LECT8_171176

LECT8_171176 - Ch. 17: Current & Resistance Current:...

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
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)
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 2
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
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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.
Background image of page 4
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.
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 6
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.
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 32

LECT8_171176 - Ch. 17: Current & Resistance Current:...

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online