Download Electric Current

Electric Current
Current
• Rate at which charge flows through a
surface.
dQ
I
dt
Units - Ampere (A)
• Ex. If the charge on a capacitor changes by
the function Q(t)=6(1-e-0.1t), what is the
charge on the capacitor and current leaving
the capacitor at t = 5s?
Drift Velocity
• Factors that affect current flow
Q
I
 nqvd A
t
n – carriers/volume
q – charge/carrier
A·vd = AL/t– volume/time
• n depends on density of material and number of
free electrons/atom.
• In a typical conductor drift velocity is ~0.25 mm/s.
How does electrical devices respond so quickly?
Ohm’s Law
• Current Density is the amount of current per
unit area.
I
A
ee-
J
A
 nqvd
e-
e-
e-
e-
• Ohm’s Law - The ratio of current density
and electric field are constant.


J  E
 - conductivity
Battery
12 V
I
Resistor
250 W
Resistance
• The common form of Ohm’s law is
V  I R
• Resistance must then be related to length,
area, and conductivity.
l
l
R

Units - Ohm’s (W)
A
A
Temperature Dependence
1

 - resistivity   0 (1   (T - T0 ))

T0 = 20 °C
Resistors
• Ex. What is the resistance of a cylindrical rod of
carbon that is 2.0 cm long and has a radius of 0.50
mm? If 25 volts is applied across this resistor,
how much current will flow?
 carbon  3.5 10 -5 W  m
What is the resistance at 40 °C?
 carbon  -0.5 10 C
-3
-1
Color Code
• Resistors have four color bands setting its value.
Color Value
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Gray
8
White
9
1st 2nd × 103rd ± tolerance
Tolerance Value
Gold
5%
Silver
10%
None
20%
Ex. What is the value of the resistor?
Power
• Power is energy used per time.
U qV
P 
 I V
t
t
2
V
P  I R 
R
2
• How much power is used by a 0.89 W
resistor when 25 volts is applied over it?
Defibrillator
• From the diagram can the current, voltage,
resistance and power be calculated?
Semiconductors
• Silicon and Germanium have 4 valence electrons and form
4 covalent bonds with its neighbors giving a neutral crystal
structure.
• Doping – Using atoms with a valence of 3 or 5 give a
deficit or surplus of electrons in the crystal structure.
P-type (missing e-)
Un-doped
N-type (extra e-)
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
B
Si
Si
Si
Si
Si
P
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
 <0.1 W m
 =23 W m
 <0.1 W m
PN Junctions
• Near the junction excess e- from N-type diffuse to places
of missing e- in P-type semiconductors.
E field
Diode
P-type
N-type
P-type
Depletion
Zone
N-type
P-type
P-type
Depletion
Zone
N-type
N-type