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Physics 19: DC Circuits

Physics 19: DC Circuits. Christopher Chui. Resistors in Series and in Parallel. Series circuit: voltages add; current is the same in each resistance R Equivalent R in series: R eq = R 1 + R 2 + R 3 Parallel circuit: currents add; voltages are the same across each R

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Physics 19: DC Circuits

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  1. Physics 19: DC Circuits Christopher Chui Physics 19: DC Circuits - Christopher Chui

  2. Resistors in Series and in Parallel • Series circuit: voltages add; current is the same in each resistance R • Equivalent R in series: Req = R1 + R2 + R3 • Parallel circuit: currents add; voltages are the same across each R • Resistance in //: 1/Req = 1/R1 + 1/R2 + 1/R3 Physics 19: DC Circuits - Christopher Chui

  3. EMF, Terminal Voltage, & Kirchhoff Laws • A battery transforms chemical energy into electrical energycalled source of electromotive force, emf • Battery voltage is not constant because of internal resistance • Vab = E – Ir • Kirchhoff’s Junction Rule: at any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction • Kirchhoff’s Loop Rule: the sum of the changes in potential around any closed path of a circuit must be zero Physics 19: DC Circuits - Christopher Chui

  4. Problem Solving: Kirchhoff Laws • Label + and – for each battery • Label the current in each branch of the circuit with a symbol and an arrow • Apply Kirchhoff junction law at each junction, and the loop law for 1 or more loops • Follow each loop in one direction only. Pay attention to signs • Solve the equations algebraically for the unknowns • Check your answers by plugging them into the original equations not previously used, loop or junction rule equation Physics 19: DC Circuits - Christopher Chui

  5. Circuits with Batteries & Capacitors • Pay attention to battery terminals with signs • Capacitance in //: Ceq = C1 + C2 + C3 • Capacitance in series: 1/Ceq = 1/C1 + 1/C2 + 1C3 • When a capacitor is in series with a resistor, the capacitor will be charged: V = E(1 – e –t/RC ) • Capacitor discharge: V = Vo e –t/RC • RC is the time constant of the circuit • Leakage current is hazardous; beware of wet skin • 120 mA can be fatal • DC ammeter uses shunt resistor; ammeter is connected in series; while voltmeter is connected in parallel Physics 19: DC Circuits - Christopher Chui

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