Basic EE Definitions, Units,
Conventions, and Circuit
Elements
Volts, Current, Energy, Power, Sign
Conventions, and Sources
Kevin D. Donohue, University of Kentucky
1
Unit Prefixes
10-12
pico (p)
¾ nano (n) 10-9
¾ micro (µ) 10-6
¾ milli (m) 10-3
¾
kilo (k) 103
¾ mega (M) 106
¾ giga (G) 109
¾ tera (T) 1012
¾
Kevin D. Donohue, University of Kentucky
2
Basic Definitions
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Electric Circuit - an interconnection of electrical
components through which charge can be moved.
Current - the rate of change of charge with respect
to time (coulombs per second).
1 Ampere = 1 coulomb per second.
1 Volt = 1 joule per coulomb = 1 newton-meter
per coulomb.
1 Watt = 1 Ampere-Volt = 1 joule per second
Kevin D. Donohue, University of Kentucky
3
Flow Conventions
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¾
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Positive current denotes
the direction of positive
charge.
Describe the direction of
charge in both circuits.
Indicate which elements
are supplying and which
are absorbing energy.
I1=2
A
B
I2=-1
A
Kevin D. Donohue, University of Kentucky
B
4
Math Notation and Relationships
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¾
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Current, charge, and time
dq (t )
i (t ) =
dt
t
q (t ) = ∫ i ( x)dx
−∞
dw
dq
Voltage, energy, and charge
v=
Voltage, current, and power
vi =
dw  dq  dw
=p
 =
dq  dt  dt
t2
t2
t1
t1
w2,1 = ∫ pdt = ∫ vidt
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Power supplied equals power absorbed for all
elements in a circuit.
Kevin D. Donohue, University of Kentucky
5
Passive Sign Convention
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It is assumed that positive
charge entering the positive
terminal of an element implies
power absorbed by the element.
Therefore, charge leaving the
positive terminal of an element
implies power supplied or
delivered by the element.
If the words absorbed or
supplied are not given with a
power value, power absorbed
will be assumed. (A negative
sign will imply power supplied).
Kevin D. Donohue, University of Kentucky
I
V
I
V
6
Example Problems
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A 10 volt battery is connected to a 20 watt
light bulb for 30 minutes. How much
charge is lost from the battery?
¾
5 coulombs of charge pass from point A to
point B at a constant rate. If 100 joules is
absorbed, what is the voltage across AB?
Kevin D. Donohue, University of Kentucky
7
Independent Sources
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Independent sources can deliver power in a
circuit. An ideal source will have either a
fixed voltage OR current independent of
other elements in the circuit.
i(t)
+
v(t)
_
+
V
_
Kevin D. Donohue, University of Kentucky
8
Power Delivered/Absorbed Examples
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Find the power absorbed in each element
assuming the passive sign convention
a
I
c
I
+
V
V
+
b
V
I
d
V
I
a1) I = 2 A, V = 3 V
a2) I= -1 A, V = 4 V
b1) I = 6 A, V = -2 V
b2) I = -2A, V = -9 V
c1) I = 7 A, V = 1 V
c2) I = -3A, V = -2V
d1) I = 1 A, V = -1 V
d2) I = -2 A, V= 6 V
Kevin D. Donohue, University of Kentucky
9
Power Circuits
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In a circuit power delivered must equal power absorbed
over all elements.
Find power delivered and absorbed in each element.
- 6V +
1
0.5 A
1A
+
2 18 V
-
1
+
-
5V
Kevin D. Donohue, University of Kentucky
+
2
1V
-
10
Dependent Sources
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Dependent sources can deliver power in a
circuit. The voltage or current of a dependent
source depends on the current or voltage in
other circuit elements.
CCCS
CCVS
F(i1)
+ H ( i1 ) -
VCCS
VCVS
G ( v1 )
Kevin D. Donohue, University of Kentucky
+ E ( v1 ) 11
Circuit Examples
¾
Find power absorbed/supplied by each
element in the circuit.
+
11 V
-
+ 6V a
0.5 A
i1
b
+ 1V c
+
5 V 4i1
-
Kevin D. Donohue, University of Kentucky
-0.5 A
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