PT4E: Resistance in Electrical Systems

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PT4EResistance in Electrical Systems
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Conductors

• Charge can flow readily through conductors
  because they contain free electrons.
• Ex Metals, some liquids, and plasma
• Superconductors resistance disappears at very
  low temperatures.

Charge flows because electrons are free.
If they are free, then why charge?
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Insulators

• Do not readily conduct electricity.
• Ex wood, plastic, glass, rubber, and some
  minerals
• Semiconductors
• Have an intermediate ability to conduct charge.
• Ex silicon, germanium, gallium, and arsenic
• Resistance decreases as temperature increases.

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Electrical Resistance

• Electrical resistance is the result of collisions
  between electrons and atoms.
• Free electrons do not move in a straight line.

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Ohms Law
? VE

• RE electrical resistance (?)
• ?VE Voltage drop (V)
• I Current (A)

RE
I
1 ? 1 V/A
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Ex. 1 Calculate resistance
RE ?VE I 10V 2 A 5 V/A 5?
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• Ex. 2 Calculate current
• Calculate the current in a 6 V battery that is
  connected to a 15 ohm resistor.

I ?VE RE 6V 15? 0.4 A
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• Ex. 3 Calculate voltage
• Calculate the voltage in a circuit with a 30 ohm
  resistor and 5.5 amps of current.

?VE RE x I 30? x 5.5A 165 V
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Conductor vs. Semiconductor

• Conductor that obeys Ohms Law

• Semiconductor that does not obey Ohms Law

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Resistance Video
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Each person is a resistor!
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Resistivity

• Resistivity is a measure of the capacity of a
  material to resist electric charge flow.

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Factors Affecting Resistance

• Length - the Longer wire, greater resistance.
• Radius - the Smaller radius, greater resistance.
• Material composition - the Higher the resistivity
  of the material, greater resistance.

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Resistivity increases with temperature because
vibrating molecules interfere with flow of charge.
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Series Circuit

• Two lamps connected in series. Current flows from
  the negative terminal to the positive terminal.

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Four Rules for Series Circuits

• The current is the same throughout the series
  circuit IT VT RT
• The total resistance is the sum of the individual
  resistances (lamps).
• RT R1 R2 R3
• Find voltage drop (?V) across each lamp
  (resistor) is the product of R x I
• V1 (R1 x IT), V2 (R2 x IT), V3 (R3 x IT)
• The total voltage drop across the entire circuit
  equals the sum of voltage drops.
• VT V1 V2 V3

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Ex. 1 Resistance in Series

• The lamps have resistances of 90 ? and 70 ?. The
  power supply produces 120 V.
• What is the total current through the circuit?
• 1. Calculate the total resistance
• 2. Calculate the current
• B) What is the voltage drop across each lamp? (V
  R x I)

RT R1 R2 90 ? and 70 ? 160 ?
I VT RT 120 160 0.75 A
V R x I V1 90 ? x 0.75 A 67.5 V V2 70 ? x
0.75 A 52.5 V VT 67.5 52.5 120 V ?
Check your work VT V1 V2
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Ex. 2 Series

• An 8 ? resistor and a 16 ? resistor are connected
  in series with a 12-V battery.
• A) Find the total resistance.
• B) Find the total current through the circuit?
• C) Find the voltage drop across each resistor? (V
  R x I)

RT R1 R2 8 ? and 16 ? 24 ?
I VT RT 12 24 0.5 A
V R x I V1 8 ? x 0.5 A 4V V2 16 ? x 0.5 A
8 V VT 4 8 12 V ?
Check your work VT V1 V2
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Parallel Circuits

• Part of the current from the power supply flows
  through one lamp and part flows through the other
  lamp.

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Four Rules for Parallel Circuits

• The total current through the circuit is the sum
  of the currents IT I1 I2 I3
• The total resistance for the lamps is less than
  the resistance of either lamp. RT-1 R1-1
  R2-1 R3-1
• The current through each resistor is the ratio
  I1 (V R1), I2 (V R2), I3 (V R3)
• The voltage drop across all lamps is the same as
  the power supply. VT RT x IT

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Ex. 1 Resistance in Parallel

• The lamps have resistances of 90? and 70?. The
  power supply produces 120 V.
• A) What is the total current?
• 1. Calculate the total resistance
• 2. Calculate the current
• B) What is the voltage drop across each lamp?
• C) What is the current at each lamp? (I V R)

RT-1 R1-1 R2-1 RT-1 90-1 70-1 39.4 ?
IT V RT 120 39.4 3.04 A
Voltage doesnt change, so Voltage 120 V _at_ ea.
lamp
I V R I1 120 V 90 ? 1.33 A I2 120 V
70 ? 1.71 A IT 1.33 1.71 3.04 A ?
Check your work IT I1 I2
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Ex. 2 Parallel

• A 10 ? and a 40 ? resistor are connected in
  parallel to a 10 V battery.
• A) Find the total resistance?
• B) Find the total current?
• C) Find the voltage across each resistor.
• D) Find the current at each lamp?

RT-1 R1-1 R2-1 RT-1 10-1 40-1 8 ?
IT V RT 10 8 1.25 A
I V R I1 10 V 10 ? 1.0 A I2 10 V
40 ? 0.25 A IT 1.0 0.25 1.25 A ?
Voltage doesnt change, so Voltage 10 V _at_ ea.
lamp
Check your work IT I1 I2
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Summary
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Resistors an electrical device that has a
specific resistance.
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Interpret Resistors using Color Codes

• To calculate the value of the resistance you use
  the first three stripes only. 
• Here is the algorithm XY x 10Z
• The first stripe is X.
• The second stripe Y.
• The third stripe is the exponent.

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Resistor Examples
Resistor Color Code Calculator (click on resistor)

• (X) Brown 1 (Y) Black 0 (Z) Red 2
• XY x 10Z 10 x 102 1000 ? 5
• Tolerance Bands indicate the accuracy of the
  resistor.
• Gold band indicates the resistor is within 5 of
  what is indicated. Silver 10 and None 20.

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Practice Rcc
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• Which of the following are resistors?
• Light bulb
• LED
• Soldering iron
• Speaker
• Copper wire
• Hot plate
• Battery
• Pencil lead

Resistors are Light bulb Soldering iron Copper
wire Hot plate Pencil lead
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Current vs. Voltage

• Simple resistors obey Ohms Law.
• Linear graph

• Diodes do not obey Ohms Law.
• Charge flows in one direction only!