Chapter 5 Ohm’s Law • Objectives – After completing this chapter, the student should be able to: • • • • Identify the three basic parts of a circuit. Identify three types of circuit configurations. Describe how current flow can be varied in a circuit. State Ohm’s law with reference to current, voltage, and resistance. 2 • Solve problems using Ohm’s law for current, resistance, or voltage in series, parallel, and seriesparallel circuits. • Describe how the total flow differs between series and parallel circuits. • Describe how the total voltage drop differs between series and parallel circuits. • Describe how the total resistance differs between series and parallel circuits. 3 • State and apply Kirchhoff’s current and voltage laws. • Verify answers using Ohm’s law with Kirchhoff’s law. 4 • Electric circuits – The path that the current follows is called an electric circuit. – All electric circuits consist of: • A voltage source. • A load. • A conductor. 5 • Three types of circuits – Series circuit 6 – Parallel circuit 7 – Series-parallel circuit 8 • Closed circuit 9 • Open circuit 10 • Current flow can be varied by: – Changing the voltage applied to the circuit. • Voltage increases, current increases. • Voltage decreases, current decreases. – Changing the resistance in the circuit. • Resistance increases, current decreases. 11 • OHM’S LAW The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit. Voltage Current = Resistance or 12 E I= R I = current in amperes. E = voltage in volts. R = resistance in ohms. 13 • In a series circuit, the same current flows throughout the circuit. IT = IR1 = IR2 = IR3 . . . = IRn • The total voltage in a series circuit is equal to the voltage drop across the individual loads in the circuit. ET = ER1 + ER2 + ER3 . . . + ERn 14 • The total resistance in a series circuit is equal to the sum to the individual resistances in the circuit. RT = R1 + R2 + R3 . . . +Rn 15 • In a parallel circuit, the same voltage is applied to each branch in the circuit. ET = ER1 = ER2 = ER3 . . . = ERn • The total current in a parallel circuit is equal to the sum of the individual branch currents in the circuit. IT = IR1 + IR2 + IR3 . . . + IRn 16 • The reciprocal of the total resistance is equal to the sum of the reciprocals of the individual branch resistances. 1/RT = 1/R1 + 1/R2 + 1/R3 . . . + 1/Rn 17 • To determine unknown quantities in a circuit: – – – – – Draw a schematic of the circuit. Label all known quantities. Solve for equivalent circuits. Redraw the circuit. Solve. 18 • Kirchhoff’s Law – In 1847 G. R. Kirchhoff extended Ohm’s law with two important statements. – Kirchhoff’s current law: • The algebraic sum of all the currents entering and leaving a junction is equal to zero. • IT = I1 + I2 + I3 19 – Kirchhoff’s voltage law • The algebraic sum of all the voltages around a closed circuit equals zero. • ET - E1 - E2 - E3 = 0 or • ET = E1 + E2 + E3 20 • In summary: – Electric circuit • Voltage source • Load • Conductor – Current path • Series • Parallel • Series-parallel 21 – Current flow • Negative to positive • Varied by changing the voltage or the resistance. – Ohm’s Law – Determining unknown quantities in a circuit. – Kirchhoff’s Laws 22