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Objective: To calculate the total resistance in series and parallel circuits in order to apply Ohm’s Law.Energy Note: How do circuits differ from each other??

Drill: 1. Get a new lab from the SET. 2. Form groups. 3. Each group needs a materials tote and

BOTH circuit boards. 4. Compare your homework diagrams. 5. Wait quietly for directions.

Ohm’s LawResistance in a Complex Circuit

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Objective: To calculate the total resistance in series and parallel circuits in order to apply Ohm’s Law.Energy Note: How is the total resistance in a circuit calculated?

Drill: 1. Open your Log Book. 2. Come up with an analogy for current, voltage, and resistance. Thin

about roads, pipes, rivers, etc…

Ohm’s LawResistance in a Complex Circuit

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Activity 1: Setting the StageIf the resistance in a circuit is 12 ohms, and the voltage is 120 volts, what is the current? Show all work using proper form.

Current = Voltage ÷ ResistanceCurrent = 120V ÷ 12ΩCurrent = 10A

Are all circuits this simple? Explain your thoughts.

Ohm’s LawResistance in a Complex Circuit

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Resistance in a Series CircuitThe resistance in a series circuit is equal to the sum of the individual resistors in the circuit. Why?

Every electron must pass through each resistor.

Ohm’s LawResistance in a Complex Circuit

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Example: If there are two 3 ohm resistors and one 4 ohm resistor in a series circuit, the total resistance equals _____ ohms.

  10 Ohms

Practice Problem: If a series circuit is powered by a 20 volt battery and there are two bulbs on the circuit, each with a resistance of 4 ohms, what is the current? (Calculate the total resistance first and then apply Ohm’s Law.)

Ohm’s LawResistance in a Complex Circuit

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Resistance = 4Ω + 4Ω = 8Ω

Current = Voltage ÷ Resistance

Current = 20 V ÷ 8ΩCurrent = 2.5A

Ohm’s LawResistance in a Complex Circuit

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Activity 3: Resistance in a Parallel Circuit In a parallel circuit, what happens to the resistance when branches are added?

The total resistance decreases.

Ohm’s LawResistance in a Complex Circuit

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Resistance in a Parallel CircuitThe reciprocal of the total resistance in a parallel circuit is equal to the sum of the reciprocals individual resistors in the circuit.  This is because all of the current does not have to go through each resistor.

Ohm’s LawResistance in a Complex Circuit

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Practice Problem: If a parallel circuit is powered by a 20 volt battery and there are two bulbs on the circuit, each with a resistance of 4 ohms, what is the current? (Calculate the total resistance first and then apply Ohm’s Law.)

Ohm’s LawResistance in a Complex Circuit

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1 1 1 2

Resistance = 4Ω + 4Ω = 4Ω

Resistance = 2Ω (reciprocal of

2/4)

Current = Voltage ÷ Resistance

Current = 20 V ÷ 2ΩCurrent = 10A

Ohm’s LawResistance in a Complex Circuit

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Look at the answers to the two practice problems. What concept(s) is/are illustrated by the solutions?

1. When the same resistors are wired in series the total resistance is greater than that of the same resistors wired in parallel.

2. Therefore, the current in the parallel circuit is greater.

Ohm’s LawResistance in a Complex Circuit

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Class Practice  

HOMEWORK: Self Practice

Ohm’s LawResistance in a Complex Circuit