Download conventional current

E5 - Current
•
•
So far we’ve looked at stationary
charges – electrostatics
Now we’re ready for moving charges!!!
General Physics 2
Current & Resistance
1
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
4
Current Density and conductivity
•
Current density indicates how rapidly charge is flowing
near a certain point in a conductor
v
r
r
J  v d  nqv d
•
•
•
•
Where ρ is the density of charge carriers, and n is the
number density of charge carriers
Current density is proportional to the electric field

r
v
J  cE
σc is the conductivity
Note that σ ≠ σc
General Physics 2
Current & Resistance
6
Current Flow
•
Current is the total rate at which charge crosses a
boundary
dQ
I
dt
•
•
Unit of current is the ampere
See
Table E5.1
General Physics 2
C
1 A 1
s
Current & Resistance
7
Microscopic View of Current
•
•
•
•
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
8
Electric Current
•
conventional current - direction a positive charge
would flow
General Physics 2
Current & Resistance
9
Surface Charges Direct Flow
•
Surface charges in a conductor move so that
current is constant
General Physics 2
Current & Resistance
10
Resistance
•
Resistance of a conducting object

R
I
•
•
Where Δϕ is the potential difference between the
ends
Unit of resistance is the ohm

General Physics 2
Current & Resistance
11
Ohm’s Law: Resistors
metal
•
For a metal conductor
nonohmic device
ohm’s
law
resistance
•
Ohm () = unit of resistance
General Physics 2
Current & Resistance
12
Power Dissipated in a Conductor
•
Power is the rate at which electrostatic potential
is converted to thermal energy
P  I 

PI R
R
2
2
•

Unit of power is the watt = J/s = N m/s = kg m3/s3

General Physics 2
Current & Resistance
13
Discharging a Capacitor
•
The potential difference VC
at any time t
VC t  Ae
t RC
•
Where R is the resistance and C
is the capacitance
General Physics 2
Current & Resistance
17
Discharging a Capacitor
VC t  Ae
t RC
VC t   9e x
e
x
1
 x
e
e 1  e
e 2  e
e 3  e

1RC
RC

2RC
RC

3RC
RC
 0.37
 0.14
 0.05
General Physics 2
Current & Resistance
18