chapter27_2 - Chapter 27 Current Resistance And Resistor...

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Chapter 27 Current Resistance And Resistor
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Review The current is defined and its unit is ampere (A), a base unit in the SI system. dt dQ I = + + + + + + I A The charges passing through the area could be positive or negative or both. They are called charge carriers. Current direction is chosen to the electric field direction or the direction as the flow of positive charges.
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The motion of the charge carriers is the result of the movement driven by the electric field plus random collisions with other particles in the conductor. The average velocity of this motion is called the drift velocity Review n is the number of charge carriers per unit volume . q is the carrier charge The drift velocity is very small. What is actually prorogating in the conducting wire is the electric field. So the conducting wire is actually the carrier or guide for the electric field, which delivers the electric power. E d V arrowrightnosp The current and the drift velocity has a linear relationship. d I nq = arrowrightnosp arrowrightnosp v A
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Current Density J , the definition Current density is a vector and is in the direction of the positive charge carriers, or in the direction of the electric field. d nq V arrowrightnosp arrowrightnosp = J The current density J of the current I a conductor is defined as the current per unit area J has SI units of A/m 2 = A arrowrightnosp arrowrightnosp d I J Compare with, one has: d I nq = arrowrightnosp arrowrightnosp v A
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Conductivity and Ohm’s Law For some materials, the current density is directly proportional to the electric field: E arrowrightnosp
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