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# E7sol - Phy sics 7B WS E7(rev 3.0 Page 1 E-7 DC Circuits...

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Physics 7B WS E7 (rev. 3.0) Page 1 E-7. DC Circuits Questions for discussion 1 . The figure below shows a cutaway view of two current-carrying wires, a thick one and a thin one. In the thick wire, electrons are moving with a speed of 3 cm/sec. In the thin wire, electrons are moving faster, with a speed of 5 cm/sec. Let both wires have the same electron density (the number of electrons per unit volume) be ρ . a) First consider the thick wire. In one second, how many electrons will pass a given point in the wire? In the thick wire, the speed of the electrons is 3 cm/sec. So in one second, 3 cm worth of electrons will pass any given point. And from the diagram, it appears that each cm worth of electrons translates into 9 × 4 = 36 electrons. So in one second, 3 × 36 = 108 electrons will pass any given point in the wire. b) Next consider the thin wire. In one second, how many electrons will pass a given point in this wire? In the thin wire, the speed of the electrons is 5 cm/sec. So in one second, 5 cm worth of electrons will pass any given point. And from the diagram, it appears that each cm worth of electrons translates into

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