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Ohm s Law and Power Today s Agenda Resistance

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					Today’s Agenda

  Potentiometers
Ohm’s Law Continued
  Power & Energy
     Review from Last Week
• How is voltage related to charge and
  energy?
• What is the formula for resistance?
• What is Ohm’s Law?
• What does it mean?
             Potentiometers
• A potentiometer is a variable resistor

• The total resistance is fixed between terminals
  A and B
                                     A
• A portion of the resistance
  is between A and C
                                C
• The remainder is between
  B and C                            B

• C can be physically moved between A and B
            2 Basic Ways to Use
               Potentiometers
• As a variable resistor:      A

   – The center tap (C) is
     connected to one end
     (B)                       B/C
   – The total resistance is
     only from A to C

• As a voltage divider
  (to be covered in a later
  lecture)
           In-Class Activity
    If you have a 1k Ω potentiometer and the
  center tap, C, is set ¼ of the way between
  A and B (closer to A),
• What is the resistance between A and C
  and between B and C?
• What is the resistance R if the
  potentiometer is connected as below
  (assume C has not been moved):
                      R
                  A       B/C
 Relationship between Current and
              Voltage
• Current through a FIXED resistance
  – Increases when the voltage increases
  – Decreases when the voltage decreases
• The current changes as a result of the change
  in voltage!

       +                         +



       _                         _




       What is the value of the resistance?
 Relationship between Current and
            Resistance
• For a FIXED voltage,
   – The current decreases proportionally to an increase in
     resistance
   – The current increases proportionally to a decrease in resistance
• The current changes as a result of the change in
  resistance

          +                                   +



          _                                   _
           In-Class Activities
1. What is the effective resistance of each
   potentiometer in these circuits?

       +                          +
                    A                         A
 5V            C     R1    10 V          C    R2
       _                          _
                    B                         B



2. If R1 and R2 actually were the same
   potentiometer set to different values and R2
   corresponds to C adjusted all the way to the B
   end (i.e. total resistance value), what
   percentage of the total resistance is R1?
                            Energy
• Think of a battery like sand in an hour glass
   – Sand = charge
• Voltage is the force that moves charge
   – Think of being on the moon vs the Earth
• Energy = V.Q
   – You use much more energy to move sand on Earth than on the
     moon where gravity is 1/6th the Earth’s
            Power & Energy
• The Instantaneous Power, P, is the
  Change of Energy, E, per unit time.
  – In our sand analogy, power       E
    is a measure of how quickly   P
    the hourglass is emptying        t
• Units:   [E] = Joules (J).
           [t] = seconds (s).
                 J
            P   Watt W 
                 s
         Power & Energy
                  E
               P
                  t
The change in energy can be written as:

            E  P  t
We often assume initial energy is zero
Power in terms of Voltage and
           Current
                                      Energy
Previously you learned that Voltage 
                                      Charge
       E
or V             Using this and E    P  t
       Q
                                Q
yields V    Q  P  t   or V    P
                                t
           Q
Since   I             then   P  VI
           t
                 Power
- The amount of energy used per unit time
- The battery shown below uses 1 J/s to
  generate current – it has used 1 W of
  power.
Determining Power




     P  VI 
 Other Power Equations

P VI
V  IR
            In this example,
P I R2

            P=
Other Power Equations (continued)
  P V I
      V
  I 
      R          In this example,
      V    2


  P             P=
       R
   In-Class Activity for Power and
             Ohms Law
• In pairs, complete the following chart
        ITEM #   CURRENT   VOLTAGE   RESISTANCE   POWER




          1       10 mA                            4W



          2                 32 V                  16 mW



          3                            3.3 kΩ     231 mW



          4       15 mA     45 V



          5       24 mA                1.2 kΩ
             In-Class Activity
      Practice Problem 3.11 (p 86)
• Calculate the total energy used by a
  1500W dishwasher, a 3600W clothes
  dryer, and a 750W air conditioner that are
  all being used for 2 hours.
• Report your answer in J and Btu.
• Report your answer in kWh.
• Use the internet to find a recent cost per
  kWh and report the total cost for this
  problem.

				
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Description: Ohm s Law and Power Today s Agenda Resistance