Review 1

Review 1 - Direct Electric Current Electric Currents...

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1 Electric Currents • Chapter 18 (Giancoli) • All sections Direct Electric Current Conductors / metals In metals, the valence electrons are very weakly attached to their parent atoms. These electrons can be considered to be “ free ”. If an electric field is applied, the free electrons will move in the direction of the force applied on them. + Bound electrons Free electrons E F F Definition : The electric current is the rate of flow of charge across a surface. t Q I = t Q I = or Units for current: C/s ampère [A] The direction of the electric current is defined as the direction of motion of a positive charge. Electron drift velocity Electrons have thermal energy . For free electrons, this energy is transformed into kinetic energy. Typically, the free electrons acquire speeds of the order of 10 6 m/s . But the directions of the velocities are random . i.e. No net movement of the charges. When an E-field is applied, the free electrons move in a direction opposite to the E-field with velocities of the order of 10 -3 m/s . This is the electron drift velocity . E E = 0 Electron drift No electron drift Electrical resistance The larger the potential difference, the larger the current inside the conductor. V I E I V V = IR Ohm’s Law R is the resistance. V is the voltage across the resistor. I is the current passing through the resistor. Units for R: V/A Ohm [ ] © Z. Altounian
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2 Meaning of resistance : For a given voltage, a larger R will produce a smaller current. i.e. “Resistance” to current flow. Symbol for R: R How to measure V and I Voltmeter (analog or digital) or a CRO can measure the voltage across a resistor.
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Review 1 - Direct Electric Current Electric Currents...

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