Download U3e L3 Electrical Technology 3

February 25, 2010
DRILL
1. Why are the + and – symbols
unnecessary when drawing the power
source in an electric schematic?
Short circuit occurs when the positive
2. A ______
terminal is connected directly to the
negative terminal by a conductor, with no
resistance.
1. The length of the lines tell you
which is positive and which is negativeU3e-L3
Electric Schematics
1. What is a schematic?
A schematic is a diagram that
represents the elements of a system
without using realistic pictures
2. Which of the following is a schematic?
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Problems – Joule’s Law
Complete problem #5 on your worksheet
U3e-L3
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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Problem #5 – 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
U3e-L3
Problem #5 – 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=?
5V
.7A
3) Rearrange, Substitute, and Solve
P=VxI
I=P/V
I = 20,000W / 144V
R = 138.9 A
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Problems – Joule’s Law
Homework Problems #2 and 3 Review
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Homework Problem #2
1) Write the given
information:
V = 9 Volts
I = .02 Amps
R=?
2) Sketch a schematic
diagram of the
circuit:
R=?
9V
.02A
3) Rearrange, Substitute, and Solve
V=IxR
R=V/I
R = 9V / .02A
R = 450 W
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Homework Problem #3
1) Write the given
information:
V = 120 Volts
I = 5 Amps
R=?
2) Sketch a schematic
diagram of the
circuit:
R=?
120V
5A
3) Rearrange, Substitute, and Solve
V=IxR
R=V/I
This is how alternating current (AC)
power sourcesRare
represented
= 120V
/ 5A
schematically R = 20 W
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Direct Current
• The unidirectional flow of electrons.
one
• Direct current is current travelling in 1 direction –
from the negative terminal to the positive terminal.
• Direct current is used in almost all electronic
systems as the power supply.
• Batteries are a sign of direct current.
I
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Direct Current
• Measuring the voltage of a direct current power
supply:
1.5 V
• The voltage is constant.
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Direct Current
• If voltage is constantly 1.5 V, what does its
graph over time look like?
2
1
Voltage (volts)
3
1.5 V
0
2
4
6 8 10 12 14
Time (seconds)
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Alternating Current
• Electrons flow back and forth through an electric
circuit.
• Most artifacts you plug in.
R=?
120V
+- 120 V
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Alternating Current
0
120
+/-120 V
-120
Voltage (volts)
• If voltage is alternating 120 V to -120 V, what
does its graph over time look like?
0
2
4 6 8 10 12 14
Time (seconds)
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Resistance Calculations
• Resistors in series are added:
R1
R2
R3
Total Resistance:
R T = R1 + R2 + R3
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Resistance Calculations
• Resistors in parallel: add their reciprocals
R1
R2
R3
Total Resistance:
1
1 1
1
=
R T R1 + R2 + R3
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Resistance Calculations
• Calculate the resistance in the following circuit:
100 W
100 W
100 W
Total Resistance:
R T = R1 + R2 + R3
R = 100 W + 100 W + 100 W
T
R = 300 W
T
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Resistance Calculations
• Calculate the resistance in the following circuit:
100 W
100 W
100 W
Total Resistance:
1
1 1
1
=
R T R1 + R2 + R3
U3e-L3
Resistance Calculations
1
1 1
1
1
1
1
=
=
+
+
R T R1 + R2 + R3 100
100
100
1 =
.01 + .01 + .01
RT
1
(Multiply both sides by Rt)
=
.03
R
T
1 = .03 x Rt
.03
.03
Rt = 33.3 W
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1. Complete the worksheet on
electrical problems using Joule’s
and Ohm’s Laws.
2. Solve for V, I, R, and P for each
of the six problems.
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