Goal: To explore the usefulness of Resistors and to understand their applications to electronic circuits.

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Goal To explore the usefulness of Resistors and
to understand their applications to electronic
circuits.

• Objectives
• What is a resistor and how is it useful?
• How resistances are found or made
• 3) The different resistor combinations
• a) In series
• b) In Parallel
• c) Hybrid
• Learn how to calculate how much the resistors
  affect the flow of current.
• Power usage by resistors.

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• Why are resistors useful?

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(No Transcript)
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Resistors allow us to limit the flow of current
in a circuit.
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How calculated?

• Larger conduits offer less resistance.
• Some materials provide better flows than others.
• The further something has to flow the more
  resistance it will encounter.
• So, R ? L / A
• Here ? is not density but is a substances
  resistivity (which can be looked up for any
  substance).
• While there is a little dependence on temperature
  the resistivity mostly depends on the substance
  and only the substance.
• Examples will come in recitation.

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Resistors in series
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Resistors in series
Reff R1 R2 R3 R4
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Sample

• You have a circuit with 3 resisters
• R1 4 Ohms
• R2 3 Ohms
• R3 9 Ohms
• What is the effective resistance of the circuit?

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Sample

• You have a circuit with 3 resisters
• R1 4 Ohms
• R2 3 Ohms
• R3 9 Ohms
• What is the effective resistance of the circuit?
• R R1 R2 R3 4 3 9 16 Ohms

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Resistors in parallel
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Resistors in parallel
1 1 1_ Reff R1 R2
If you remember to yesterday with that math trick
this would go to R R1 R2 / (R1 R2)
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Resistors in parallel
You have 2 resistors in parallel. R1 5 Ohms,
R2 10 Ohms. What is the effective resistance?
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Resistors in parallel
If just two resistors this simplifies to
R1 R2
Reff R1 R2
So, R 10 5 / (10 5) R 3.3 Ohms Note that
the resistance is lowered.
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Hybrid resistor combinations
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Hybrid resistor combinations
When faced with a hybrid of in series and in
parallel, start by solving for the parallel
portions, then solve for the in series portions.
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Hybrid resistor combinations
Reff RAB R3 R45eff
R4 R5 Reff RAB R3 R4 R5 We will do
an example in recitation.
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Calculating the current

• V i R or i V / R
• So, in series
• i V / Rtotal
• In parallel
• For each branch, ibranch V / Rbranch
• And the total current is still i V / Rtotal

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Calculating the current for hybrids

• For hybrid i V / Rtotal
• For each branch of the
• parallel portion, the
• current is found by
• R45
• i4 i R4
• R4 R5
• R45 R4 R5
• R5 _
• i4 i R4 R5

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Power

• Resisters use power.
• Power is an energy per time.
• This is measured in W
• Energy Power Time
• Energy is in Joules or Watt Hours
• Your electric company charges you on the energy
  you use (in kW-hrs).

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Power for Resister

• For a Resistor the power it uses is just
• P IV I2 R V2 /R
• Thats all there is too it!
• For an entire circuit you use I, V, R for the
  entire circuit.
• For a single resistor or group of resistors, you
  use the I, V, R for those resistors.

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Conclusion

• Resistors are very useful in controlling the
  amount of current present in a circuit.
• Resistors can be used in series or in parallel
  (to allow the same circuit to have different
  currents in different places).
• The total current in any circuit with just
  resitors is just
• i V / Rtotal