PHY132_L09

# PHY132_L09 - Classical Physics II PHY132 Lecture 9 Currents...

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Lecture 9 1 Classical Physics II PHY132 Lecture 9 Currents and Resistance 02/15/2010

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02/15/2010 Lecture 9 2 Current – Flow of Charges In the preceding notes, we have claimed that the electric field E inside a conductor must always be zero – if not, the mobile charges will re-arrange themselves such as to cancel the electric field E : – note that only a small percentage of the charges (electrons here) need to rearrange themselves in order to cancel even a strong external electric field… If a net electric field exists inside a conductor (i.e. a Vol- tage gradient : E x = dV/dx , …) an electric current must flow: – Current I dQ/dt , units: A (mpere) = C/s – this current dies out very quickly, unless the build-up of charges at the ends is prevented by continuously delivering ( source- ing) and removing ( sink -ing) them at the ends! – A well-charged capacitor, or a BATTERY may provide such… + + + + + + + - - - - - - - + + + + + + + - - - - - - - E=0 E E E inside = E + E =0 V
02/15/2010 Lecture 9 3 Resistance Let’s assume we have somehow arranged (e.g. using a battery as source and sink of charges) for a permanent uniform electric field E inside a non-ideal conducting wire of cross section A and length L : then there must exist a potential difference (voltage difference) between the ends of the wire: V = 0 L E · d l = EL – An electric field acting on a mobile charge q exerts a force F = q E , thus we expect the charge carriers to be accelerated (if no friction!)

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PHY132_L09 - Classical Physics II PHY132 Lecture 9 Currents...

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