LECT8_171176

# LECT8_171176 - Ch 17 Current Resistance Current Rate at...

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Current: Rate at which charge flows through an area A (cross-section of a wire) Flow is assumed to be perpendicular to area. Units = Coul/sec = Amp. Remember: I is defined as the direction in which positive charges will travel (in metal, the charge carriers are actually electrons)

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Potential difference sets up E- field to drive Current V 1 –V 2 = Δ V Example: Terminals of a battery V 1 V 2 I
Example: A flashlight bulb carries a current of 0.1 A. Find the charge that passes through the bulb in 0.5 seconds: I = Δ Q/ Δ T : Δ Q = I × Δ T = 0.1C/s × 0.5s = 0.05 C How many electrons does this correspond to? Δ Q = N × e N = Δ Q/e = 0.05C / (1.6 × 10 -19 C/e ) = 3.1 × 10 17 e ’s

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Amp-hour Unit of charge charge = current × time Ex.: Ni-metal hydride battery: How much charge (in C) is equal to 2100 mAh? Charge = (2100 × 10 –3 A) (1 hour) = (2100 × 10 -3 C/s)(3600s) = 7560 C.
Amp-hour If one of these batteries is used to power a device which draws 0.15 Amps, how long will the battery last? I = Δ Q/ Δ T Δ T = Δ Q / I = (2100 × 10 -3 Amp × hr) / 0.15 Amps = 14 hours.

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Drift Velocity, v d Volume = A Δ x n = density of charge carriers = # of charge carriers per unit vol. N = Total # of charge carriers = n A Δ x Total charge in this volume: Δ Q = N × charge/carrier = n A Δ x q Δ x = v d Δ t Δ Q = nA v d Δ t q I = Δ Q/ Δ t = n A v d q
Drift Velocity, v d Electrons undergo repeated collisions and move randomly. Typical velocity for Cu is 2 × 10 6 m/s In the presence of an external field, the average motion is a slow drift Electric signal travels very fast -- almost at the speed of light: electrons interact and "push" other electrons in the conductor.

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Example: Find the drift velocity of electrons in a copper conductor whose diameter is 2 mm when the applied current is 0.5 A. The mass density of Cu is ρ = 8.95g/cm 3 . Each Cu atom contributes 1 electron. One mole of Cu has a mass of 63.5 gm. Soln: Need to calculate density of charge carriers (# of e
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LECT8_171176 - Ch 17 Current Resistance Current Rate at...

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