Transcript U3e L2 Electrical Technology 2

February 28, 2011
DRILL
Moving electrons have _______.
Energy
Load of an electric circuit uses
The _______
the electrons’ ability to do work.
• The three basic units in electricity are
Voltage
(V), Current (I), Resistance (R)
_________________________________.
•
•
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TERMINOLOGY
• Voltage (V) – the electric potential between two
points of opposite charge.
• Voltage is the potential energy of the electrical
power source.
• Voltage is measured in Volts (V).
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TERMINOLOGY
• Current (I) is the rate that charges flow
during a given time
– Current = Charge/Time
– Current is measured in ampere (amps).
• Amp = Coulomb/second
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TERMINOLOGY
Power source: An artifact that provides a supply of “free
electrons to a circuit. A power source has
a positive (+) and a negative (-) terminal.
The Power Source is the origin of the Voltage in a circuit.
Two types of Power Source:
1.
Direct Current (DC) Power Source.
Schematic symbol:
Realistic picture
2.
Alternating Current (AC) Power Source.
Schematic symbol:
Realistic picture
Power Source Symbols
DC Power Source:
+ (Positive Terminal - Long line)
+
-
-
(Negative Terminal – Short line)
This is where the symbol
comes from
“Wet cell”- car
battery
TERMINOLOGY
• Resistance (R) –a measure of the degree to which
an object opposes an electric current through it .
• Resistance often generates heat.
• Resistance is measured in Ohms (W – the Greek
letter Omega)
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Ohm’s Law
• George Ohm (1789 – 1854) – a high school and
university mathematics instructor
• Discovered ~1827
• Voltage = Current x Resistance
The formula:
V
=
I
x
R
What are the units of measure of this formula?
Voltage
Current
Resistance
Volts(v) = Amperes(amps.) x Ohms(Ω)
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Electrical Calculations – What is Ohm’s Law?
I=
3V
2Ω
I = 1.5 amps
Schematics
A schematic is a diagram that
represents the elements of a
system using symbols instead of
realistic pictures.
Remember: A complete circuit consists of
power source, conductor, and load.
Schematic vs. Realistic
Diagrams
Load
Power source
+
_
Power
source
Load
Conductor
Conductor
Which one is easier to draw?
SCHEMATIC DIAGRAMS
a.k.a. Schematics
CONDUCTOR
(D.C.)
POWER
SOURCE
(+)
(-)
LOAD
CURRENT
(arrow shows the direction of electron flow)
PROBLEM SOLVING
1.
2.
3.
4.
5.
6.
Draw the diagram.
List the table of knowns and unknowns.
Write the formula.
Substitute.
Calculate.
Box in answer.
An Ipod uses a 0.5 volt battery. The current in the ear bud
circuit is 0.2 amperes. What is the resistance of the ear bud?
Step #1 Diagram
Step #2 List Information
V = 0.5 V
I = 0.2amps.
V = 0.5v
R = ?
R = ?
I = 0.2amps.
Step #3 Formula
Step #4 Substitute
Step #5 Solve
V = I x R
0.5 V = 0.2 A x R
2.5 Ω = R
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What voltage is needed to produce a 4 Amp circuit with a 5
Ohm resistor?
Step #1 Diagram
Step #2 List Information
V = ?
I = 4 Amp
R = 5 Ohm
V = ?
R = 5 Ohm
I = 4 amp
Step #3 Formula
Step #4 Substitute
Step #5 Solve
V = I x R
V = 4 A x 5 W
V = 20 Volts
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Solve the following circuit problems:
(USE ALL 6 STEPS!)
1.
V=6v
I = 2 amps.
R=?
2.
V = 8.5 v
I=?
R = 2.125 Ω
3.
V=?
I = 9 amps.
R = 10 Ω
4.
The heater fan on the #33 bus is on a 7 amp. circuit.
The bus has a 24 v battery. What is the resistance
of the fan motor?
Problems – Ohm’s Law
1) Write the given information:
V = 5 volts
I = .7 amps
R=?
2) Sketch a schematic diagram of the circuit:
a)
b)
c)
d)
Start with power source
Add conductors
R=?
Add resistance
Add current
+
V = 5V
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I = .7A
Problems – Ohm’s Law
Copy the steps to solve this problem on your
worksheet:
1. A cell phone charger converts the wall outlet
voltage to 5 Volts to charge the cell phone. The
amount of current that the charger outputs is .7
Amperes. Sketch a schematic diagram of this
circuit. What is the resistance of the cell phone
battery?
schematic
a diagram that
1)AWrite
the given is
information:
represents the
of a system
V = 5elements
volts
without usingI =realistic
.7 amps pictures
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R=?
Problems – Ohm’s Law
1) Write the given
information:
V = 5 volts
I = .7 amps
R=?
2) Sketch a schematic
diagram of the
circuit:
+
R=?
-
V = 5V
I = .7A
3) Rearrange, Substitute, and Solve
V=IxR
R=V/I
R = 5V / .7A
R = 7.14 W
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Electrical Power
• Electric Power – the rate at which electrical energy
is transferred by an electric circuit.
• Units of power are the Watt (W)
• A human climbing a flight of stairs is doing work at
a rate of about 200 watts.
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Joule’s Law
• James Joule (1818 – 1889) – showed that heat
and electrical energy are interchangeable.
• Joule’s Law:
Power = Voltage x Current
P=VI
Watts = Volts x Amperes
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Problems – Joule’s Law
Let’s look at problem #4, which is very similar to
problem #1:
4. A cell phone charger converts the wall outlet
voltage to 5 Volts to charge the cell phone. The
amount of current that the charger outputs is .7
Amperes. Sketch a schematic diagram of this
circuit. What is the power used by the cell phone
in recharging its battery?
schematic
a diagram that
1)AWrite
the given is
information:
represents the
of a system
V = 5elements
volts
without usingI =realistic
.7 amps pictures
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P=?
Problems – Joule’s Law
1) Write the given information:
V = 5 volts
I = .7 amps
P=?
2) Sketch a schematic diagram of the circuit:
a)
b)
c)
d)
Start with power source
Add conductors
P=?
Add resistance
Add current
+
V = 5V
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I = .7A
Problems – Joule’s Law
1) Write the given
information:
V = 5 volts
I = .7 amps
P=?
2) Sketch a schematic
diagram of the
circuit:
+
P=?
-
V = 5V
I = .7A
3) Write Formula, Substitute, and Solve
P=VI
P = 5V x .7A
P = 3.5 W
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Problems – Joule’s Law
Complete problem #5 on your worksheet
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Problems – Joule’s Law
5. A Honda Insight (hybrid) has a 144 Volt battery
system in its trunk. The electric motor that the
batteries power can generate 20,000 Watts of
power. Sketch a schematic diagram of this circuit.
What is the current traveling through this high
voltage circuit?
1) Write the given information:
144
volts
A schematicVis= a
diagram
that
P = 20,000 watts
represents the
elements of a system
I=?
without using realistic pictures
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Problems – Joule’s Law
1) Write the given information:
V = 144 Volts
P = 20,000 Watts
I=?
2) Sketch a schematic diagram of the circuit:
Start with power source
Add conductors
P = 20,000 W
Add load
Add current
I=?
+
V = 144V
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Problems – Joule’s Law
1) Write the given
information:
V = 144 Volts
P = 20,000 Watts
I=?
2) Sketch a schematic
diagram of the
circuit:
+
R=?
-
V = 5V
I = .7A
3) Rearrange, Substitute, and Solve
P=VxI
I=P/V
I = 20,000W / 144V
R = 138.9 A
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Complete the worksheet on
electrical problems using Joule’s
and Ohm’s Laws
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