Chapter 25 – Current, Resistance and
Electromotive Force
- Current
- Resistivity
- Resistance
- Electromotive Force and Circuits
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
- Positive charges would move with the electric field, electrons move in opposition.
- The motion of electrons in a wire is analogous to water coursing through a river.
