Ohm�s Law and Resistance by a74QIjce

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									Ohm’s Law and
  Resistance

    AP Physics C
Montwood High School
      R. Casao
        Current Density J
• For a conductor of cross-sectional area
  A carrying current I, the current
  density J is:

             I
          J    n  q  vd
             A
• The current density is in the direction
  of motion of the charges for positive
  charge carriers and opposite the
  direction of motion for negative charge
  carriers.
              Ohm’s Law
• A current density J and an electric
  field E are established in a conductor
  when a potential difference is
  maintained across the conductor.
  – If the potential difference is constant, the
    current in the conductor will also be
    constant.
• Ohm’s law: J = ·E
            = conductivity
             Ohm’s Law
• Ohm’s law states that for many
  materials (including most metals), the
  ration of the current density and
  electric field is a constant , which is
  independent of the electric field
  producing the current.
• The most common form of Ohm’s law
  is:
         V
      I
         R
Resistance
       • The motion of a ball
        rolling down an
        inclined plane and
        bouncing off pegs in
        its path is
        analogous to the
        motion of an
        electron in a
        metallic conductor
        with an electric field
        present.
                 Resistance
• If some of the pegs were removed, the ball
    would experience less resistance to its
    movement. If more pegs were added the ball
    might experience more resistance to its
    downhill motion.
•   At the atomic level, currents are pictured as
    the flow of the outer electrons of atoms
    through the material.
•   Resistance results from collisions of
    electrons with other electrons and with
    atoms.
•   Resistance is the opposition to the flow of
    charge in a conductor.
                 Resistance
                                               1V
• Resistance is measured in ohms .       1Ω 
                                               1A
• The inverse of the conductivity  of a material is its
  resistivity ; units ·m.
                                   1
                                ρ
                                   σ
• Resistivity is related to the nature of the material.
  Good conductors have low resistivity (or high
  conductivity). Poor conductors have high
  resistivity (or low conductivity).
• Resistance:                 Resistivity:
                   ρl
                R             = o + o ··(T – To)
                    A
                 Resistance
• The resistance of a conductor is proportional to the
  length.
   – Resistance increases with increased length.
• The resistance of a conductor is inversely
  proportional to the cross-sectional area of the
  conductor.
   – Resistance decreases with increased cross-
     sectional area.
• Resistance is also dependent upon the temperature
  of the conductor. Collisions of electrons with other
  electrons and with atoms raises the temperature of
  a material as the added heat energy causes the
  electrons to move faster and hence collide more
  often. This increases the resistance of the
  conductor.
Factors Affecting Resistance
Resistance and Temperature
• R = Ro· + Ro··(T – To)
•  is the temperature coefficient of
  resistivity.
• All electric appliances have a fixed
  resistance.
• Electric circuits make use of resistors
  to control the current level in the
  circuit.
              Web Sites

• Resistors and Resistor Color Code
  Calculator
• Resistor Color Code
• Ohm’s Law
• Voltage Circuit Simulator

								
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