Chapter 5- Ohm’s Law

Landstown High School Governors STEM & Technology Academy

• 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

Open and Closed Circuits

• 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 LAWThe current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit.

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Resistance

VoltageCurrent

REI

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

Finding the Current (I) Value

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E= I x R

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

Finding the Voltage (E) Value

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R = E/I

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

Finding the Resistance (R) Value

• 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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Rules

3 Formulas

•   E = I x R  (Voltage = Current multiplied by Resistance)

• R = E / I  (Resistance = Voltage divided by Current)

•  I = E / R  (Current = Voltage Divided by Resistance)

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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 . . . + IRn15

• 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.

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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 (I) 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 (E) around a

closed circuit equals zero.• ET = E1 + E2 + E3

• 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.

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