Physics 201 Midterm I Review: Current model

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Physics 201Midterm I ReviewCurrent model
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Experiment 1
R
R
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Experiment 2
R
R
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Adding in series

• What happens to the current through the battery,
  Ibatt, if a bulb or network of bulbs is inserted
  between these two bulbs?

A
Ibatt
B
A) it goes up B) it goes down C) unchanged
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Adding in series

• What happens to the current through the battery,
  Ibatt, if a bulb or network of bulbs is inserted
  between these two bulbs?

A
B

• In all cases, it goes down. Why?
• Clog up an existing path,
• Increase the resistance of the circuit, and
• Decrease the current through the battery.

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Summary adding in series

• When a bulb or network of bulbs is added in
  series, the resistance of the circuit increases.

A
B

• The important thing is not what is added but how
  it is added.

• If you have to break the circuit, you are adding
  something in series.

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Adding in parallel

• What happens to Ibatt when a bulb (or network) is
  added in parallel to bulb B?

A

• It goes up. Why?
• Add a new path (opportunity),
• Decrease the resistance of the circuit, and
• Increase the current through the battery.

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Summary adding in parallel

• When a bulb or network of bulbs is added in
  parallel, the resistance of the circuit
  decreases.

A

• Again, the important thing is not what is added
  but how it is added.

• If you dont have to break the circuit, you are
  adding something in parallel.

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Which circuit has more resistance?
B
A
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Which circuit has more resistance?
B
A
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Rank the resistances
A
B
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Rank the resistances
A
C
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Rank the resistances
B
C
A
1) CgtAgtB 2) BgtCgtA 3) AgtBgtC
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Rank the resistances
R
R
R
B
C
A
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Rank the bulbs

• Rank all the bulbs below for brightness. Explain
  your reasoning.

A gets all of the current B C 1/2 of all
(the current) A gt B C
B and C share equally since each branch has equal
resistance.
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Rank the bulbs

• Which is brighter, C or D? Explain.

• Current favors the branch of least resistance.

B C less than ½ of all D more than ½ of
all A all
Ranking A gt D gt B C
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Rank the bulbs

• Rank all the bulbs below for brightness. Explain
  your reasoning. Batteries and bulbs are all the
  same.

A F gt B C D E Both circuits have the
same current. Which comes first does not matter.
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Rank the bulbs
Compare A to D
AgtD
Bulb F is added in series so resistance
increases, reducing the total current in the
bottom circuit. A is getting all of more and D
is getting all of less current so AgtD
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Rank the bulbs
Compare A to D
AltD
Bulb F is added in parallel so resistance
decreases, increasing the total current in the
bottom circuit. A is getting all of less and D
is getting all of more current so AltD
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Short circuits
A
B
What happens to the brightness of each bulb when
the switch is closed?
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Compare bulbs A and B. What happens to A and B
when S is closed?
A characteristic of an ideal battery is that
branches connected directly across it are
independent
A unchanged
B brighter, b/c closing S adds extra pathway in
parallel
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Kirchhoffs rule

• The total current out of a node is equal to the
  total current into a node

I1
I
I2
I I1 I2
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Practice Problem
What is the current through each of the bulbs?
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Practice Problem
Rank all the bulbs using the current model!
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Practice Problem
Find the equivalent resistance of branch I and
branch II, which branch has more? Rank the
branches according to current, which one has more
current?