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Electric field ohm s law_ etc.ppt

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    • 1. Electric Current, Ohm’s Law, and Electric Circuits ISAT 241 Fall 2002 David J. Lawrence
    • 2. Electric Current
      • Consider a bar of material in which positive charges are moving from left to right:
      • Electric current is the rate at which charge
      • passes through the surface, I avg =  Q/  t ,
      • and the instantaneous current is I = dQ/dt .
      imaginary surface I
    • 3. Electric Current
      • SI unit of charge: Coulomb (C)
      • SI unit of current: Ampere (1A= 1C/s)
      • A current of 1 ampere is equivalent to 1 Coulomb of charge passing through the surface each second.
    • 4. Electric Current
      • By definition , the direction of the current is in the direction that positive charges would tend to move if free to do so, i.e., to the right in this example.
      • In ionic solutions (e.g., salt water) positive charges (Na + ions) really do move. In metals the moving charges are negative , so their motion is opposite to the conventional current.
      • In either case, the direction of the current is in the direction of the electric field.
    • 5. Electric Current
      • Na + ions moving through salt water
      • Electrons moving through copper wire
      E I E I
    • 6.  
    • 7. Electric Current
      • The electric current in a conductor is given by
      • where
      • n = number of mobile charged particles
      • (“carriers”) per unit volume
      • q = charge on each carrier
      • v d = “drift speed” (average speed) of
      • each carrier
      • A = cross-sectional area of conductor
      • In a metal, the carriers have charge q  e.
    • 8.  
    • 9. Electric Current
      • The average velocity of electrons moving through a wire is ordinarily very small ~ 10 -4 m/s.
      • It takes over one hour for an electron to travel 1 m!!!
      E I
    • 10.  
    • 11. Ohm’s Law
      • For metals, when a voltage (potential difference) V ba is applied across the ends of a bar, the current through the bar is frequently proportional to the voltage.
      • The voltage across the bar is denoted:
      • V ba = V b  V a .
      area A V b V a E I
    • 12. Ohm’s Law
      • This relationship is called Ohm’s Law.
      • The quantity R is called the resistance of the conductor.
      • R has SI units of volts per ampere. One volt per ampere is defined as the Ohm (  . 1  =1V/A.
      • Ohm’s Law is not always valid!!
    • 13.  
    • 14.  
    • 15.  
    • 16.  
    • 17. Ohm’s Law
      • The resistance can be expressed as
      • where
        •  is the length of the bar (m)
        • A is the cross-sectional area of the bar (m 2 )
        •  , “Rho”, is a property of the material called the
        • resistivity. SI units of ohm-meters (  -m).
       area A V b V a E I
    • 18. Ohm’s Law
      • The inverse of resistivity is called conductivity :
      • So we can write
    • 19. Resistance and Temperature
      • The resistivity of a conductor varies with temperature (approximately linearly) as
      • where
          •  resistivity at temperature T ( o C)
          •  o  resistivity at some reference temperature T o (usually 20 o C)
          •  “ temperature coefficient of resistivity”.
      • Variation of resistance with T is given by
    • 20.  
    • 21. Electrical Power
      • The power transferred to any device carrying current I (amperes) and having a voltage (potential difference) V (volts) across it is
      • P = VI
      • Recall that power is the rate at which energy is transferred or the rate at which work is done.
      • Units: W (Watt) = J/s
    • 22.  
    • 23.  
    • 24. Electrical Power
      • Since a resistor obeys Ohm’s Law
      • V = IR , we can express the power dissipated in a resistor in several alternative ways:
    • 25.  
    • 26.  
    • 27.  
    • 28.