Resistance

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Resistance

• Ohms Law
• The current flowing through the resistor is
  proportional to the potential difference
  (voltage)
• It is not a fundamental law
• Valid for ohmic devices mainly metallic
  conductors at constant temperature.
• Definition of resistance
• I V/R
• Unit of resistance ohm ? V/A

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Resistance

• Valid for ohmic devices mainly metallic
  conductors at constant temperature.

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Wires

• Elements with negligible resistance. We assume
  that there is no potential difference across the
  wire.
• Connecting both electrodes (ends) of the voltage
  source with a wire (not advised) is called short.

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• Let me relate something that happened to a friend
  of mine, a dentist by trade who likes to work on
  his sports car occasionally. In preparation for
  some other work, he elected to remove the battery
  from the car, using a box wrench to loosen the
  battery clamps. The wrench made contact with his
  wedding ring (he's lefthanded), and the ring in
  turn made contact with the battery holddown. A
  massive short circuit welded his ring to the
  holddown and to the wrench, which in turn was
  welded to the positive terminal. Car batteries
  store an enormous amount of energy, and they're
  optimized to deliver it in a very short period of
  time. A shorted car battery can easily deliver
  several hundred amps more than an arc-welding
  machine. Within a second, my friend's wedding
  ring was almost red-hot, and only a fast reaction
  with his other hand to break the connection by
  hitting the wrench prevented it from remaining
  connected long enough to melt. The ring had to be
  sawed off his finger at the emergency room, and
  it was more than a month before the burn
  specialist was sure he wouldn't lose the finger
  altogether.

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Fuses

• Demo Nail burner

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Circuit elements
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Electric Circuits
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Electrical Hazards

• Feel 1mA
• pain few mA
• deadly over 70mA

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Electrical Hazards

• Fig. 25.28

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Grounded high voltage line
1 000 000 V
0 V
50 000 V
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Electrical Power

• P VI
• Unit Watt (W) VA
• kW, MW, GW
• Electrical energy
• J Ws
• kWh
• GJ

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Electrical power on the ohmic device

• P VI
• V RI (Ohms Law)
• P V2/R I2R

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Alternating current

• DC AC
• V V0sin(2?ft) Peak voltage V0
• I I0sin(2?ft) Peak current I0

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Kirchhoff's rules

• At any junction point, the sum of all currents
  entering the junction must equal the sum of all
  currents leaving the junction.
• ( Conservation of charge)
• The Sum of the changes in potential around any
  closed path of the circuit must be zero

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Simulation http//phet.colorado.edu/new/simulatio
ns/sims.php?simCircuit_Construction_Kit_DC_Only
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Combinations of resistors

• Resistors in series
• V V1V2V3 IR1 IR2 IR3
• Req V/I R1 R2 R3

R1
R2
R3
V
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Combinations of resistors

• Resistors in parallel
• I I1I2I3 V/ R1 V/ R2 V/R3
• I V/Req
• 1/Req 1/ R1 1/ R2 1/R3

R1
R2
R3
V
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Combinations of resistors

• Resistors in parallel
• 1/Req 1/ R1 1/ R2 1/R3
• Req R1R2R3 / (R1R2 R2R3 R1R3)
• Notice Req is smaller than any one of them!

R1
R2
R3
V
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Demos Bulbs in series and in parallel