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Current, Resistance and Electromotive Force
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Chapter 25 – Current, Resistance andElectromotive Force
- Current
- Resistivity
- Resistance
- Electromotive Force and Circuits
- Energy and Power in Electric Circuits
- Theory of Metallic Conduction
,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 Current (I) = 0, but not all charges
are at rest, free electrons can move (v ~ 106 m/s). Electrons are attracted to
+ ions in material do not escape.
Electron motion is random no net charge flow
Non-electrostatic: E ≠ 0 inside conductor F = q E
Charged particle moving in vacuum steady acceleration // F
Charged particle moving in a conductor 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 net current in conductor.
Drift velocity (vd) = 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
transfer of KE to the conductor through
collisions with ions increase in
vibrational energy of ions 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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