Lecture_14_(Chap.19)

# Lecture_14_(Chap.19) - Field due to the Battery E Ebends In...

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Field due to the Battery E E bends In the steady state there must be some other charges somewhere that contribute to the net electric field in such a way that the electric field points upstream everywhere. 1

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Field due to the Battery nAuE i Surface charge arranges itself in such a way as to produce a pattern of electric field that follows the direction of the wire d h h it d th t t i th l th and has such a magnitude that current is the same along the wire. 2
Field due to Battery E Smooth transition from + surface charge to – to provide constant E . The amount of surface charge is proportional to the voltage. 3

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Amount of Surface Charge The average magnitude of E in a closed circuit can vary from ~.01 V/m in copper wire to more than 100 V/m in Nichrome wire - due to a much different electron mobility. Surface charge at the end of a battery is ~10 6 electrons. Compare to tape: ~10 10 electrons What is easy: to draw E and i hat is complex: draw surface What is complex: to draw surface charge distribution 4
Connecting a Circuit When making the final connection in a circuit, feedback forces a rapid rearrangement of the surface charges leading to the steady state. This period of adjustment before establishing the steady state is called the initial transient. The initial transient 5

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Connecting a Circuit The initial transient E other E net Before the gap is closed, the net field in the wire must be zero, because the system is in static equilibrium. 6
Connecting a Circuit 1. Static equilibrium: nothing moving o current) (no current) 2. Initial transient: short-time process leading to the steady state Speed of light: 30 cm/ns In just a few nanoseconds the 3. Steady state: constant current rearrangement of the surface charges will extend all the way around the circuit.

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Lecture_14_(Chap.19) - Field due to the Battery E Ebends In...

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