Download Electricity_basics

Electricity
Basics
1
Electron
• The smallest particle caring electrical charge.
video
• Electrical charge is measured in Coulombs
1Coulomb  6.24  10 electrons ch arg es
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2
Electrical current
• Electrical current is flow of electrical charges
(by water analogy)
• Electrical current is measured in amperes
1Ampere  1Coulomb per sec ond
one ampere is an electrical current that one
Coulomb of electric charge is transferred in
one second
• Electrical current is denoted with letter “I”
3
Materials and electricity
• Conductive materials (conductors):
Metals
Non metals – Carbon, electrolytes
etc.
• Non conductive materials (insulators):
Plastics
Glass
Ceramic
etc.
4
A circuit
• Circuit is a continuous electrical path
• Closed circuit – circuit is unbroken and
electricity flows
• Open circuit – circuit is interrupted,
preventing electricity to flow
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The task
Our main task is to control the electrical flow for
useful purposes.
• To predict the flow of electrons through electrical
circuits so to organize consumers and sources
of electricity in a properly working circuits
• To prevent electrical system from failure
• To predict size of the sources of electricity
• To prevent people from accidents
• Etc.
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Useful purposes on boats (consumers)
Continuous
Long duration
Sort duration
•Standby power
taken by VHF
or the SSB
•Refrigerator
•Freezer
•Instruments:
boat speed
wind speed
fuel available
tachometer
•Navigation lights
•Depth finder
•Marine charging
•Autopilot
•Navigation: Radar,
GPS
•Cabin lights
•Water maker
•Air conditioning
•Entertainment
•Engine starter
•Pumps
•Trim and tilt
system
•Electric winches
and capstan
• Bow thrusters
•Microwave
•Washing machine
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Factors controls current (I)
in a circuit are:
• The electrical driving forces, or voltage (V)
(current increases when voltage increases)
• The resistance to flow of the circuit
materials (R)
(current decreases when resistance
increases)
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Ohm’s Law
V
I
R
where
I – Current measured in amperes [A]
V – Voltage measured in volts [V]
R – Resistance measured in ohms [W]
(Works only when electric current flaws)
9
Schematic of a circuit
V=12v
R=2W
V 12
I 
 6
R 2
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Ohm’s Law
V
I
R
• Ohm’s Law can be rearranged:
V  I R
V
R
I
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Loads in series
• Resistive loads in series
act like one continuous
load of a total resistance
equal to the sum on the
individual resistance
RR R R
1
2
3
I
R
1
 etc.
V
V
I
R1  R2  R3  etc.
R
V1
2
R
3
Example:
V=12V
R1=2W
R2=5W
R3=3W
I=?
12
Answer
12V
12
I

 1.2 A
2W  5W  3W 10
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Loads in parallel
I
I1
V
R
1
V
I1 
R1
Total current is:
I3
I2
R2
R
3
The same voltage acts
across each of the
loads. The currents
through the loads are:
V
I2 
R2
V
I3 
R3
I  I1  I 2  I 3
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Continue from the previous slide
 1
V
V
V
1
1 
I


 V   
 
R1 R2 R3
 R1 R2 R3 
Remember Ohm’ low?
V
1
I  V 
R
R
• In other words:
1
1
1
1



R R1 R2 R3
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Example:
V=12 V
R1=2W
R2=5W
R3=3W
R=?
I=?
1 1 1 1
    1.033
R 2 5 3
I
I1
V
R
1
I3
I2
R2
R
3
1
R
 0.97W
1.033
V
12
I 
 12.4 A
R 0.97
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Loads in series/parallel
V=12 V; R1=2W
R2=5W
R3=3W
Solution:
R=?
I=?
I1,I2,I3=?
1
1
1 1 1


   0.53
R23 R2 R3 5 3
R2  3 
1
 1.875W
0.53
I1
V1
R
V
1
I3
I2
R  R1  R23  2  1.875  3.875W
V
12
I 
 3.1A
R 3.875
I
R2
R
3
I1  I  3.1A
V1  I1  R1  3.1 2  6.2V
V23  I 23  R23  3.1 1.875  5.8V
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Monitoring electrical systems
• THE MULTIMETER (VOM)
The multimeter may also known as
the Volt-Ohm-Milliammeter or
VOM, or the DMM (Digital MultiMeter). Because other functions
may be included; thus the name
multi-function meter or more
simply; multimeter
The multimeter is an instrument used
to measure:
Voltage in volts, Current in amperes,
Resistance in ohms
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ANALOG MULTIMETERS
The analogue multimeter is easily
recognised by:
1. A needle-like pointer
2. The semicircular scale
both of which appear on the front
panel.
The pointer travels the scale and
indicates the measurement on the
scale directly beneath it. The scales
are related to the settings of the
function and range switches.
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DIGITAL MULTIMETERS
The digital multimeter is easily
identified by:
1. Numerical readout panel
2. Absence of a scale
The measurement appears as an
ordinary number within the
panel so that anyone who can
read numbers can read the
indicated value.
Reading errors (long associated
with analogue meters) are
mostly eliminated by digital
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multimeters.
OPERATING CONTROLS
The multimeter has two main operating
controls:
1. The function switch
2. The range switch....
These are sometimes combined into one
dual-purpose switch
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Measuring current
• The multimeter is connected to the circuit
in series
• The function switch should be on correct
function (amps)
• The range has to be chosen on expected
range of the value
22
Measuring resistance
• A resistor or other load measured must be
isolated from the circuit
• The multimeter is connected to both ends
of the item
• The function switch should be on correct
function (Ohms)
• The range has to be chosen on expected
range of the value
23
Measuring voltage
• The multimeter is connected to the circuit
in paralel
• The function switch should be on correct
function (voltage)
• The range has to be chosen on expected
range of the value
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Sources in series
I
V  V1  V2  V3
V1
Therefore:
I
V1  V2  V3 
V2
R
R
V3
Example:
V1=1.5V
V2=1.5V
V3=3.0V
V
R=4W
I=?

1.5  1.5  3.0
I
 1.5 A
4
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Sources in series
• Caution!
Batteries work in series must have the
same capacity and the same state of
charge. Otherwise the battery with the
greatest capacity may drive the voltage of
the weakest battery negative, which will
destroy the weaker battery
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Sources in parallel
I
V1
V2
V
R
• Provided batteries are the same voltage. The
net result is a single battery of capacity equal to
the sum of the individual capacities.
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Sources in parallel
• If they are not of the same voltage, the
higher voltage battery will discharge into
the lower voltage battery, possibly
overcharging and destroying it
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Energy
• Energy is the ability to do work
Energy can be neither created nor
destroyed.
Energy can be only transformed form one
form to another.
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Power
• Power is the rate of doing work
• Work is done when a force applied
on a body causes it to move.
In electrical circuit the power can be
easily calculated:
W V I
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Electrical measurements and units:
• Voltage is measured
in Volts (V)
W
V
I
• Current is measured
in Amps (A)
W
I
V
• Power is measured in
Watts (W)
W V I
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Example:
• What is the power of a light bulb that draws 1.25
amps at 12 Volts?
W  V  I  12 1.25  15 _ Watts
• How many amps would a 20Watt lamp draw in a
12 Volt circuit?
W 20
I

 1.67 Amper
V 12
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Resolve the problem:
• What is the resistance of a light bulb with a
label:
12V 15W
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