Lecture_13_(Chap.19) - Chapter 19 A Microscopic View of...

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Chapter 19 A Microscopic View of Electric Circuits 1
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Current in a Circuit A microscopic view of electric circuits: • Are charges used up in a circuit? • How is it possible to create and maintain a nonzero electric field inside a wire? • What is the role of the battery in a circuit? Steady state and static equilibrium In an electric circuit the system does not reach equilibrium! Static equilibrium : • no charges are moving Steady state : • charges are moving •their velocities at any location do not change with time • no change in the deposits of excess charge anywhere 2
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Current in Different Parts of a Circuit What happens to the charges that flow through the circuit? Is the current the same in all parts of a series circuit? What would be I A compared to I B ? 1) I B = 0 all electrons are used up in the bulb 2) I B < I A some of the electrons are used up in the bulb ) = I 3) I B I A 3
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Current in Different Parts of a Circuit 1) I B = 0 all electrons are used up in the bulb t d i i t h l i h t b l b a) electrons disappear in the light bulb requires nuclear reaction E e mc 2 9 10 31 kg 3 10 8 m/s 2 8.1 10 14 J b) electrons accumulate in the light bulb P NE e I / e E e 0.2A /1.6 10 19 C 8.1 10 14 J ~ 100 kW will notice charge 2) I B < I A some of the electrons are used up in the bulb 3) I B = I A 4
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Current in Different Parts of a Circuit 3) I B = I A Test: Can use compass needle deflection for wire A and B 1. Can use compass needle deflection for wire A and B 2. Run wires A and B together above compass A B 5
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Current in Different Parts of a Circuit 3) I B = I A in a steady ate circuit state circuit What is used up in the light bulb? We cannot get something for nothing! Energy is transformed from one form to another Electric field – accelerates electron Friction – energy is lost to heat Battery – chemical energy is used up Closed circuit – energy losses to heat: not an isolated system!
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This note was uploaded on 04/23/2011 for the course PHYS 272h taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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Lecture_13_(Chap.19) - Chapter 19 A Microscopic View of...

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