ERHS PHYSICS

Chapter 23.2 Notes

Resistance
  • Electrical resistance is the opposition to the flow of electrons in a circuit.
  • Electrical resistance is present in all parts of an electric circuit
  • The unit of electrical resistance is the ohm. An ohm is equal to one volt/ampere
  • Ohm’s law, R = V/I, can be used to solve for resistance (R), voltage difference (V) and current (I) in electrical circuits.
  • Wire resistance in electrical circuits is similar to fluid resistance in pipes.
  • The total resistance (Rtot) of two resistors (R1 and R2) connected in series is Rtot = R1 + R2
  • The total resistance (Rtot) of two resistors (R1 and R2) connected in parallel is
  • or:
  • Conductors, semiconductors and insulators are material classifications based on resistivities. Resistivity means the capacity to resist electric current.

    Resistivity

    • Constant for any material
    • Resistance is proportional to the resistivity and length of the material, and inversely proportional to the cross-sectional area of the material.
    • Resistance based on resistivity is given by:

    Where:

      1. R = resistance (W)
      2. r = resistivity (W·cm)
      3. l = length (cm)
      4. A = cross-sectional area (cm2)

    Examples of resistivity:

    Homework: Set #1: Ch 23 probs Questions 1-6, Problems 21-24

    Set#2: Read 23.2 Questions 7-11, problems 26-28, 32

    study this internet tutorial: http://library.thinkquest.org/10796/ch14/ch14.htm

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