Physics 220 Class 10

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Physics 220 Class 10

• Today you will learn
• what series and parallel combinations of
  resistors are.
• how to do series-parallel reduction to solve
  circuits.
• how to deal with real batteries in circuits.

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Reminders

• Chapter Test 3 is due today!
• Review is next Monday

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Resistance and Geometry

• One block has V, I, R.

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Resistance and Geometry

• Take two blocks with I going through each.
• Voltage is Current is
• Resistance is

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Resistance and Geometry

• Take two blocks with I going through each.
• Voltage is Current is
• Resistance is

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Resistance and Resistivity

• ? is the resistivity. The units of resistivity
  are Om.
• s 1/ ? is the conductivity

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Resistance and Temperature

• a is the temperature coefficient of resistivity
  (resistance).
• a is usually positive.
• a is negative for graphite.

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Resistors in SeriesHave the Same Current

• Take two resistors with I going through each.
• Voltage is Current is
• Resistance is

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Resistors in Series

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Resistors in ParallelHave the Same Voltage

• Take two blocks with I going through each.
• Voltage is Current is
• Resistance is

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Resistors in ParallelHave the Same Voltage

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Quizzes on Series and Parallel Combinations of
Resistors
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Series- Parallel Reduction

• Find a combination in series or parallel.
• Combine resistors into a single equivalent
  resistor.
• Repeat until there is only one resistor.
• The voltage across the resistor is the same as
  the voltage across the battery.

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Series-Parallel Reduction

• Find V, I, R, P for the last step.
• Bootstrap your way back to the beginning, diagram
  by diagram.

• What if there are resistors that arent in
  series or parallel?
• --- Youll need to use Kirchoffs Laws which
  well learn next time.

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Now well work some examples
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Find all the currents, voltages, and powers

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2A
24V,2A
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2A
2A
2A
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2A, 4V
2A
2A, 20V
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4V
4V
2A
2A, 20V
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4V, 2/3 A
4V, 4/3 A
2A
2A, 20V
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2/3 A
2/3 A
4V, 4/3 A
2A
2A
2A
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2/3 A, 8/3 V
2/3 A, 4/3 V
4V, 4/3 A
2A
2A, 4V
2A, 16V
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16/9 W
8/9 W

2/3 A, 8/3 V
2/3 A, 4/3 V
16/3 W
4V, 4/3 A
2A
2A, 4V
48 W
2A, 16V
8 W
32 W
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Using Meters

• Ammeters measure current. they must be paced in
  series with other circuit elements so current
  flows through them. Ammeters should have very
  small voltage.

• Voltmeters measure voltage. To measure the
  voltage between two points, you connect the two
  leads of the meter to those points. Therefore
  voltmeters are placed in parallel. Voltmeters
  should have large voltage.

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Real Batteries

• Real batteries have internal resistance. When
  they are placed in a circuit we can represent
  them as a resistor in series with an ideal
  battery.

Theres a voltage drop across the internal
resistance. This means that the full voltage of
the ideal battery isnt available to the circuit.