Download Total Resistance in a Circuit

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.
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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…
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Activity 1: Setting the Stage
If 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 ÷ Resistance
Current = 120V ÷ 12Ω
Current = 10A
Are all circuits this simple? Explain your thoughts.
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Resistance in a Series Circuit
The 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.
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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.)
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Resistance = 4Ω + 4Ω = 8Ω
Current = Voltage ÷ Resistance
Current = 20 V ÷ 8Ω
Current = 2.5A
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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.
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Resistance in a Parallel Circuit
The 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.
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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.)
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1
1
1
Resistance = 4Ω + 4Ω =
2
4Ω
Resistance = 2Ω (reciprocal of 2/4)
Current = Voltage ÷ Resistance
Current = 20 V ÷ 2Ω
Current = 10A
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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.
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Class Practice
HOMEWORK: Self Practice
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