1. Physics 18: Electric Currents Christopher Chui Physics 18: Electric Currents - Christopher Chui

2. The Electric Battery • Volta originated the battery, which transforms chemical energy into electrical energy • All batteries have electrodes and electrolytes • An electric cell is a device that has electrodes and electrolytes. Several cells connected together is a battery • In a single cell, electrons are drawn from the positive terminal. When the electrodes are connected with a conductor, then the electrons complete the circuit. Thus, the current flows. By convention, current flows from positive terminal to negative terminal Physics 18: Electric Currents - Christopher Chui

3. Electric Current and Ohm’s Law • A flow of charge from one terminal to another is called an electric current, I = DQ/Dt • The unit for current is Ampere (1775-1836), 1 A=1 C/s • Conventional current starts from +ve terminal to the –ve terminal; electron flow is opposite • Ohm’s law: V = IR, R is in ohms • Resistor color code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9 • Tolerance: gold 5%, silver 10%, no color 20% • Resistance R of a metal wire is proportional to L and inversely proportional to cross-sectional area A • R= r L/A; r is resistivity, which depends on the material • Effect of temperature: rT = ro [1 + a(T-To)] Physics 18: Electric Currents - Christopher Chui

4. Electric Power • P = power = energy transformed/time = QV/t = IV • P = I(IR) = I2R = V2/R • A short circuit means that 2 wires have crossed so that the path of the current is shortened • Fuses and circuit breakers are safety devices to prevent shorts in the household • When current flows steadily in one direction, it is DC • AC currents alternate from +ve to –ve continuously • In AC, V = Vo sin 2pft; Vo=peak voltage, I=Io sin 2pft • Average power = ½ Io2R = ½ Vo2/R = Irms2R • Irms = Io/sqrt(2) = 0.707Io and Vrms = Vo/sqrt2=0.707Vo • I = DQ/Dt = neAvd Physics 18: Electric Currents - Christopher Chui