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.
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• 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.
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• Three types of circuits
– Series circuit
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– Parallel circuit
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– Series-parallel circuit
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• Closed circuit
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• Open circuit
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• 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.
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• 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
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E
I=
R
I = current in amperes.
E = voltage in volts.
R = resistance in ohms.
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• 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
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• The total resistance in a series circuit is
equal to the sum to the individual
resistances in the circuit.
RT = R1 + R2 + R3 . . . +Rn
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• 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
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• 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
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• To determine unknown quantities in a
circuit:
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–
–
–
–
Draw a schematic of the circuit.
Label all known quantities.
Solve for equivalent circuits.
Redraw the circuit.
Solve.
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• 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
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– 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
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• In summary:
– Electric circuit
• Voltage source
• Load
• Conductor
– Current path
• Series
• Parallel
• Series-parallel
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– 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
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