Ohm’s Law
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Presentation Overview
Terms and definitions
Letters and terms used in Ohm’s Law
Ohm’s Law
Use of the Ohm’s Law circle
Applications of Ohm’s Law
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Terms and Definitions
A.
B.
C.
D.
E.
F.
G.
A.
Voltage- electrical potential; an electrical pressure created by the buildup of charge; causes charged particles to move
Volt- unit of voltage; Symbol= V
Electromotive force- a historical term used to describe voltage; Symbol= E
(No longer relevant, the definition of force is something that causes a mass to accelerate, and voltage or EMF does not fit that definition). E is now commonly used as a symbol for electric field strength.
Current- the flow or movement of electrons
Ampere- unit of current; Symbol= I
Resistance- opposition to current flow
Ohm- unit of resistance; Symbol= Ω
(Greek symbol Omega)
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Terms and Definitions (continued)
H.
I.
J.
K.
L.
Energy- the fundamental ability to do work
Joule- unit of energy; Symbol= J
Electrical Power- the rate of electrical energy used in a circuit; calculated by multiplying current times voltage, or
P = V • I
Watt- unit of measurement for power; a watt is one joule per second (J/s); Symbol= W
Ohm’s Law- a formula describing the mathematical relationship between voltage, current, and resistance; one of the most commonly used equations in all of science
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Terms and Definitions (cont)
M.
N.
Directly proportional- having a constant ratio; a situation where one variable moves in the same direction as another variable when other conditions are constant
• Example- current doubles when voltage is doubled if resistance is held constant; thus, voltage and current are directly proportional
Inversely proportional- having a constant but inverse ratio; a situation where one variable moves in the opposite direction from another variable when other conditions remain constant
• Example- with a constant voltage, current decreases when resistance increases; thus, current and resistance are inversely proportional
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Review of Letters and Terms used in Ohm’s Law
•
•
•
•
•
•
I – Electrical current in amperes
R – Resistance in ohms
V – Represents voltage in volts
A – Represents amperes
Ω – Represents ohms
E – Electromotive force (emf) in volts, sometimes used as an alternate symbol for voltage
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Ohm’s Law
• A mathematical formula typically expressed as,
(Read as “current equals voltage divided by resistance.”)
• This formula can be rearranged in two more ways: and
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The Ohm’s Law Circle
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Using The Ohm’s Law Circle
Cover the value you want to solve for
The remaining two terms give the equation
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Example One
Solve for Current
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Example One
Cover the current symbol
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Example One
Cover the current symbol
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Example One
Cover the current symbol
Current equals voltage divided by resistance
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Example Two
Solve for Voltage
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Example Two
Cover the voltage symbol
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Example Two
Cover the voltage symbol
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Example Two
Cover the voltage symbol
Voltage equals current times resistance
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Example Three
Solve for Resistance
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Example Three
Cover the resistance symbol
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Example Three
Cover the resistance symbol
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Example Three
Cover the resistance symbol
Resistance equals voltage divided by current
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Three Forms of Ohm’s Law
1. Calculating circuit resistance
Example: R = V / I
2. Calculating circuit amperage
Example: I = V / R
3. Calculating circuit voltage
Example: V = IR
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Ohm’s Law
One volt is required to push one amp through one ohm resistance.
E = IR
Volts = Amperes x Ohms
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Common Metric Values
Ohm’s Law uses results in both large and small numbers, so a review of metric prefix values is important.
Prefix
Giga
Mega kilo
centi milli micro nano
Symbol
G
M k
c m
μ n
Fraction
1,000,000,000
1,000,000
1,000
1
Exponent Decimal
1,000,000,000
1,000,000
1,000
1
0.01
0.001
0.000001
0.000000001
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Practice Problem One
You have a circuit with a source voltage of 12 V
DC and a circuit resistance of 150 Ω. Solve for the unknown value using Ohm’s Law.
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Practice Problem One
You have a circuit with a source voltage of 12 V
DC and a circuit resistance of 150 Ω. Solve for the unknown value using Ohm’s Law.
To solve, first write down all values
V = 12 v
R = 150 Ω
I = ?
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Practice Problem One
Next, look at the Ohm’s Law Circle
V = 12 v
R = 150 Ω
I = ?
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Practice Problem One
Cover what you are solving for
V = 12 v
R = 150 Ω
I = ?
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Practice Problem One
Cover what you are solving for
V = 12 v
R = 150 Ω
I = ?
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Practice Problem One
Cover what you are solving for
V = 12 v
R = 150 Ω
I = ?
I = 0.08 A
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Summary of Problem One or
I = 80 mA
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Practice Problem Two
You have a circuit with a source voltage of 9 V DC and a circuit resistance of 360 Ω
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Practice Problem Two
You have a circuit with a source voltage of 9 V DC and a circuit resistance of 360 Ω
Write down all of the values
V = 9 v
R = 360 Ω
I = ?
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Practice Problem Two
You have a circuit with a source voltage of 9 V DC and a circuit resistance of 360 Ω
Write down all of the values
V = 9 v
R = 360 Ω
I = ?
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Summary of Problem 2
V = 9 v
R = 360 Ω
I = ?
or
I = 25 mA
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Practice Problem Three
You have a circuit with a resistance of 900 Ω and a current of 30 mA
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Practice Problem Three
You have a circuit with a resistance of 900 Ω and a current of 30 mA
R = 900 Ω
I = 30 mA
V = ?
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Practice Problem Three
You have a circuit with a resistance of 900 Ω and a current of 30 mA
R = 900 Ω
I = 30 mA
V = ?
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Practice Problem Three
You have a circuit with a resistance of 900 Ω and a current of 30 mA
R = 900 Ω
I = 30 mA
V = ?
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Summary of Problem Three
R = 900 Ω
I = 30 mA
V = ?
or
V = 27 v
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Practice Problem Four
I = 59 mA
V = 19 v
R = ?
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Practice Problem Four
I = 59 mA
V = 19 v
R = ?
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Practice Problem Four
I = 59 mA
V = 19 v
R = ?
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Practice Problem Four
I = 59 mA
V = 19 v
R = ?
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Presentation Summary
Terms and definitions
Letters and terms used in Ohm’s Law
Ohm’s Law
Ohm’s law in circular expression
Use of Ohm’s Law
Let’s do some problems!
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