Current Studies - General Physics I - Lecture Slides, Slides of Physics

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General Physics 2 Current & Resistance 1

E5 - Current

• So far we’ve looked at stationary

charges – electrostatics

• Now we’re ready for moving charges!!!

General Physics 2 Electric Field 2

Motion of q in a uniform E field

The figure shows an electron entering a uniform electric field of

1500. N/C. What is the acceleration of the e

- 

once inside the field? m e = 9.11 x 10 - kg q e = 1.602 x 10 - C

General Physics 2 Current & Resistance 4

Drude Model

• When an electric field is present, the electron’s collisions with atoms results in a drift velocity opposite to the direction of the electric field

General Physics 2 Current & Resistance 5

Drift Velocity

• The average drift speed is proportional to E
• In between collisions, the acceleration is proportional to the electric field

F

e

= m

e

a

a =

F

e

m

e

− e

E

m

e €

v (^) ( t ) =

v 0

at =

v 0

e

E

m e t €

v (^) ( t ) = 0 +

at = 0 − e

E

m e t

v

d

e

m

e

E τ

General Physics 2 Current & Resistance 7

Current Flow

• Current is the total rate at which charge crosses a boundary
• Unit of current is the ampere
• Insert Table E5.1€ I ≡ dQ dt

1 A = 1

C

s

General Physics 2 Current & Resistance 8

Microscopic View of Current (L)

• Current is the amount of charge in coulombs passing a given point per second.
• How fast are individual electrons moving?
• Electrons move due to electric field inside wire, but collisions with other particles slow them down.
• e- are moving to the left. Which way is E? Which way is I?

General Physics 2 Current & Resistance 10

Surface Charges Direct Flow

• Surface charges in a conductor move so that current is constant

General Physics 2 Current & Resistance 11

Resistance

• Resistance of a conducting object
• Where Δϕ is the potential difference between the ends
• Unit of resistance is the ohm € R ≡ Δ φ I

General Physics 2 Current & Resistance 13

Power Dissipated in a Conductor

• Power is the rate at which electrostatic potential is converted to thermal energy
• Unit of power is the watt = J/s = N m/s = kg m 3 /s 3 € P = I Δ φ € P = I 2 R = Δ φ 2 R

General Physics 2 Current & Resistance 14

Plug & Chug

• (a) Determine the resistance of, and current through, a 75-W lightbulb connected to its proper source voltage of 120 V.

General Physics 2 Current & Resistance 16

Think-Pair-Share

• How many kWh of energy does a 550-W toaster use in the morning if it is in operation for a total of 15 min? At a cost of 9.0 cents/kWh, estimate how much this would add to your monthly electric bill if you made toast four mornings per week.

General Physics 2 Current & Resistance 17

Discharging a Capacitor

• The potential difference V C at any time t
• Where R is the resistance and C is the capacitance

V

C

( t ) =^ Ae

− t RC