Transcript Resistors - Web Physics

Goal: To explore the usefulness of
Resistors and to understand their
applications to electronic circuits.
Objectives:
1) What is a resistor and how is it useful?
2) How resistances are found or made
3) The different resistor combinations:
a) In series
b) In Parallel
c) Hybrid
4) Learn how to calculate how much the resistors affect
the flow of current.
5) Power usage by resistors.
Why are resistors useful?
Resistors allow us to limit
the flow of current in a
circuit.
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 = ρ L / A
• Here ρ 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.
Resistors in series:
Resistors in series:
Reff = R1 + R2 + R3 + R4
Sample:
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You have a circuit with 3 resisters:
R1 = 4 Ohms
R2 = 3 Ohms
R3 = 9 Ohms
What is the effective resistance of the
circuit?
Sample:
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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
Resistors in parallel:
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)
Resistors in parallel:
You have 2 resistors in
parallel. R1 = 5 Ohms, R2 =
10 Ohms. What is the
effective resistance?
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.
Hybrid resistor combinations:
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.
Hybrid resistor combinations:
Reff = RAB + R3 + R45eff
Reff = RAB + R3 +
R4 * R5
R4 + R5
We will do an example in recitation.
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
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
R 4 * R5
R45 =
R 4 + R5
i4 = i *
R5 _
R4 + R5
Power
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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).
Power for Resister
• For a Resistor the power it uses is just:
• P = IV = I2 R = V2 /R
• That’s 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.
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