Electronics

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

– 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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One Way To Visualize Ohm’s Law

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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