of electron is e = –1.60 x 10-19 C. Any charge can only exist as an integral multiple of this value. i.e. Q = Ne Examples of charged particles or charged carriers in conductors are • negative electrons in copper wire • positive and negative ions in an electrolyte • positive ions and negative electrons in a discharge tube • negative electrons and positive holes in a semiconductor 1.2 Electric current Electric current I is the rate of flow of charge. i.e. Q t=I The SI unit of current is ampere (A). Hence, in a circuit carrying a constant current I, the charge Q which flows in time t, is given by charge = current x time i.e. Q = It The coulomb is defined as the charge which passes a point in a circuit when a constant current of one ampere flows for one second. If N electrons, each of