u3e-l2 moving electrons have _______. the _______ of an electric circuit uses the electrons’...
TRANSCRIPT
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U3e-L2
• Moving electrons have _______.• The _______ of an electric circuit uses
the electrons’ ability to do work.• The three basic units in electricity are
_________________________________.
February 28, 2011
DRILL
Voltage (V), Current (I), Resistance (R)
EnergyLoad
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• 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).
TERMINOLOGY
U3e-L2
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TERMINOLOGY
U3e-L2
• 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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TERMINOLOGYPower 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:
2. Alternating Current (AC) Power Source. Schematic symbol:
Realistic picture
Realistic picture
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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
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• 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 ( – the Greek letter Omega)
TERMINOLOGY
U3e-L2
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U3e-L1
• 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?
Volts(v) = Amperes(amps.) x Ohms(Ω)
Ohm’s Law
Voltage Current Resistance
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Electrical Calculations – What is Ohm’s Law?
I =
3 V2 Ω
I = 1.5 amps
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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.
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Schematic vs. Realistic Diagrams
Which one is easier to draw?
+_
Power source
Conductor
Load
Power source
Conductor
Load
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SCHEMATIC DIAGRAMSa.k.a. Schematics
(+)
(-)
POWER
SOURCE
CONDUCTOR
LOAD
CURRENT
(arrow shows the direction of electron flow)
(D.C.)
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PROBLEM SOLVING
1. Draw the diagram.
2. List the table of knowns and unknowns.
3. Write the formula.
4. Substitute.
5. Calculate.
6. Box in answer.
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U3e-L2
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 DiagramStep #2 List Information
Step #3 Formula
Step #4 Substitute
Step #5 Solve
V = 0.5v
I = 0.2amps.
R = ?
V = 0.5 V
I = 0.2amps.
R = ?
V = I x R
0.5 V = 0.2 A x R
2.5 Ω = R
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U3e-L2
What voltage is needed to produce a 4 Amp circuit with a 5 Ohm resistor?
Step #1 DiagramStep #2 List Information
Step #3 Formula
Step #4 Substitute
Step #5 Solve
V = ?
I = 4 amp
R = 5 Ohm
V = ?
I = 4 Amp
R = 5 Ohm
V = I x R
V = 4 A x 5
V = 20 Volts
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Solve the following circuit problems:
(USE ALL 6 STEPS!)
1. V = 6 v 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 resistanceof the fan motor?
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U3e-L2
1) Write the given information:V = 5 volts
I = .7 ampsR = ?
2) Sketch a schematic diagram of the circuit:
a) Start with power sourceb) Add conductorsc) Add resistance d) Add current
Problems – Ohm’s Law
V = 5V
I = .7A
-
+R = ?
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U3e-L2
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?
1) Write the given information:V = 5 volts
I = .7 ampsR = ?
Problems – Ohm’s Law
A schematic is a diagram that represents the elements of a system without using realistic pictures
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1) Write the given information:
V = 5 volts I = .7 ampsR = ?
3) Rearrange, Substitute, and Solve
Problems – Ohm’s Law
V = 5V
I = .7A
-
+R = ?
2) Sketch a schematic diagram of the circuit:
V = I x R
R = 5V / .7A
R = 7.14 W
R = V / I
U3e-L2
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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.
U3e-L2
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U3e-L1
Joule’s Law
• James Joule (1818 – 1889) – showed that heat and electrical energy are interchangeable.
• Joule’s Law:
Power = Voltage x Current
P = V I
Watts = Volts x Amperes
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U3e-L2
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?
1) Write the given information:V = 5 volts
I = .7 ampsP = ?
Problems – Joule’s Law
A schematic is a diagram that represents the elements of a system without using realistic pictures
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U3e-L2
1) Write the given information:V = 5 volts
I = .7 ampsP = ?
2) Sketch a schematic diagram of the circuit:
a) Start with power sourceb) Add conductorsc) Add resistance d) Add current
Problems – Joule’s Law
V = 5V
I = .7A
-
+P = ?
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P = V I P = 5V x .7A
P = 3.5 W
1) Write the given information:
V = 5 volts I = .7 ampsP = ?
3) Write Formula, Substitute, and Solve
Problems – Joule’s Law
V = 5V
I = .7A
-
+P = ?
2) Sketch a schematic diagram of the circuit:
U3e-L2
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Complete problem #5 on your worksheet
Problems – Joule’s Law
U3e-L2
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U3e-L2
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:V = 144 volts
P = 20,000 watts I = ?
Problems – Joule’s Law
A schematic is a diagram that represents the elements of a system without using realistic pictures
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1) Write the given information:V = 144 Volts
P = 20,000 Watts I = ?
2) Sketch a schematic diagram of the circuit:
Start with power sourceAdd conductorsAdd load Add current
Problems – Joule’s Law
V = 144V
I = ?
-
+P = 20,000 W
U3e-L2
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1) Write the given information:
V = 144 VoltsP = 20,000 Watts
I = ?
3) Rearrange, Substitute, and Solve
Problems – Joule’s Law
V = 5V
I = .7A
-
+R = ?
2) Sketch a schematic diagram of the circuit:
P = V x I
I = 20,000W / 144V
R = 138.9 A
I = P / V
U3e-L2
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U3e-L2
Complete the worksheet on electrical problems using Joule’s
and Ohm’s Laws