Chap25_PHY2049

# Chap25_PHY2049 - Chapter 25 Current Resistance and...

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Chapter 25 – Current, Resistance and Electromotive Force - Current - Resistivity - Resistance - Electromotive Force and Circuits - Energy and Power in Electric Circuits - Theory of Metallic Conduction

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1. Current Electric current: charges in motion from one region to another. Electric circuit: conducting path that forms a closed loop in which charges move. In these circuits, energy is conveyed from one place to another. Electrostatics: E = 0 within a conductor b Current (I) = 0, but not all charges are at rest, free electrons can move (v ~ 10 6 m/s). Electrons are attracted to + ions in material b do not escape. Electron motion is random b no net charge flow Non-electrostatic: E 0 inside conductor b F = q E Charged particle moving in vacuum b steady acceleration // F Charged particle moving in a conductor b collisions with “nearly” stationary massive ions in material change random motion of charged particles. Due to E, superposition of random motion of charge + slow net motion (drift) of charged particles as a group in direction of F = q E b net current in conductor. Drift velocity (v d ) = 10 -4 m/s (slow)
Direction of current flow: - In the absence of an external field, electrons move randomly in a conductor. If a field exists near the conductor, its force on the electron imposes a drift. - E does work on moving charges b transfer of KE to the conductor through collisions with ions b increase in vibrational energy of ions b increase T. - Much of W done by E goes into heating the conductor, not into accelerating charges faster and faster. Metal: moving charges – Ionized gas (plasma) or ionic solution : moving charges + or – Semiconductor: electron + hole (vacancy) conduction

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- Positive charges would move with the electric field, electrons move in opposition. - The motion of electrons in a wire is
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Chap25_PHY2049 - Chapter 25 Current Resistance and...

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