Resistors s04 by L9k7v8N5

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									                 Resistors

        Ohm’s Law and Combinations of
                   Resistors
            See Chapters 1 & 2 in
          Electronics: The Easy Way
               (Miller & Miller)


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                      Electric Charge

 Electric charge is a fundamental property of some
  of the particles that make up matter, especially (but
  not only) electrons and protons.
 Charge comes in two varieties:
        Positive (protons have positive charge)
        Negative (electrons have negative charge)
 Charge is measured in units called Coulombs.
        A Coulomb is a rather large amount of charge.
        A proton has a charge 1.602  10-19 C.

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                        ESD

 A small amount of charge can build up on one’s
  body – you especially notices it on winter days in
  carpeted rooms when it’s easy to build a charge and
  get or give a shock.
 A shock is an example of electrostatic discharge
  (ESD) – the rapid movement of charge from a place
  where it was stored.
 One must be careful of ESD when repairing a
  computer since ESD can damage electronic
  components.
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                             Current

 If charges are moving, there is a current.
 Current is rate of charge flowing by, that is, the
  amount of charge going by a point each second.
 It is measured in units called amperes (amps) which
  are Coulombs per second (A=C/s)
        The currents in computers are usually measured in
         milliamps (1 mA = 0.001 A).
 Currents are measured by ammeters.

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                 Ammeter in EWB




                   Ammeters are connected in series.

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                  Current Convention

 Current has a direction.
 By convention the direction of the current is the direction in
  which positive charge flows.
    The book is a little unconventional on this point.

 If negative charges are flowing (which is often the case), the
   current’s direction is opposite to the particle’s direction
Current moving to right         Negative charges moving to left

                                               e-
         I                                          e-
                                          e-
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            Potential Energy and Work

 Potential energy is the ability to due work, such as
  lifting a weight.
 Certain arrangements of charges, like that in a
  battery, have potential energy.
 What’s important is the difference in potential
  energy between one arrangement and another.
 Energy is measured in units called Joules.


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                       Voltage

 With charge arrangements, the bigger the charges,
  the greater the energy.
 It is convenient to define the potential energy per
  charge, known as the electric potential (or just
  potential).
 The potential difference (a.k.a. the voltage) is the
  difference in potential energy per charge between
  two charge arrangements
 Comes in volts (Joules per Coulomb, V=J/C).
 Measured by a voltmeter.
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                 Volt = Joule / Coulomb




                     =


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                 Voltmeter in EWB




                 Voltmeters are connected in parallel.
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                 Voltage and Current

 When a potential difference (voltage) such as
  that supplied by a battery is placed across a
  device, a common result is for a current to
  start flowing through the device.




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                     Resistance

 The ratio of voltage to current is known as
  resistance
                  R = V
                         I
 The resistance indicates whether it takes a lot of
  work (high resistance) or a little bit of work (low
  resistance) to move charges.
 Comes in ohms ().
 Measured by ohmmeter.

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                 Multi-meter being used as
                   ohmmeter in EWB




       A resistor or combination of resistors is removed from
       a circuit before using an ohmmeter.
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                 Conductors and Insulators

 It is easy to produce a current in a material
  with low resistance; such materials are called
  conductors.
        E.g. copper, gold, silver
 It is difficult to produce a current in a
  material with high resistance; such materials
  are called insulators.
        E.g. glass, rubber, plastic

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                     Semiconductor

 A semiconductor is a substance having a
  resistivity that falls between that of
  conductors and that of insulators.
        E.g. silicon, germanium
 A process called doping can make them more
  like conductors or more like insulators
        This control plays a role in making diodes,
         transistors, etc.

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                       Ohm’s Law

 Ohm’s law says that the current produced by
  a voltage is directly proportional to that
  voltage.
        Doubling the voltage, doubles the current
        Resistance is independent of voltage or current

                   I
                                     Slope=I/V=1/R


PHY 202 (Blum)            V                                16
                     V=IR



                 =




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                            Ohmic

 Ohm’s law is an empirical observation
        “Empirical” here means that it is something we
         notice tends to be true, rather than something that
         must be true.
        Ohm’s law is not always obeyed. For example, it
         is not true for diodes or transistors.
        A device which does obey Ohm’s law is said to
         “ohmic.”

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                          Resistor

 A resistor is an Ohmic device, the sole
  purpose of which is to provide resistance.
        By providing resistance, they lower voltage or
         limit current




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                   Example

 A light bulb has a resistance of 240  when
  lit. How much current will flow through it
  when it is connected across 120 V, its normal
  operating voltage?
V=IR
 120 V = I (240 )
 I = 0.5 V/ = 0.5 A

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                 Series

                    Two resistors are in
                     series if a charge
                     passing through the
                     first resistor must pass
                     through the second
                     resistor.
                    It has nowhere else to
                     go.


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                       Resistors in series

 Each resistor obeys Ohm’s law
        V1 = I1 R1         and   V2 = I2 R2
 The current through the resistors is the same
        I1 = I2 = I
                       V1           V2

a                R1                R2          b

                  I1                I2 
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          Equivalent resistance (series)

 The equivalent resistance is the value of a single
  resistor that can take the place of a combination
        Has same current and voltage drop as combo
   Vab = V1 + V2 (the voltages add up to the total)
   Vab = I1R1 + I2R2
   Vab = I (R1 + R2)
   Vab = I Req
   Req = R1 + R2

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                 Resistors in series

 Resistors in series add.
 The equivalent resistance is larger than either
  individual resistance.
 If there are more things one has to go
  through, it will be more difficult.



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                 Equivalent Resistance




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                 Parallel

                     Two resistors are in
                      parallel if the top ends
                      of the two resistors are
                      connected by wire and
                      only wire and likewise
                      for the bottom ends.
                     A charge will pass
                      through one or the
                      other but not both
                      resistors.
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                  Resistors in parallel

 The voltage across the resistors is the same
        V1 = V2 = Vab
 The current is split between the resistors
        I = I1 + I2

                         R1

                         R2


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       Equivalent resistance (parallel)

 I = I1 + I2
    Vab              V1       V2
                 =        +        V’s are
    Req              R1       R2
                                   same, so
                                   they cancel
       1             1        1
                 =        +
      Req            R1       R2


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                 Resistors in parallel

 Resistors in parallel add reciprocally.
 The equivalent resistance will be smaller
  than either individual resistance.
 It is always easier if one has a choice of what
  one has to go through.



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                 Equivalent Resistance




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                 Fire in a theater analogy

 If it bothers you that the resistance of two resistors
  in parallel is lower than either resistor, consider the
  following.
 A fire starts in a packed theatre and there is one
  door through which everyone must exit. It’s a
  difficult task to get everyone out. A second exit is
  found, the second exit is narrower and fewer people
  can use it. However, the theater can be emptied
  much faster using two exits than one – even if a
  given person can only use one of the exits.
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                 Series/Parallel Recap

 Series
        Resistors in series have the same current.
        Their voltages add up to the total voltage.
        Rs = R1 + R2
 Parallel
        Resistors in parallel have the same voltage.
        Their currents add up to the total current.
        1/Rp = 1/R1 +1/R2


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        Serial and parallel connections

 A connection is said to be serial if all of the
  bits entering follow exactly the same path,
  bits then arrive one-by-one.
 A connection is said to be parallel if there are
  a set of paths, bits can then take different
  paths and groups of bits can arrive
  simultaneously.

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                    Multi-meter

 A multi-meter can serve as a voltmeter, ammeter or
  ohmmeter depending on its setting.
 To measure the voltage across a resistor, the
  voltmeter is placed in parallel with the resistor.
 To measure the current through a resistor, the
  ammeter is placed in series with the resistor.
 To measure the resistance of a resistor, the resistor
  is removed from the circuit and each end is
  connected to an end of the ohmmeter.

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       Voltmeter in parallel with 1-k
                  Resistor




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          Ammeter in series with 1-k
                   Resistor




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       Ohmmeter measuring resistance of
       1-k and 2 -k resistors in series




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                 Checking continuity

 A wire or cable is metal (a conductor) on the
  inside and thus has a low resistance.
 A broken cable has a high resistance.
 To check a cable,
        remove the cable,
        set the multi-meter to ohmmeter
        Check each wire for “continuity” (should find a
         low resistance).

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                       Heat

 A basic principle of physics is that energy is
  conserved, that is, energy is never lost or
  gained but only rearranged and put in
  different forms.
 When we have a simple resistor circuit, the
  potential energy that was in the battery
  becomes heat which is another form of
  energy.

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                 Cooling off

 When you run a computer, heat is constantly
  being generated because current is passing
  through circuits that have resistance.
 Too much heat can damage the circuits.
 The heat sink and the fan are used to reduce
  the amount of heat.


PHY 202 (Blum)                               40

								
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