Homework

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Homework

• WebAssign Simple Electricity(1)
• Read Chapter 22 (Notes)

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A flow of charged particles in a closed system.
(Which can be very large)
Electricity
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Electron Energy Kinetic Energy
Current (I) Definition
V Definition
Resistance (R) Definition
Potential Difference
Positive and Negative Current
Ohms / Mhos
Work
Franklin
SuperConductor
Schematic
V IR
kWH and Usage
Circuit
Parallel
P IV
RT 1/R11/R2 1/R3
P I2R
Ohms Law
Series
Material Resistivity / Reference Tables
Graphs VI(Linear) IV(Linear) VIR Light
bulb R(t)
Tools Voltmeter Ammeter VOM DMM Rheostat Potentio
meter Battery
RT R1 R2
R r(T)L/A
RR(T)
Battery
Internal Resistance
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Voltage, Current Circuits

• Voltage is the potential difference that drives
  the movement of charged particles. (Named after
  Volta, who rediscovered the battery)
• Current is the flow of charged particles.
• An electric circuit is composed of three
  elements
• 1) conductor
• 2) potential difference (V), supplied by
• battery, solar cell, thunderstorm,
• 3) closed loop

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Voltage, Current Circuits

• Voltage is the potential difference that drives
  the movement of charged particles.
• Voltage is analogous to height in a gravitational
  field
• gh
• Multiply by m ? mgh and you now have
  gravitational potential energy (Joules).
• Electrical potential energy is W qV Joules.
  The unit of V is Joules / coulomb

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Current

• Electric current is the rate of flow of charge
• I ?q/ ?t
• Ampere Coulomb per second (passing by a point
  in a circuit)
• Ampere made discoveries regarding the
  interrelationships between electricity and
  magnetism in the early 1800s

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Current which way does it go?

• Two types of current
• Conventional current Current flows from the
  positive terminal to the negative terminal
• Electron flow Follow the electrons! Which are
  going the wrong way (by definition)
• Why are these not the same?
• What famous American guessed wrong?
• When was the electron discovered?

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Current which way does it go?

• Two types of current
• Conventional current Current flows from the
  positive terminal to the negative terminal
• Electron flow Follow the electrons! Which are
  going the wrong way (by definition)
• Why are these not the same?
• What famous American guessed wrong?
• When was the electron discovered?
• 1898 (JJ Thompson) with his cathode ray tube.
  Long after current was defined.

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How do I visualize what is going on in an
electric circuit?
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How do I visualize what is going on in an
electric circuit?
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What is driving the current?
What is driving the flow?
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Ancient Video
http//www.stmary.ws/highschool/physics/home/notes
/electricity/circuits/default.htm
http//www.stmary.ws/highschool/physics/home/notes
/electricity/circuits/OhmsLawEquation.htm
http//www.stmary.ws/highschool/physics/home/notes
/electricity/circuits/CurrentBasics.htm
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Ohms Law Ohm's Law says that, for many
materials under a wide range of conditions, the
voltage, V, and current, I, are linearly related,
which implies resistance, R, is independent of V
and I.
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Linear equation with zero intercept y mx V
IR
The slope is the resistance, R
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Resistance

• Resistance
• the ratio of V/I
• Units Ohms
• Its inverse is conductance, unit is mhos
• Georg Ohm promulgated his famous law in 1827
• He was a high school Physics teacher!

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Which is the dependent and which the independent
variable?
The slope is the inverse of the resistance, 1/R
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Ohms Law V IR

• Why do I care?
• Fix wiring problems in my home
• Fix wiring problems in my car
• Fix my kids toys
• Figure out why my circuit breaker / fuse blows
• Do my job at work (the one I have now, not the
  one I had for the last 27 years)
• Disclaimer Consult your parents or a qualified
  electrician before trying this yourself.

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Ohms Law Mnemonic
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V IR
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V ----- I R
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V ----- R I
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Ohms Law
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Ohms Law Ohm's Law says that, for many
materials under a wide range of conditions, the
voltage, V, and current, I, are linearly related,
which implies resistance, R, is independent of V
and I.

• When does it not apply?
• Circuit elements that change temperature
• Examples?
• Circuit elements with large capacitance or strong
  magnetic fields (that are changing)
• Semiconductors materials that are natural
  insulators that are made to be somewhat
  conductive
• Diode like a one-way path

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Example

• Units
• Volt
• Current
• Resistance

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Example

• A 10V battery is placed across a 5 Ohm resistor.
  What is the current?
• A 10A current flows through a 10 Ohm resistor.
  What is the voltage?
• A 10V battery drives a 1A current through a
  resistor. What is the resistance?

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Power

• Power is the rate of doing work (Watts)
• Power Work / time
• Power Volt q / t but q/t I
• Electrical Power Voltage Current VI
• Electrical Energy Power Time VIt

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Resistance and Ohms Law

• Resistance is the impedance to the flow of charge
• Resistance is defined as the ratio of potential
  difference to current
• R V/I Unit is the Ohm, O
• A device is said to obey Ohms Law of its
  resistance is independent of the Voltage

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Find Resistance of a material

• Depends on four factors
• R 8 Temp
• R 8 RHO (?) resitivity
• R 8 Length
• R 8 A-1 (cross-sectional Area)

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Low resistance

• Short
• Fat
• cold

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High Resistance

• Long
• Thin
• Hot

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Problem

• Find the resistance of a 20 meters length of
  Aluminum with a diameter of 12 mm

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Givens

• Length 20 m
• RHO 2.82 10 -8 O m
• Area p r 2
• D 12 mm .012 m r .006 m
• A p (.006m)2 m2

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Diagramming circuits
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• Series Connection
• Within the circuit,
• ammeters are always in the circuit, they measure
  flow of charge
• Ammeters should always have low resistance

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• Parallel Connection
• Provides multiple paths for flow
• Voltmeter is always placed parallel to the device
  you wish to measure the potential difference and
  have very high resistance

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More Electric Energy Equations

• P W/t Watt
• P VI V2/R I2R
• Electric Energy (W) Pt VIt V2/Rt I2Rt
• Joules for all types of Energy

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Kirchoffs Junction Rule

• Conservation of charge

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Kirchoffs Loop Rule

• Conservation of Energy
• The sum of the changes in potential around any
  closed path (loop) of a circuit must be zero.

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Series Circuit

• Current One path for the flow of charge
• ItI1I2I3
• Resistance must increase because length increases
• Rt-R1R2R3
• Voltage must add to Zero
• Vt-V1-V2-V30 VtV1V2V3

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Series Diagram
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VIRP Table

• Vt V1 V2
• It I1 I2
• Rt R1 R2
• Pt P1 P2

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Practice Series Circuits

• /www.stmary.ws/physics/home/animations3/electricit
  y/ElectricPowerChallenge.html.net/ed1_files/circui
  ts1.html
• http//www.stmary.ws/highschool/physics/home/notes
  /electricity/circuits/default.htm

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Parallel Circuit

• More than one path for the flow of charge
• More room for the flow of charge so Resistance
  goes down
• Voltage must stay the same since all charges have
  the same drop

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Parallel Equations
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Parallel Diagram