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International System of Units (SI)
Table 1. SI base units
SI base unit
Name
Symbol
meter
m
Base quantity
Length
Mass
Time
Electric current
Thermodynamic
temperature
Amount of
substance
Luminous
intensity
kilogram
second
ampere
kg
s
A
kelvin
K
mole
mol
candela
cd
Table 5. SI prefixes
Factor Name
1024 yotta
1021 zetta
1018 exa
1015 peta
1012 tera
109
giga
6
10
mega
103
kilo
2
10
hecto
1
10
deka
Symbol
Y
Z
E
P
T
G
M
k
h
da
Factor Name
10-1
deci
-2
10
centi
10-3
milli
-6
10
micro
-9
10
nano
-12
10
pico
-15
10
femto
10-18 atto
10-21 zepto
10-24 yocto
Symbol
d
c
m
µ
n
p
f
a
z
y
Force, Work, and Power
□ Work is done by a force on an object if the force acts on the object in it moves
through a distance parallel to the force.
□ Work = Force times Distance moved
□ W=Fd
□ The unit for work is the Joule (J) which is equivalent to Newton-meter
□ 1 J = 1N m = 1 Kg m2 / s2
□ Energy
□ Things have energy if they are able to do work. A human body has energy; so
does a tank of gas and a falling stone.
□ Energy is the capacity to do work.
□ Energy exists in a variety of forms:
□ Chemical Energy, Potential Energy, Nuclear Energy, Thermal (Heat)
Energy etc…
□ Work
□ Work is done whenever energy is changed from one form into another.
□ The amount of energy changed from one form to another is known as the
energy transferred:
□ work done = energy transferred
□ Power
□ Power is equal to the amount of work done per unit time.
□ The unit for power in Standard Units is the Watt (W) which is equivalent
to Joule/second
□ 1 W = 1J / s
□ Another unit for power is horsepower (hp)
□ 1 hp = 746 W
Derived SI Units
□ The derived units follow the mathematical expressions which relate the
quantities.
□ From “force equals mass times acceleration,” the newton (N) is
defined as the unbalanced force that imparts an acceleration of 1
meter per second squared to a 1-kilogram mass. Thus,
□
Quantity
Symbol
SI Unit
Abbreviation
electric charge
Q, q
coulomb
C
electric potential
V, v
volt
V
resistance
R
ohm

conductance
G
siemens
S
inductance
L
henry
H
capacitance
C
farad
F
frequency
f
hertz
Hz
force
F,f
newton
N
energy, work
W, w
joule
J
power
P, p
watt
W

weber
Wb
B
tesla
T
magnetic flux
magnetic flux density
Constant Acceleration Motion:
Let us consider the case where a particle accelerates from rest at a constant
acceleration a:
The initial velocity is
v0  V0
The final velocity is
v f  V0  at
The average velocity is
v  (v0  v f ) / 2  V0  at / 2
Thus the displacement is
d  vt  V0t  at 2 / 2
Kinetic Energy
KE  mv 2 / 2
EXAMPLE
In simple rectilinear motion a 10-kg mass is given a constant acceleration of
2m / s 2 (a) Find the acting force F. (b) If the body was at rest at t  0, x  0 find the
position, kinetic energy, and power for t = 4s.
F  ma  10kg  2m/s 2  20kg  m/s 2  20 N
(b) At t  4s,
x  at 2  2m/s 2  (4s) 2  16m
v f  V0  at  0  2m/s 2  4s  8m/s
KE  mv 2 / 2  10kg  (8m/s) 2  (320kg  m/s 2 )  m  320J
It can be shown:
P  Fv f  20N  8m/s  160W
Resistive Circuits
□ This are circuits in which there are not energy storing components other than the
voltage or current sources they do not contain capacitors or inductors.
□ The definitions of Work, Energy and Power do not change but the nature of the
Involved forces does.
□ An Electric Circuit is a model that approximates the behavior of an actual Electric
System
□ Electric Effects Happen Instantaneously Throughout a System
□ The net charge on every component of the system is always zero
□ There is no magnetic coupling between any two different components in a
system
□ Circuit theory provides simple solutions to problems that would be otherwise
extremely complicated
Electric Charge
The unit of electric charge is the coulomb. Ordinary matter is made up of atoms which
have positively charged nuclei and negatively charged electrons surrounding them.
Charge is quantized as a multiple of the electron or proton charge:
m  9.11  10 31 kg
# e in 1C 6.24E+18
Mass of 1C 5.69E-12
□ Like charges repel, unlike charges attract. The electric force acting on a point
charge q1 as a result of the presence of a second point charge q2 is given by
Coulomb's Law:
 0 is the permitivit y of space
r  1m
q1  q 2  1C
F  9  10 9 N  9  10 9 / 9.8  9.18 kgf  1000 tons
□ The electric field of a point charge can be obtained from Coulomb's law:
Gravitational Force
F G
Mm
Mm
; U  G
G=
2
r
r
□ Potential energy can be defined as the capacity for doing work which arises from
position or configuration.
PE  mgh
□ If a positive charge Q is fixed at some point in space, any other positive charge
which is brought close to it will experience a repulsive force and will therefore
have potential energy. The potential energy of a test charge q in the vicinity of
this source charge will be:
□ Voltage is electric potential energy per unit charge, measured in joules per
coulomb ( = volts). It is often referred to as "electric potential", which then must
be distinguished from electric potential energy by noting that the "potential" is a
"per-unit-charge" quantity.
□
□
The unit of current, the ampere (A), is defined as the constant current in
two parallel conductors of infinite length and negligible cross section, 1
meter apart in vacuum, which produces a force between the conductors of
2.0 x10 7 newtons per meter length.
A more useful concept, however, is that current results from charges in
motion, and 1 ampere is equivalent to 1 coulomb of charge moving across
a fixed surface in 1 second.
w
q
w q w
;i
; v i 


; hence,
q
t
q t
t
p  v i
v
The Ideal Basic Circuit Element
□ Has Only two terminal
□ It is described mathematically in terms of current and/or voltage
□ It cannot be subdivided into other elements
Passive Sign Convention
Whenever the reference direction for the current in an element is in the direction of the
reference voltage drop across the element, use a positive sign in any expression relating
voltage and current
Notation
□ Time varying quantities - lower case
e.g. v(t), i(t)
□ sometimes assume time - v(t) = v
□ Time invariant quantities - upper case
e.g. V, R,
□ Remember to include units of measure
e.g. 15 Volts
Voltage & Current Sources
□ An ideal voltage source maintains its stated voltage regardless of the load attached
□ An ideal current source supplies its stated current regardless of the load attached
□ Short Circuit.
□ Basic Circuit element whose voltage is always 0. (Resistance =0)
□ Symbol
□ Open Circuit
□ Basic Circuit element whose current is always 0. (Resistance = infinity)
□ Symbol