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ELECTRICAL TECHNOLOGY
EET 103/4
Define and explain the meaning of
current, voltage, resistance,
power, conductor, and insulator
1
Electric charge (Q)
Atoms and their structure
•
Nucleus
– Protons
– Electrons
– Neutrons
The orbiting electron carries a
negative charge equal in magnitude
to the positive charge of the proton.
2
Electric charge (Q)
The atomic structure of any stable atom has an equal
number of electrons and protons.
3
Electric charge (Q)
4
Electric charge (Q)
Ions are imbalance atoms (+ve or –ve)
5
Electric charge (Q)
• Opposite charges attract; same charges
repel
• Unit for electric charge is coulomb (C).
1 coulomb (C) is equal to the total charge
of 6.242 x 1018 electrons.
6
Electric charge (Q)
• Coulomb is not normally used because it
is too big a unit.
• The common units are millicoulomb (mC)
and microcoulomb (C):
1 mC = 10-3 C
1 C = 10-6 C
7
Voltage (V)
• The flow of charge is established by an external
“pressure” derived from the energy that a mass has
by virtue of its position: Potential energy
• Energy: the capacity to do work
– If a mass (m) is raised to some height (h) above a
reference plane, it has a measure of potential
energy expressed in joules (J) that is determined
by
– W (potential energy) = mgh
where g is the gravitational acceleration (9.8 m/s2)
8
Voltage (V)
• The unit of measurement is volt (V)
• A potential difference of 1 volt (V) exists
between two points if 1 joule (J) of energy is
exchanged in moving 1 coulomb (C) of
charge between the two points
9
Voltage (V)
For large values of voltage, kilovolt (kV) and megavolt
(MV) are used:
1 kV = 103 V
1 MV = 106 V
For low values of voltage, millivolt (mV) and microvolt
(V) are used:
1 mV = 10-3 V
1 V = 10-6 V
10
Voltage (V)
W
V
Q
V  volt (V)
W  joules (J)
Q  coulomb (C)
11
Voltage (V)
• A potential difference or voltage is always
measured between two points in the system.
Changing either point may change the
potential difference between the two points
under investigation.
• Potential difference between two points is
determined by: V = W/Q (volts)
12
Voltage (V)
• Notations for sources of voltage and loss of
potential
– E - Voltage sources (volts)
– V - Voltage drops (volts)
• Potential – The voltage at a point with
respect to another point in the electrical
system. Typically the reference point is the
ground, which is at zero potential.
13
Voltage (V)
• Potential difference: The algebraic difference
in potential (or voltage) between two points of
a network.
• Voltage: When isolated, like potential, the
voltage at a point with respect to some
reference such as ground.
14
Voltage (V)
• Voltage difference: The algebraic difference
in voltage (or potential) between two points
of a system. A voltage drop or rise is as the
terminology would suggest.
• Electromotive force (emf): The force that
establishes the flow of charge (or current) in
a system due to the application of a
difference in potential.
15
Voltage Sources
DC voltage source
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Voltage Sources
DC voltage source
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Voltage Sources
DC voltage source
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Voltage Sources
DC generator
19
Voltage Sources
Symbol
OR
E
+
_
20
Current (I)
• The free electron is the charge carrier in a
copper wire or any other solid conductor of
electricity
• With no external forces applied, the net flow
of charge in a conductor in any one direction
is zero
21
Current (I)
Motion of negatively charged electrons in a
copper wire when placed across battery
terminals with a difference in potential of
volts (V).
22
Current (I)
• Basic electric circuit
23
Current (I)
• Unit for electric current is ampere (A).
• 1 ampere (A) = 1 coulomb (C) per second (s)
Q
I
t
24
Current (I)
1 kiloampere (kA) = 103 A
1 milliampere (mA) = 10-3 A
1 microampere (A) = 10-6 A
25
Current (I)
•
Safety considerations
– Even small levels of current through the human
body can cause serious, dangerous side effects
– Any current over 10 mA is considered dangerous
– currents of 50 mA can cause severe shock
– currents over 100 mA can be fatal
– Treat electricity with respect – not fear
26
Current Source
An ideal current source supplies a
constant current to any load irrespective
of the value of the load.
Symbol
I
27
Conductors and Insulators
• Conductors are those materials that permit
a generous flow of electrons with very little
external force (voltage) applied
In addition,
• Good conductors typically have only one
electron in the valance (most distant from
the nucleus) ring.
28
Conductors and Insulators
• Insulators are those materials that have
very few free electrons and require a large
applied potential (voltage) to establish a
measurable current level
• Insulators are commonly used as covering
for current-carrying wire, which, if
uninsulated, could cause dangerous side
effects
29
Conductors and Insulators
• Rubber gloves and rubber mats are used
to help insulated workers when working
on power lines
• Even the best insulator will break down
if a sufficiently large potential is applied
across it
30
Conductors and Insulators
31
Conductors and Insulators
• Table 2.1 shows the relative conductivity of
various materials
32
Conductors and Insulators
• Table 2.2 shows breakdown strength of some
common insulators
33