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Material Type: Notes; Class: General Physics: Electricity and Magnetism; Subject: Physics; University: Penn State - Main Campus; Term: Unknown 1989;
Typology: Study notes
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rate of positive charge passing through a rate of positive charge passing through a
surface surface
I=dQ/dt
dQ/dt
units units Coulomb/Second = Ampere
Coulomb/Second = Ampere (C/s = A)
(C/s = A)
Current Flow of Charge
Atomic Model for current Atomic Model for current
x
A
ne
Q
Δ
=
Δ
n = number of charges/ n = number of charges/vol
vol
e = magnitude of charge e = magnitude of charge
A
t
x
ne
t
Q
Δ
Δ
=
Δ
Δ
take limit as take limit as
t t goes to 0
goes to 0
A
d
t
dx
ne
d
t
dQ
=
A
nev
I
d
=
d
d
v
ne
J
r
r
=
Ohm Ohm
’ ’
s Law s Law
E
J
r
r
∝
Conductivity and Conductivity and resistivity
resistivity
E
J
r
r
σ
=
Ohm Ohm’
’s Law vector form
s Law vector form
σ
ρ
Resistance and Resistance and resistivity
resistivity
L A E L A J
)
(
1
r
r
r
r
⋅
=
⋅
ρ
L
V
A
I
=
A
L
I
V
=
IR
V
=
Resistance Resistance
A
R
l
ρ
=
Units:
Units: Ohm = Volt / Ampere
Ohm = Volt / Ampere (
(
Ω Ω
= V / A
= V / A)
)
Sample problem 26 Sample problem 26
4 4
Find resistance ofrectangular block ofiron……
Temperature Dependence of Resistance Temperature Dependence of Resistance
Define temp coefficient of Define temp coefficient of resistivity
resistivity
dT
d
ρ
ρ
α
1
=
If If
α α
is small and constant is small and constant
(
)
[
]
o
o
T
T
−
=
α
ρ
ρ
1
Units of Units of
α α
is is (
o o
1
Low temperature behavior of Low temperature behavior of
resistance
resistance
Electric Power Electric Power
dt
d
P
W
=
(
)
dt
qV
d
P
=
dt
dq
V
P
=
VI
P
=
Units: Units: watt = amp
watt = amp
volt volt
Substituting Substituting
Ohm Ohm’
’s Law
s Law
yields: yields:
R
I
P
2
=
R
V
P
2
=
