Ch6,7+current+&+resistance

Ch6,7+current+&+resistance - CURRENT, RESISTANCE,...

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C URRENT , R ESISTANCE , AND C IRCUITS A LI A LZAHRANI
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C URRENT Moving charges create an electric current which is defined as the rate of change of electric charges. The SI unit of current is the Ampere (A) which is 1 A = 1 C/s. By convention, the direction of the current is the direction of the flow of positive charges. The actual charge carriers are electrons; hence they move in the opposite direction to current. Current density is the amount of current per unit area. Mathematically it is It is worthy to note that the current density is a vector quantity whereas current is a scalar quantity. dt dq i A i J
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R ESISTANCE When electrons move under the influence of an electric field, their randomly speed (drift speed) is defined as. where (Ne ) is the electronic charge density (electronic charge per unit volume) . It is quite important to note that the drift velocity has the same direction of electric field if the carrier charge is positive. Otherwise they have opposite directions. Resistance is the ability to resist the flow of current through a resistor (any wire). For a wire of length L, cross-sectional area of A, and resistivity , the resistance R is The SI unit of resistance is Ohm ( ). Its symbol in circuits is J Ne v d 1 A L R
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O HM S L AW When a potential V is applied across a resistor R, the electric current will be Here we note that larger resistance leads to smaller current. Also we find a new unit for resistance which is 1 = 1 Volt (V)/Ampere (A). Resistivity ( itsinverse conductivity ) can be now defined as The unit of resistivity is .m i V R R V i   E J J E J E i A L V L A i V L RA 1 1
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Ch6,7+current+&+resistance - CURRENT, RESISTANCE,...

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