Transcript Basic Concepts - Oakland University

Basic Concepts
Discussion D1.2
Chapter 1
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Basic Concepts
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System of Units
Charge
Current and Voltage
Power and Energy
Ideal Circuit Elements
2
International System of Units
(SI)
Quantity
Basic Unit
Length
meter
Mass
kilogram
Time
second
Electric current
ampere
Thermodynamic temperature
kelvin
Luminous intensity
candels
Symbol
m
kg
s
A
K
cd
3
SI Prefixes
Multiplier
1012
109
106
103
10-1
10-3
10-6
10-9
10-12
Pefix
terra
giga
mega
kilo
deci
mili
micro
nano
pico
Symbol
T
G
M
K
d
m
m
n
p
4
Examples
Express 1 ms/1000 in SI
units.
1 X 10-3 / 1 X 10 3 = 1 X 10-6 = 1 m s
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Example 2
Not too long ago, computer disk space
was measured in Mbytes. Modern
disks have about 10,000 times more
storage capacity. In SI units, what is
the capacity unit of modern disks?
1 X 106 X 1 X 104 = 10 X 109
= tens of Gbytes
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e12
Charge, q
q2
r12
Recall Coulomb’s Law
q1
Force F1 on charge q2 due to charge q1 is given by
q1q2
F1 
e12
2
4 0 r12
1
1
4 0
 107 c 2  8.99 109
Charge on an electron
(proton) is negative (positive)
and equal to 1.602 x 10-19 C
e12  unit vector pointing from q1 to q2
r12  distance between charge q1and q2
Unit: Newton meter2 / coulomb2
volt meter / coulomb
Note: Positive force is repulsive, negative force is attractive
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Electric Current, i
Current (in amperes) (A) is the time rate of change of charge q
dq
i
dt
1 A = 1 C/s
t
q(t )   idt  q(t0 )
t0
Charge flowing past a point in the interval [t0, t] is qT (t ) 

t
t0
idt
Convention: Direction of current flow is that of positive charges,
opposite to the direction of electron flow
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Voltage
The energy in joules (w) required to move a
charge (q) of one coulomb through an element is
1 volt (V).
dw
v
dq
1 volt = 1 joule/coulomb = 1 newton meter/coulomb
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Power and Energy
Power (p), in watts (W), is the time rate of expending or
absorbing energy (w) in joules
dw
p
dt
dw  dw   dq 
p
      vi
dt  dq   dt 
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Power and Energy
Change in energy from time t1 to time t2
t2
t2
t1
t1
w   pdt  vidt
Passive sign convention:
i (t )
+
p  vi
If p > 0 power is absorbed
by the element
v(t )
-
If p < 0 power is supplied
by the element
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Circuit Elements
+
vs  t 
a
vab
+
-
vab  vs (t )
i?
i
-b
Independent Voltage Sources
12
Circuit Elements
+
a
vab
is  t 
i
vab  ?
i  is (t )
-b
Independent Current Source
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Circuit Elements
i
+
vi
-
+
+
-
i
a
ii
v  m vi vab  m vi
i?
-b
a
+
+
-
v  rii vab  rii
i?
-b
Dependent Voltage Sources
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Circuit Elements
i
+
vi
-
a
ii
i
+
i  gvi
vab
a
+
i   ii
vab
-
-
b
b
i  gvi
vab  ?
i   ii
vab  ?
Dependent Current Sources
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Passive Circuit Elements
+
+
i
v
R
-
v  iR
1
i v
R
v
i
+
i
+
L
-
di
vL
dt
1
i   vdt
L
C
v
-
-
1
v   idt
C
dv
iC
dt
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