Chapter 5 Ohm’s Law

2 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.

3 Solve problems using Ohm’s law for current, resistance, or voltage in series, parallel, and series- parallel 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.

4 State and apply Kirchhoff’s current and voltage laws. Verify answers using Ohm’s law with Kirchhoff’s law.

5 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.

6 Three types of circuits –Series circuit

7 –Parallel circuit

8 –Series-parallel circuit

9 Closed circuit

10 Open circuit

11 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.

12 OHM’S LAW The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit. or

13 I = current in amperes. E = voltage in volts. R = resistance in ohms.

14 In a series circuit, the same current flows throughout the circuit. I T = I R1 = I R2 = I R3... = I Rn The total voltage in a series circuit is equal to the voltage drop across the individual loads in the circuit. E T = E R1 + E R2 + E R E Rn

15 The total resistance in a series circuit is equal to the sum to the individual resistances in the circuit. R T = R 1 + R 2 + R R n

16 In a parallel circuit, the same voltage is applied to each branch in the circuit. E T = E R1 = E R2 = E R3... = E Rn The total current in a parallel circuit is equal to the sum of the individual branch currents in the circuit. I T = I R1 + I R2 + I R I Rn

17 The reciprocal of the total resistance is equal to the sum of the reciprocals of the individual branch resistances. 1/R T = 1/R 1 + 1/R 2 + 1/R /R n

18 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.

19 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. I T = I 1 + I 2 + I 3

20 –Kirchhoff’s voltage law The algebraic sum of all the voltages around a closed circuit equals zero. E T - E 1 - E 2 - E 3 = 0 or E T = E 1 + E 2 + E 3

21 In summary: –Electric circuit Voltage source Load Conductor –Current path Series Parallel Series-parallel

22 –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