Download Electrical - Ohmn`s Law

Contextual Lesson Planning Guide
Title: Ohm’s Law
Time Required: Three 30 to 40 minute sessions
Lesson Submitted By: Vivian Humphrey
[email protected]
Lesson Contextualized Toward: Career Pathway: Electrical
Content Standards Benchmarks Taught: M1.2.1, M2.2.1, M2.2.2, M2.2.3, M5.3.2, M5.4.9a,SC 3.8.2
Digital Literacy Skills Taught: T1.1.6, T6.2.4
Objectives/Learning Goals:
Interpret simple circuit diagrams using symbols., Apply Ohm’s law and the power equation o simple
DC circuits: V=IR , Learn how circuits work, Learn how Current Voltage and Resistance interact,
Explain simple series circuits in terms of Ohm’s Law (Challenge parallel series), Review solving simple
equations, Be able to express equations in another form by solving for different variables, Explain
meaning of a variable in context of a problem, Encourage interaction in a team setting.
Rationale:
Ohm’s Law and how circuits function is in the basic curriculum of any occupation or career involved
with electrical circuitry. Ohm’s Law would be in general preparatory classes for secondary education.
Knowing about basic circuits could help prevent or understand overloads. One might even be able to
repair a flashlight.
Materials/Resources Needed:
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Smart board
Computer connectable projection equipment or other means of providing a place for students to
demonstrate math solutions or diagrams.
Computers
Handouts that students can use in class, take home, and make notes for study
Procedure/Instructional Outline:
Prerequisite: Solve simple equations with one variable
Session 1
Background:
Formulas are relationships that can be used over and over again by changing the values that are put
into them. As we have seen the letters represent the same quantity--not the same value--all of the
time. To solve the formula, you substitute numbers in for the letters. Care must be taken to substitute
the correct value in for the correct letter. Care must also be taken to have the correct units with the
values that are substituted.
We have used formulas to find areas and volumes. There are many different formulas that you will
have to work with in the heating and air conditioning field. We will look at formulas that look at
electrical value.
Current is produced by electrons traveling from one point to another point. It is the flow of electric
charge. The unit measure for electrical current is amperes.
The flow of electrons is dependent upon the voltage of the system and the resistance. Resistance is
the opposition to the flow of electric charge. It is measured in ohms. The voltage is the force applied to
cause the electrons to flow through the resistance. It I measured in volts.
The understanding of circuits is basic to any career or course of study involving electricity i. e. Heating
and air, construction, music production, IT sight management, license for electrician . . .
Introduction
Handout 1
See TeacherView session one and have “Circuit construction kit” at http://phet.colorado.edu/
downloaded on class computers or laptops.
Give a general overview of flow of current.
Watch https://www.youtube.com/watch?v=J4Vq-xHqUo8 There is space on Handout 1 for notes
students record. Have discussion as guided on the handout. Students reflect, in number 5, on what
they now know. Have a voluntary board secretary to list the classes knowledge obtained from the
video on circuits.
Students go by ones or pairs.to the circuit construction Kit site on the computers to experiment with
constructing circuits. Have them take note of what works and what doesn’t work at first try.
Session 2
Have Visual Teacher View session 2 so whole class can see it.
Review equations by solving for different variables to find a new formula. Use something familiar
maybe A=LW. Solve for L. Solve for W.
Have laptops for students to go to https://phet.colorado.edu/en/simulation/ohms-law and change
values of voltage and resistance to see how it effects current. After a little time for individual
experience, you may have small groups discuss their conclusions to be shared with the class.
Challenge the groups to figure out three ways Ohm’s formula might be expressed.
If some need more challenge, point out that the current (I) is directly proportional to the force applied
to produce the current (V) and indirectly, or inversely, proportional to the resistance (R) of the flow of
electrons. This means that the higher the voltage, the higher the current, and the smaller the
resistance the higher the current. Did the simulation show this? (Direct and Inverse Functions)
Ohmn’s Law
Show that the formula for Ohmn’s law is
(V is also sometimes used for voltage)
Voltage (E) equals current (I) times resistance (R)
Note: E was first used because the voltage was identified as sudden energy. Why wasn’t V used?
Label paper plates E, I, and R, X. Put students in groups. Have 2 sets of these plates for each group.
The groups are to manipulate the formula to solve for currant and resistance. On one side of the room
tape a plate on the wall that says Current and other side a plate that says Resistance. Asks groups to
display simultaneously to tape up their plates on the respective walls to display their formulas. Have
groups discuss their formulas until all are in agreement.
Distribute Handout 2
It would be good to have extra sheet of paper so that only one illustration at a time is showing. If you
have the equipment show the first illustration.
Work out the currant for the circuit at the top of page HD2-2.
If this goes well have student groups review the finding of the currant and voltage in the last examples
and plan their explanation as a short news clip. Video the clips with a smart phone.
Email the clips to be opened on a computer and if available to be viewed on a Smart Board.
At the end of class have all students check for understanding. Can they write the formula for Ohm’s
Law in at least one form, and can they explain the relationship of the three factors?
Session 3
Start by asking to think about what they know about Ohm’s laws and circuits. Show clips from last
session on the smart board.
As review have class to quickly do 1 and 2 on Handout 3. If they know it, show it! Discuss results and
give those not finished a helping hand.
Do Before Handout 4
Put summary section below in your own words or see if the class can come up with a good description
on how circuits work.
Ohm's law describes the way current flows through a resistance when a different electric current
(voltage) is enforce at both ends of the resistance. One way to think of this is as water flowing through
a pipe. The voltage is the water pressure, the current is the amount of water flowing through the pipe,
and the resistance is the size of the pipe or some blockage in the pipe. More water will flow through
the pipe (current) the more pressure is applied (voltage) and the less blockage free and/or bigger the
pipe is (lower the resistance).
Do these example problems with the class
1. If the resistance of an electrical circuit is increased, what will happen to the current assuming the
voltage remains the same?
Answer: The current will decrease.
2. If the voltage across a resistance is doubled, what will happen to the current?
Answer: The current will double as well. Explanation: If you look at the equation V= IR, if R stays the
same then if you multiple V*2 (double the voltage), you must also double the current for the equation
to remain true.
3. What is the voltage V in the circuit shown? Have a visual for the students to view.
,
V = I R V = 2 x 13, V = 28
Give out Handout 4. There is a student copy and a teacher copy with answers.
Go over the four steps to solve a problem. Illustrate the steps as you solve number 1.
Act as a mute scribe, and have the class led you through the four steps to solve number 2.
Groups continue to follow instruction three on handout 3 to do handout 4. If a group finishes they can
come back to number 4 on handout 3. If a group doesn’t take the time to reread and follow the
instructions for doing handout 4, don’t clue them. They get to do them for homework. Occasionally
call on a group or someone within a group to put up a worked exercise for all to see. If a person
doesn’t want to come up, remind them they have a group to help. Ask the other groups if they agree
or disagree. Ask if anyone solved it differently.
Homework: Students are to bring to class all unfinished odd, numbered exercises. Everyone is to do
the remaining even numbered exercises (which should be even numbers 4 to 16) for individual
practice. Next class, check and review. Challenge students to look for circuits and how they work, for
example a flashlight. Tell them, “Caution: Stay away from High Voltage.”
Teacher View Session 1
Circuit Construction Kit (AC+DE) (3.20)
Download at http://phet.colorado.edu Circuit Construction kit (AC+DC) (3.20)
Current, Voltage, Resistance
1.
Handout 1
HD2
http://www.allaboutcircuits.com/textbook/direct-current/chpt-2/voltage-current-resistance-relate/
Notice the symbols and units of measurement.
2. Notes for https://www.youtube.com/watch?v=J4Vq-xHqUo8 Use a separate sheet if needed.
3. Discussion: What did you see?
Unexpected:
Expected:
New Information:
Need More information:
4. Assigned pairs get a laptop and make circuits with the Circuit Construction Kit.
(Note: Instructors pre download http://phet.colorado.edu Circuit Construction kit (AC+DC) (3.20 on
laptops)
Current, Voltage, Resistance
Handout 1-2
5. What do we know about Current, Voltage, Resistance? Divide students into 3 groups each
assigned one of the preceding key terms. Divide the white board into three sections. Students line up
and upon a signal write all they know about their team’s term in a 5minute period. Team members
can help each other if they blank. After time is up each group presents the information in their section.
The class reviews each section’s information and illustrations and may make additional data.
6. Use https://phet.colorado.edu/en/simulation/ohms-law . Record the given settings. See what
happens when to the Current (I) when you change the Resistance (R). Go back to original settings
and see what happens when you change the Voltage (V).
Reflections:
Interview students to reveal their experience with a basic circuit when current or voltage change.
Teacher View Session 2
Ohm’s Law Simulation
https://phet.colorado.edu/en/simulation/ohms-law
Ohm’s Law
Handout 2
The first, and perhaps most important, relationship between current, voltage, and resistance is called
Ohm’s Law, discovered by Georg Simon Ohm and published in his 1827 paper, The Galvanic Circuit
Investigated Mathematically. Ohm’s principal discovery was that the amount of electric current through a
metal conductor in a circuit is directly proportional to the voltage impressed across it, for any given
temperature. Ohm expressed his discovery in the form of a simple equation, describing how voltage,
current, and resistance interrelate:
Note: E or V May be used as the voltage variable in equations. If you understand
that V is used both for the variable (Voltage) in the equation and for the measurement symbol
(Volt), you may change the “E”s to “V”s in the exercises.
In this algebraic expression, voltage (E) is equal to current (I) multiplied by resistance (R). Using algebra
techniques, we can manipulate this equation into two variations, solving for I and for R, respectively:
Let’s see how these equations might work to help us analyze simple circuits:
In the above circuit, there is only one source of voltage (the battery, on the left) and only one source of
resistance to current (the lamp, on the right). This makes it very easy to apply Ohm’s Law. If we know the
values of any two of the three quantities (voltage, current, and resistance) in this circuit, we can use Ohm’s
Law to determine the third.
HD2-2
In this first example, we will calculate the amount of current (I) in a circuit, given values of voltage (E) and
resistance (R):
What is the amount of current (I) in this circuit?
HD2-3
In this second example, we will calculate the amount of resistance (R) in a circuit, given values of voltage
(E) and current (I):
What is the amount of resistance (R) offered by the lamp?
In the last example, we will calculate the amount of voltage supplied by a battery, given values of current
(I) and resistance (R):
What is the amount of voltage provided by the battery?
Ohm’s Law is a very simple and useful tool for analyzing electric circuits. It is used so often in the study of
electricity and electronics that it needs to be committed to memory by the serious student.
http://www.allaboutcircuits.com/textbook/direct-current/chpt-2/voltage-current-resistance-relate/
Ohm’s Law
Handout 3
1. Review: What three basic components are needed for a simple circuit? What is Ohm’s law?
2. Draw two circuits that will not work for different reasons.
3. Get in assigned groups and do the odd number exercises on Handout 4. All students should have a personal
copy of solutions and be able to explain the solutions. Help each other to this end.
4. If you have a chance find out what other careers might use Ohm’s Law . See if you can find out why
R, E, V, or I where used for variables in the formula. Post your findings on the back cork board.
Ohm’s Law Handout 4-1
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http://www.mcffa.com/uploads/4/4/8/0/4480777/ohms_law_worksheet.pdf
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Show all four steps for each problem
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Step 1 – Record Info
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Step 2 – Write Equation
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Step 3 – Substitute in the Equation
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Step 4 – Solve
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Ohm’s Law and Power Equation Practice Worksheet
1. Find the current through a 12-ohm resistive circuit when 24 volts is applied.
2. Find the resistance of a circuit that draws 0.06 amperes with 12 volts applied.
3. Find the applied voltage of a circuit that draws 0.2 amperes through a 4800-ohm resistance.
4. Find the applied voltage of a telephone circuit that draws 0.017amperes through a resistance of
15,000 ohms.
5. A 20-volt relay has a coil resistance of 200 ohms. How much current does it draw?
6. A series circuit has 1200-ohms of total resistance with 12 V as the power supply. What is the total
current of this circuit?
7. What is the increase of current when 15 V is applied to 10000-ohm. rheostat, which is adjusted to
1000-ohm value?
8. A transformer is connected to 120 volts. Find the current if the resistance is 480-ohms?
9. A resistive load of 600-ohms is connected to a 24 V power supply. Find the current through the
resistor.
10. A circuit consists of a 12 V battery connected across a single resistor. If the current in the circuit
is 3 A, calculate the size of the resistor.
11. If a small appliance is rated at a current of 10 amps and a voltage of 120 volts, the power rating
would be ______ Watts. (P = I V )
12. If a blender is plugged into a 110 V outlet that supplies 2.7 A of current, what amount of power is
used by the blender?
13. If a clock expends 2 W of power from a 1.5 V battery, what amount of current is supplying the
clock?
14. Tommy runs his juicer every morning. The juicer uses 90 W of Power and the current supplied is
4.5 A. How many volts are necessary to run the juicer?
15. Amanda’s hair dryer requires 11A of current from a 110 V outlet. How much power does it use?
16. A DC electric motor transforms 1.50 kW of electrical power into mechanical form. If the motor's
operating voltage is 300 volts, how much current does it "draw" when operating at full load (full
power output)
Ohm’s Law Handout 4-2
Challenge
Calculate the amount of power dissipated by this electric heating element, if the generator's output
voltage is 110 volts and the heater's resistance is 2.5 ohms: ( P = V / R )
Now, calculate the power dissipated by the same heater if the generator's output voltage is doubled.
Source: http://www.mcffa.com/uploads/4/4/8/0/4480777/ohms_law_worksheet.pdf
Ohm’s Law and Power Equation Practice Worksheet Answers
Teacher-Ohm’s Handout 4-1
1. I=E/R = 24/12= 2 amperes
2. R=E.I = 12/.06 = 200 ohms
3. E = IR = (0.2)(4800) = 960 volts
4. E=IR = (.017)(15000) = 255 volts
5. I=0.5Aor45mA
6. I= 0.01A or 10mA
7. I= 0.0135 A or 13.5 mA
8. I=0.25Aor250mA
9. I=0.04Aor40mA
10. R=4^
11. P=IV=(10)(120v)=1200 W
12. P=IV=(2.7)(110v)= 297 W
13. P=IV;I=P/V=2W/1.5V= 1.3A
14. P=IV; V=P/I= 90W/4.5A= 20V
15. P=IV=(11 A)(110V)=1210 W
16. P=IV;I=P/V=1500W/300V= 5A
Challenge Answer Key
T-Ohm’s Handout 4-2
Challenge 110 v
P=V /R= (110v) /2.5ohms=4840Watts \
Doubled voltage—220v P=V /R= (220v) /2.5ohms = 19360Watts wow!
Sources in Order of Use:
http://phet.colorado.edu/
https://www.youtube.com/watch?v=J4Vq-xHqUo8
https://phet.colorado.edu/en/simulation/ohms-law
http://phet.colorado.edu/ Circuit Construction kit (AC+DC) (3.20)
http://www.allaboutcircuits.com/textbook/direct-current/chpt-2/voltage-current-resistance-relate/
http://www.mcffa.com/uploads/4/4/8/0/4480777/ohms_law_worksheet.pdf
http://www.uoguelph.ca/~antoon/gadgets/resistors/resistor.htm