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Vincent Selhorst-Jones

Vincent Selhorst-Jones

Electric Current

Slide Duration:

Table of Contents

I. Motion
Math Review

16m 49s

Intro
0:00
The Metric System
0:26
Distance, Mass, Volume, and Time
0:27
Scientific Notation
1:40
Examples: 47,000,000,000 and 0.00000002
1:41
Significant Figures
3:18
Significant Figures Overview
3:19
Properties of Significant Figures
4:04
How Significant Figures Interact
7:00
Trigonometry Review
8:57
Pythagorean Theorem, sine, cosine, and tangent
8:58
Inverse Trigonometric Functions
9:48
Inverse Trigonometric Functions
9:49
Vectors
10:44
Vectors
10:45
Scalars
12:10
Scalars
12:11
Breaking a Vector into Components
13:17
Breaking a Vector into Components
13:18
Length of a Vector
13:58
Length of a Vector
13:59
Relationship Between Length, Angle, and Coordinates
14:45
One Dimensional Kinematics

26m 2s

Intro
0:00
Position
0:06
Definition and Example of Position
0:07
Distance
1:11
Definition and Example of Distance
1:12
Displacement
1:34
Definition and Example of Displacement
1:35
Comparison
2:45
Distance vs. Displacement
2:46
Notation
2:54
Notation for Location, Distance, and Displacement
2:55
Speed
3:32
Definition and Formula for Speed
3:33
Example: Speed
3:51
Velocity
4:23
Definition and Formula for Velocity
4:24
∆ - Greek: 'Delta'
5:01
∆ or 'Change In'
5:02
Acceleration
6:02
Definition and Formula for Acceleration
6:03
Example: Acceleration
6:38
Gravity
7:31
Gravity
7:32
Formulas
8:44
Kinematics Formula 1
8:45
Kinematics Formula 2
9:32
Definitional Formulas
14:00
Example 1: Speed of a Rock Being Thrown
14:12
Example 2: How Long Does It Take for the Rock to Hit the Ground?
15:37
Example 3: Acceleration of a Biker
21:09
Example 4: Velocity and Displacement of a UFO
22:43
Multi-Dimensional Kinematics

29m 59s

Intro
0:00
What's Different About Multiple Dimensions?
0:07
Scalars and Vectors
0:08
A Note on Vectors
2:12
Indicating Vectors
2:13
Position
3:03
Position
3:04
Distance and Displacement
3:35
Distance and Displacement: Definitions
3:36
Distance and Displacement: Example
4:39
Speed and Velocity
8:57
Speed and Velocity: Definition & Formulas
8:58
Speed and Velocity: Example
10:06
Speed from Velocity
12:01
Speed from Velocity
12:02
Acceleration
14:09
Acceleration
14:10
Gravity
14:26
Gravity
14:27
Formulas
15:11
Formulas with Vectors
15:12
Example 1: Average Acceleration
16:57
Example 2A: Initial Velocity
19:14
Example 2B: How Long Does It Take for the Ball to Hit the Ground?
21:35
Example 2C: Displacement
26:46
Frames of Reference

18m 36s

Intro
0:00
Fundamental Example
0:25
Fundamental Example Part 1
0:26
Fundamental Example Part 2
1:20
General Case
2:36
Particle P and Two Observers A and B
2:37
Speed of P from A's Frame of Reference
3:05
What About Acceleration?
3:22
Acceleration Shows the Change in Velocity
3:23
Acceleration when Velocity is Constant
3:48
Multi-Dimensional Case
4:35
Multi-Dimensional Case
4:36
Some Notes
5:04
Choosing the Frame of Reference
5:05
Example 1: What Velocity does the Ball have from the Frame of Reference of a Stationary Observer?
7:27
Example 2: Velocity, Speed, and Displacement
9:26
Example 3: Speed and Acceleration in the Reference Frame
12:44
Uniform Circular Motion

16m 34s

Intro
0:00
Centripetal Acceleration
1:21
Centripetal Acceleration of a Rock Being Twirled Around on a String
1:22
Looking Closer: Instantaneous Velocity and Tangential Velocity
2:35
Magnitude of Acceleration
3:55
Centripetal Acceleration Formula
5:14
You Say You Want a Revolution
6:11
What is a Revolution?
6:12
How Long Does it Take to Complete One Revolution Around the Circle?
6:51
Example 1: Centripetal Acceleration of a Rock
7:40
Example 2: Magnitude of a Car's Acceleration While Turning
9:20
Example 3: Speed of a Point on the Edge of a US Quarter
13:10
II. Force
Newton's 1st Law

12m 37s

Intro
0:00
Newton's First Law/ Law of Inertia
2:45
A Body's Velocity Remains Constant Unless Acted Upon by a Force
2:46
Mass & Inertia
4:07
Mass & Inertia
4:08
Mass & Volume
5:49
Mass & Volume
5:50
Mass & Weight
7:08
Mass & Weight
7:09
Example 1: The Speed of a Rocket
8:47
Example 2: Which of the Following Has More Inertia?
10:06
Example 3: Change in Inertia
11:51
Newton's 2nd Law: Introduction

27m 5s

Intro
0:00
Net Force
1:42
Consider a Block That is Pushed On Equally From Both Sides
1:43
What if One of the Forces was Greater Than the Other?
2:29
The Net Force is All the Forces Put Together
2:43
Newton's Second Law
3:14
Net Force = (Mass) x (Acceleration)
3:15
Units
3:48
The Units of Newton's Second Law
3:49
Free-Body Diagram
5:34
Free-Body Diagram
5:35
Special Forces: Gravity (Weight)
8:05
Force of Gravity
8:06
Special Forces: Normal Force
9:22
Normal Force
9:23
Special Forces: Tension
10:34
Tension
10:35
Example 1: Force and Acceleration
12:19
Example 2: A 5kg Block is Pushed by Five Forces
13:24
Example 3: A 10kg Block Resting On a Table is Tethered Over a Pulley to a Free-Hanging 2kg Block
16:30
Newton's 2nd Law: Multiple Dimensions

27m 47s

Intro
0:00
Newton's 2nd Law in Multiple Dimensions
0:12
Newton's 2nd Law in Multiple Dimensions
0:13
Components
0:52
Components
0:53
Example: Force in Component Form
1:02
Special Forces
2:39
Review of Special Forces: Gravity, Normal Force, and Tension
2:40
Normal Forces
3:35
Why Do We Call It the Normal Forces?
3:36
Normal Forces on a Flat Horizontal and Vertical Surface
5:00
Normal Forces on an Incline
6:05
Example 1: A 5kg Block is Pushed By a Force of 3N to the North and a Force of 4N to the East
10:22
Example 2: A 20kg Block is On an Incline of 50° With a Rope Holding It In Place
16:08
Example 3: A 10kg Block is On an Incline of 20° Attached By Rope to a Free-hanging Block of 5kg
20:50
Newton's 2nd Law: Advanced Examples

42m 5s

Intro
0:00
Block and Tackle Pulley System
0:30
A Single Pulley Lifting System
0:31
A Double Pulley Lifting System
1:32
A Quadruple Pulley Lifting System
2:59
Example 1: A Free-hanging, Massless String is Holding Up Three Objects of Unknown Mass
4:40
Example 2: An Object is Acted Upon by Three Forces
10:23
Example 3: A Chandelier is Suspended by a Cable From the Roof of an Elevator
17:13
Example 4: A 20kg Baboon Climbs a Massless Rope That is Attached to a 22kg Crate
23:46
Example 5: Two Blocks are Roped Together on Inclines of Different Angles
33:17
Newton's Third Law

16m 47s

Intro
0:00
Newton's Third Law
0:50
Newton's Third Law
0:51
Everyday Examples
1:24
Hammer Hitting a Nail
1:25
Swimming
2:08
Car Driving
2:35
Walking
3:15
Note
3:57
Newton's Third Law Sometimes Doesn't Come Into Play When Solving Problems: Reason 1
3:58
Newton's Third Law Sometimes Doesn't Come Into Play When Solving Problems: Reason 2
5:36
Example 1: What Force Does the Moon Pull on Earth?
7:04
Example 2: An Astronaut in Deep Space Throwing a Wrench
8:38
Example 3: A Woman Sitting in a Bosun's Chair that is Hanging from a Rope that Runs Over a Frictionless Pulley
12:51
Friction

50m 11s

Intro
0:00
Introduction
0:04
Our Intuition - Materials
0:30
Our Intuition - Weight
2:48
Our Intuition - Normal Force
3:45
The Normal Force and Friction
4:11
Two Scenarios: Same Object, Same Surface, Different Orientations
4:12
Friction is Not About Weight
6:36
Friction as an Equation
7:23
Summing Up Friction
7:24
Friction as an Equation
7:36
The Direction of Friction
10:33
The Direction of Friction
10:34
A Quick Example
11:16
Which Block Will Accelerate Faster?
11:17
Static vs. Kinetic
14:52
Static vs. Kinetic
14:53
Static and Kinetic Coefficient of Friction
16:31
How to Use Static Friction
17:40
How to Use Static Friction
17:41
Some Examples of μs and μk
19:51
Some Examples of μs and μk
19:52
A Remark on Wheels
22:19
A Remark on Wheels
22:20
Example 1: Calculating μs and μk
28:02
Example 2: At What Angle Does the Block Begin to Slide?
31:35
Example 3: A Block is Against a Wall, Sliding Down
36:30
Example 4: Two Blocks Sitting Atop Each Other
40:16
Force & Uniform Circular Motion

26m 45s

Intro
0:00
Centripetal Force
0:46
Equations for Centripetal Force
0:47
Centripetal Force in Action
1:26
Where Does Centripetal Force Come From?
2:39
Where Does Centripetal Force Come From?
2:40
Centrifugal Force
4:05
Centrifugal Force Part 1
4:06
Centrifugal Force Part 2
6:16
Example 1: Part A - Centripetal Force On the Car
8:12
Example 1: Part B - Maximum Speed the Car Can Take the Turn At Without Slipping
8:56
Example 2: A Bucket Full of Water is Spun Around in a Vertical Circle
15:13
Example 3: A Rock is Spun Around in a Vertical Circle
21:36
III. Energy
Work

28m 34s

Intro
0:00
Equivocation
0:05
Equivocation
0:06
Introduction to Work
0:32
Scenarios: 10kg Block on a Frictionless Table
0:33
Scenario: 2 Block of Different Masses
2:52
Work
4:12
Work and Force
4:13
Paralleled vs. Perpendicular
4:46
Work: A Formal Definition
7:33
An Alternate Formula
9:00
An Alternate Formula
9:01
Units
10:40
Unit for Work: Joule (J)
10:41
Example 1: Calculating Work of Force
11:32
Example 2: Work and the Force of Gravity
12:48
Example 3: A Moving Box & Force Pushing in the Opposite Direction
15:11
Example 4: Work and Forces with Directions
18:06
Example 5: Work and the Force of Gravity
23:16
Energy: Kinetic

39m 7s

Intro
0:00
Types of Energy
0:04
Types of Energy
0:05
Conservation of Energy
1:12
Conservation of Energy
1:13
What is Energy?
4:23
Energy
4:24
What is Work?
5:01
Work
5:02
Circular Definition, Much?
5:46
Circular Definition, Much?
5:47
Derivation of Kinetic Energy (Simplified)
7:44
Simplified Picture of Work
7:45
Consider the Following Three Formulas
8:42
Kinetic Energy Formula
11:01
Kinetic Energy Formula
11:02
Units
11:54
Units for Kinetic Energy
11:55
Conservation of Energy
13:24
Energy Cannot be Made or Destroyed, Only Transferred
13:25
Friction
15:02
How Does Friction Work?
15:03
Example 1: Velocity of a Block
15:59
Example 2: Energy Released During a Collision
18:28
Example 3: Speed of a Block
22:22
Example 4: Speed and Position of a Block
26:22
Energy: Gravitational Potential

28m 10s

Intro
0:00
Why Is It Called Potential Energy?
0:21
Why Is It Called Potential Energy?
0:22
Introduction to Gravitational Potential Energy
1:20
Consider an Object Dropped from Ever-Increasing heights
1:21
Gravitational Potential Energy
2:02
Gravitational Potential Energy: Derivation
2:03
Gravitational Potential Energy: Formulas
2:52
Gravitational Potential Energy: Notes
3:48
Conservation of Energy
5:50
Conservation of Energy and Formula
5:51
Example 1: Speed of a Falling Rock
6:31
Example 2: Energy Lost to Air Drag
10:58
Example 3: Distance of a Sliding Block
15:51
Example 4: Swinging Acrobat
21:32
Energy: Elastic Potential

44m 16s

Intro
0:00
Introduction to Elastic Potential
0:12
Elastic Object
0:13
Spring Example
1:11
Hooke's Law
3:27
Hooke's Law
3:28
Example of Hooke's Law
5:14
Elastic Potential Energy Formula
8:27
Elastic Potential Energy Formula
8:28
Conservation of Energy
10:17
Conservation of Energy
10:18
You Ain't Seen Nothin' Yet
12:12
You Ain't Seen Nothin' Yet
12:13
Example 1: Spring-Launcher
13:10
Example 2: Compressed Spring
18:34
Example 3: A Block Dangling From a Massless Spring
24:33
Example 4: Finding the Spring Constant
36:13
Power & Simple Machines

28m 54s

Intro
0:00
Introduction to Power & Simple Machines
0:06
What's the Difference Between a Go-Kart, a Family Van, and a Racecar?
0:07
Consider the Idea of Climbing a Flight of Stairs
1:13
Power
2:35
P= W / t
2:36
Alternate Formulas
2:59
Alternate Formulas
3:00
Units
4:24
Units for Power: Watt, Horsepower, and Kilowatt-hour
4:25
Block and Tackle, Redux
5:29
Block and Tackle Systems
5:30
Machines in General
9:44
Levers
9:45
Ramps
10:51
Example 1: Power of Force
12:22
Example 2: Power &Lifting a Watermelon
14:21
Example 3: Work and Instantaneous Power
16:05
Example 4: Power and Acceleration of a Race car
25:56
IV. Momentum
Center of Mass

36m 55s

Intro
0:00
Introduction to Center of Mass
0:04
Consider a Ball Tossed in the Air
0:05
Center of Mass
1:27
Definition of Center of Mass
1:28
Example of center of Mass
2:13
Center of Mass: Derivation
4:21
Center of Mass: Formula
6:44
Center of Mass: Formula, Multiple Dimensions
8:15
Center of Mass: Symmetry
9:07
Center of Mass: Non-Homogeneous
11:00
Center of Gravity
12:09
Center of Mass vs. Center of Gravity
12:10
Newton's Second Law and the Center of Mass
14:35
Newton's Second Law and the Center of Mass
14:36
Example 1: Finding The Center of Mass
16:29
Example 2: Finding The Center of Mass
18:55
Example 3: Finding The Center of Mass
21:46
Example 4: A Boy and His Mail
28:31
Linear Momentum

22m 50s

Intro
0:00
Introduction to Linear Momentum
0:04
Linear Momentum Overview
0:05
Consider the Scenarios
0:45
Linear Momentum
1:45
Definition of Linear Momentum
1:46
Impulse
3:10
Impulse
3:11
Relationship Between Impulse & Momentum
4:27
Relationship Between Impulse & Momentum
4:28
Why is It Linear Momentum?
6:55
Why is It Linear Momentum?
6:56
Example 1: Momentum of a Skateboard
8:25
Example 2: Impulse and Final Velocity
8:57
Example 3: Change in Linear Momentum and magnitude of the Impulse
13:53
Example 4: A Ball of Putty
17:07
Collisions & Linear Momentum

40m 55s

Intro
0:00
Investigating Collisions
0:45
Momentum
0:46
Center of Mass
1:26
Derivation
1:56
Extending Idea of Momentum to a System
1:57
Impulse
5:10
Conservation of Linear Momentum
6:14
Conservation of Linear Momentum
6:15
Conservation and External Forces
7:56
Conservation and External Forces
7:57
Momentum Vs. Energy
9:52
Momentum Vs. Energy
9:53
Types of Collisions
12:33
Elastic
12:34
Inelastic
12:54
Completely Inelastic
13:24
Everyday Collisions and Atomic Collisions
13:42
Example 1: Impact of Two Cars
14:07
Example 2: Billiard Balls
16:59
Example 3: Elastic Collision
23:52
Example 4: Bullet's Velocity
33:35
V. Gravity
Gravity & Orbits

34m 53s

Intro
0:00
Law of Universal Gravitation
1:39
Law of Universal Gravitation
1:40
Force of Gravity Equation
2:14
Gravitational Field
5:38
Gravitational Field Overview
5:39
Gravitational Field Equation
6:32
Orbits
9:25
Orbits
9:26
The 'Falling' Moon
12:58
The 'Falling' Moon
12:59
Example 1: Force of Gravity
17:05
Example 2: Gravitational Field on the Surface of Earth
20:35
Example 3: Orbits
23:15
Example 4: Neutron Star
28:38
VI. Waves
Intro to Waves

35m 35s

Intro
0:00
Pulse
1:00
Introduction to Pulse
1:01
Wave
1:59
Wave Overview
2:00
Wave Types
3:16
Mechanical Waves
3:17
Electromagnetic Waves
4:01
Matter or Quantum Mechanical Waves
4:43
Transverse Waves
5:12
Longitudinal Waves
6:24
Wave Characteristics
7:24
Amplitude and Wavelength
7:25
Wave Speed (v)
10:13
Period (T)
11:02
Frequency (f)
12:33
v = λf
14:51
Wave Equation
16:15
Wave Equation
16:16
Angular Wave Number
17:34
Angular Frequency
19:36
Example 1: CPU Frequency
24:35
Example 2: Speed of Light, Wavelength, and Frequency
26:11
Example 3: Spacing of Grooves
28:35
Example 4: Wave Diagram
31:21
Waves, Cont.

52m 57s

Intro
0:00
Superposition
0:38
Superposition
0:39
Interference
1:31
Interference
1:32
Visual Example: Two Positive Pulses
2:33
Visual Example: Wave
4:02
Phase of Cycle
6:25
Phase Shift
7:31
Phase Shift
7:32
Standing Waves
9:59
Introduction to Standing Waves
10:00
Visual Examples: Standing Waves, Node, and Antinode
11:27
Standing Waves and Wavelengths
15:37
Standing Waves and Resonant Frequency
19:18
Doppler Effect
20:36
When Emitter and Receiver are Still
20:37
When Emitter is Moving Towards You
22:31
When Emitter is Moving Away
24:12
Doppler Effect: Formula
25:58
Example 1: Superposed Waves
30:00
Example 2: Superposed and Fully Destructive Interference
35:57
Example 3: Standing Waves on a String
40:45
Example 4: Police Siren
43:26
Example Sounds: 800 Hz, 906.7 Hz, 715.8 Hz, and Slide 906.7 to 715.8 Hz
48:49
Sound

36m 24s

Intro
0:00
Speed of Sound
1:26
Speed of Sound
1:27
Pitch
2:44
High Pitch & Low Pitch
2:45
Normal Hearing
3:45
Infrasonic and Ultrasonic
4:02
Intensity
4:54
Intensity: I = P/A
4:55
Intensity of Sound as an Outwardly Radiating Sphere
6:32
Decibels
9:09
Human Threshold for Hearing
9:10
Decibel (dB)
10:28
Sound Level β
11:53
Loudness Examples
13:44
Loudness Examples
13:45
Beats
15:41
Beats & Frequency
15:42
Audio Examples of Beats
17:04
Sonic Boom
20:21
Sonic Boom
20:22
Example 1: Firework
23:14
Example 2: Intensity and Decibels
24:48
Example 3: Decibels
28:24
Example 4: Frequency of a Violin
34:48
Light

19m 38s

Intro
0:00
The Speed of Light
0:31
Speed of Light in a Vacuum
0:32
Unique Properties of Light
1:20
Lightspeed!
3:24
Lightyear
3:25
Medium
4:34
Light & Medium
4:35
Electromagnetic Spectrum
5:49
Electromagnetic Spectrum Overview
5:50
Electromagnetic Wave Classifications
7:05
Long Radio Waves & Radio Waves
7:06
Microwave
8:30
Infrared and Visible Spectrum
9:02
Ultraviolet, X-rays, and Gamma Rays
9:33
So Much Left to Explore
11:07
So Much Left to Explore
11:08
Example 1: How Much Distance is in a Light-year?
13:16
Example 2: Electromagnetic Wave
16:50
Example 3: Radio Station & Wavelength
17:55
VII. Thermodynamics
Fluids

42m 52s

Intro
0:00
Fluid?
0:48
What Does It Mean to be a Fluid?
0:49
Density
1:46
What is Density?
1:47
Formula for Density: ρ = m/V
2:25
Pressure
3:40
Consider Two Equal Height Cylinders of Water with Different Areas
3:41
Definition and Formula for Pressure: p = F/A
5:20
Pressure at Depth
7:02
Pressure at Depth Overview
7:03
Free Body Diagram for Pressure in a Container of Fluid
8:31
Equations for Pressure at Depth
10:29
Absolute Pressure vs. Gauge Pressure
12:31
Absolute Pressure vs. Gauge Pressure
12:32
Why Does Gauge Pressure Matter?
13:51
Depth, Not Shape or Direction
15:22
Depth, Not Shape or Direction
15:23
Depth = Height
18:27
Depth = Height
18:28
Buoyancy
19:44
Buoyancy and the Buoyant Force
19:45
Archimedes' Principle
21:09
Archimedes' Principle
21:10
Wait! What About Pressure?
22:30
Wait! What About Pressure?
22:31
Example 1: Rock & Fluid
23:47
Example 2: Pressure of Water at the Top of the Reservoir
28:01
Example 3: Wood & Fluid
31:47
Example 4: Force of Air Inside a Cylinder
36:20
Intro to Temperature & Heat

34m 6s

Intro
0:00
Absolute Zero
1:50
Absolute Zero
1:51
Kelvin
2:25
Kelvin
2:26
Heat vs. Temperature
4:21
Heat vs. Temperature
4:22
Heating Water
5:32
Heating Water
5:33
Specific Heat
7:44
Specific Heat: Q = cm(∆T)
7:45
Heat Transfer
9:20
Conduction
9:24
Convection
10:26
Radiation
11:35
Example 1: Converting Temperature
13:21
Example 2: Calories
14:54
Example 3: Thermal Energy
19:00
Example 4: Temperature When Mixture Comes to Equilibrium Part 1
20:45
Example 4: Temperature When Mixture Comes to Equilibrium Part 2
24:55
Change Due to Heat

44m 3s

Intro
0:00
Linear Expansion
1:06
Linear Expansion: ∆L = Lα(∆T)
1:07
Volume Expansion
2:34
Volume Expansion: ∆V = Vβ(∆T)
2:35
Gas Expansion
3:40
Gas Expansion
3:41
The Mole
5:43
Conceptual Example
5:44
The Mole and Avogadro's Number
7:30
Ideal Gas Law
9:22
Ideal Gas Law: pV = nRT
9:23
p = Pressure of the Gas
10:07
V = Volume of the Gas
10:34
n = Number of Moles of Gas
10:44
R = Gas Constant
10:58
T = Temperature
11:58
A Note On Water
12:21
A Note On Water
12:22
Change of Phase
15:55
Change of Phase
15:56
Change of Phase and Pressure
17:31
Phase Diagram
18:41
Heat of Transformation
20:38
Heat of Transformation: Q = Lm
20:39
Example 1: Linear Expansion
22:38
Example 2: Explore Why β = 3α
24:40
Example 3: Ideal Gas Law
31:38
Example 4: Heat of Transformation
38:03
Thermodynamics

27m 30s

Intro
0:00
First Law of Thermodynamics
1:11
First Law of Thermodynamics
1:12
Engines
2:25
Conceptual Example: Consider a Piston
2:26
Second Law of Thermodynamics
4:17
Second Law of Thermodynamics
4:18
Entropy
6:09
Definition of Entropy
6:10
Conceptual Example of Entropy: Stick of Dynamite
7:00
Order to Disorder
8:22
Order and Disorder in a System
8:23
The Poets Got It Right
10:20
The Poets Got It Right
10:21
Engines in General
11:21
Engines in General
11:22
Efficiency
12:06
Measuring the Efficiency of a System
12:07
Carnot Engine ( A Limit to Efficiency)
13:20
Carnot Engine & Maximum Possible Efficiency
13:21
Example 1: Internal Energy
15:15
Example 2: Efficiency
16:13
Example 3: Second Law of Thermodynamics
17:05
Example 4: Maximum Efficiency
20:10
VIII. Electricity
Electric Force & Charge

41m 35s

Intro
0:00
Charge
1:04
Overview of Charge
1:05
Positive and Negative Charges
1:19
A Simple Model of the Atom
2:47
Protons, Electrons, and Neutrons
2:48
Conservation of Charge
4:47
Conservation of Charge
4:48
Elementary Charge
5:41
Elementary Charge and the Unit Coulomb
5:42
Coulomb's Law
8:29
Coulomb's Law & the Electrostatic Force
8:30
Coulomb's Law Breakdown
9:30
Conductors and Insulators
11:11
Conductors
11:12
Insulators
12:31
Conduction
15:08
Conduction
15:09
Conceptual Examples
15:58
Induction
17:02
Induction Overview
17:01
Conceptual Examples
18:18
Example 1: Electroscope
20:08
Example 2: Positive, Negative, and Net Charge of Iron
22:15
Example 3: Charge and Mass
27:52
Example 4: Two Metal Spheres
31:58
Electric Fields & Potential

34m 44s

Intro
0:00
Electric Fields
0:53
Electric Fields Overview
0:54
Size of q2 (Second Charge)
1:34
Size of q1 (First Charge)
1:53
Electric Field Strength: Newtons Per Coulomb
2:55
Electric Field Lines
4:19
Electric Field Lines
4:20
Conceptual Example 1
5:17
Conceptual Example 2
6:20
Conceptual Example 3
6:59
Conceptual Example 4
7:28
Faraday Cage
8:47
Introduction to Faraday Cage
8:48
Why Does It Work?
9:33
Electric Potential Energy
11:40
Electric Potential Energy
11:41
Electric Potential
13:44
Electric Potential
13:45
Difference Between Two States
14:29
Electric Potential is Measured in Volts
15:12
Ground Voltage
16:09
Potential Differences and Reference Voltage
16:10
Ground Voltage
17:20
Electron-volt
19:17
Electron-volt
19:18
Equipotential Surfaces
20:29
Equipotential Surfaces
20:30
Equipotential Lines
21:21
Equipotential Lines
21:22
Example 1: Electric Field
22:40
Example 2: Change in Energy
24:25
Example 3: Constant Electrical Field
27:06
Example 4: Electrical Field and Change in Voltage
29:06
Example 5: Voltage and Energy
32:14
Electric Current

29m 12s

Intro
0:00
Electric Current
0:31
Electric Current
0:32
Amperes
1:27
Moving Charge
1:52
Conceptual Example: Electric Field and a Conductor
1:53
Voltage
3:26
Resistance
5:05
Given Some Voltage, How Much Current Will Flow?
5:06
Resistance: Definition and Formula
5:40
Resistivity
7:31
Resistivity
7:32
Resistance for a Uniform Object
9:31
Energy and Power
9:55
How Much Energy Does It take to Move These Charges Around?
9:56
What Do We Call Energy Per Unit Time?
11:08
Formulas to Express Electrical Power
11:53
Voltage Source
13:38
Introduction to Voltage Source
13:39
Obtaining a Voltage Source: Generator
15:15
Obtaining a Voltage Source: Battery
16:19
Speed of Electricity
17:17
Speed of Electricity
17:18
Example 1: Electric Current & Moving Charge
19:40
Example 2: Electric Current & Resistance
20:31
Example 3: Resistivity & Resistance
21:56
Example 4: Light Bulb
25:16
Electric Circuits

52m 2s

Intro
0:00
Electric Circuits
0:51
Current, Voltage, and Circuit
0:52
Resistor
5:05
Definition of Resistor
5:06
Conceptual Example: Lamps
6:18
Other Fundamental Components
7:04
Circuit Diagrams
7:23
Introduction to Circuit Diagrams
7:24
Wire
7:42
Resistor
8:20
Battery
8:45
Power Supply
9:41
Switch
10:02
Wires: Bypass and Connect
10:53
A Special Not in General
12:04
Example: Simple vs. Complex Circuit Diagram
12:45
Kirchoff's Circuit Laws
15:32
Kirchoff's Circuit Law 1: Current Law
15:33
Kirchoff's Circuit Law 1: Visual Example
16:57
Kirchoff's Circuit Law 2: Voltage Law
17:16
Kirchoff's Circuit Law 2: Visual Example
19:23
Resistors in Series
21:48
Resistors in Series
21:49
Resistors in Parallel
23:33
Resistors in Parallel
23:34
Voltmeter and Ammeter
28:35
Voltmeter
28:36
Ammeter
30:05
Direct Current vs. Alternating Current
31:24
Direct Current vs. Alternating Current
31:25
Visual Example: Voltage Graphs
33:29
Example 1: What Voltage is Read by the Voltmeter in This Diagram?
33:57
Example 2: What Current Flows Through the Ammeter When the Switch is Open?
37:42
Example 3: How Much Power is Dissipated by the Highlighted Resistor When the Switch is Open? When Closed?
41:22
Example 4: Design a Hallway Light Switch
45:14
IX. Magnetism
Magnetism

25m 47s

Intro
0:00
Magnet
1:27
Magnet Has Two Poles
1:28
Magnetic Field
1:47
Always a Dipole, Never a Monopole
2:22
Always a Dipole, Never a Monopole
2:23
Magnetic Fields and Moving Charge
4:01
Magnetic Fields and Moving Charge
4:02
Magnets on an Atomic Level
4:45
Magnets on an Atomic Level
4:46
Evenly Distributed Motions
5:45
Unevenly Distributed Motions
6:22
Current and Magnetic Fields
9:42
Current Flow and Magnetic Field
9:43
Electromagnet
11:35
Electric Motor
13:11
Electric Motor
13:12
Generator
15:38
A Changing Magnetic Field Induces a Current
15:39
Example 1: What Kind of Magnetic Pole must the Earth's Geographic North Pole Be?
19:34
Example 2: Magnetic Field and Generator/Electric Motor
20:56
Example 3: Destroying the Magnetic Properties of a Permanent Magnet
23:08
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Electric Current

  • Electric current (I) is a continuous flow of charge. It is defined as how much charge (q) passes by in an amount of time:
    I = q

    t
    .
    It uses the unit of amperes (usually shortened to amps) (A). One amp is one coulomb per second.
  • The amount of current that will flow between two points is connected to the electric potential difference between those two points. A large voltage means more current flow.
  • Some materials resist current flow more than others. We account for this with resistance (R).
    R = V

    I
    .
    Resistance is measured in ohms (Ω).
  • If we know a material's resistivity (ρ), we can find its resistance depending on its length (L) and cross-sectional area (A):
    R =ρL

    A
    .
  • Since energy is involved in moving charge around, and we have a continuous flow of charge, electric current is connected to power (energy per unit of time). There are many ways to express this relationship:
    P = IV ,        P = I2R ,       P = V2

    R
    .
  • If we want a sustained current, we need a sustained voltage source. The two main ways to create a long-term voltage source are through generators and batteries.
  • While electricity flows very quickly, each individual charge actually flows quite slowly. This is because all the charges start flowing at once as soon as the switch is flipped, like turning on a hose full of water.

Electric Current

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Electric Current 0:31
    • Electric Current
    • Amperes
  • Moving Charge 1:52
    • Conceptual Example: Electric Field and a Conductor
    • Voltage
  • Resistance 5:05
    • Given Some Voltage, How Much Current Will Flow?
    • Resistance: Definition and Formula
  • Resistivity 7:31
    • Resistivity
    • Resistance for a Uniform Object
  • Energy and Power 9:55
    • How Much Energy Does It take to Move These Charges Around?
    • What Do We Call Energy Per Unit Time?
    • Formulas to Express Electrical Power
  • Voltage Source 13:38
    • Introduction to Voltage Source
    • Obtaining a Voltage Source: Generator
    • Obtaining a Voltage Source: Battery
  • Speed of Electricity 17:17
    • Speed of Electricity
  • Example 1: Electric Current & Moving Charge 19:40
  • Example 2: Electric Current & Resistance 20:31
  • Example 3: Resistivity & Resistance 21:56
  • Example 4: Light Bulb 25:16

Transcription: Electric Current

Hi welcome back to educator.com. Today we’re going to be talking about electric current.0000

So far we’ve talked about stationary charges and it’s been a great way to build a foundation for understanding electricity.0006

We understand how the force the between one charge and another charge, how it interacts and what’s going on there.0011

The technology that we’re all using, it isn’t built around static stationary charge, it’s based on moving charge. It’s not that something’s just still, it’s flowing through the wires.0017

We call this continuous flow of charge electric current. Current, electric current we use the symbol i. It concerns itself with the idea of charge moving around.0027

Remember we’ve got all these electrons in the line and they’re all loosely connected. The outermost free electrons are loosely connected to copper, to all metals.0038

They’re able to slide around. If we’ve got a wire that’s connected to something, it’s going to have the possibility for those electrons to all slide simultaneously like water in a hose, all the water sliding simultaneously through the hose.0049

How do we tell how much charge is moving? How would we tell how much water is moving through the hose?0066

Well we have to set some point and then see how much charge passes by. If we were to see how many water molecules passed by we can see how much charge flows by.0071

I, the current is defined as the charge passed a point divided by the time it takes for that charge to pass.0081

Current is measured in amperes. Amperes? It’s French actually so I think I’m just ham fisting that word. Normally it’s shortened to amps, simple sound, amps.0090

It uses the symbol capital A. One amp is the same as one coulomb of charge per second. Which makes exact sense because it’s set as charge divided by time, so one amp is equal to 1c/1s.0100

We define the idea of current but what causes the charges to move around? One way we could look at it is electric fields. How we were looking at electric force previously.0114

If we had a conductor and an electric field, the free outermost electrons of each atom would want to move under the influence of that field.0122

Each of those electrons would see the field and it would be pushed. Remember once again, electric field goes in the motion of positive, so if we had an electron here, it would go this way.0129

In general we’re going to talk about positive charge moving, we’re going to pretend that there is this positive charge action moving along.0140

In reality what we’re seeing is the opposite to the electrons movement in the opposite direction. It’s okay, we’re able to deal with the idea of positive charge, just as the electrons in way leave a void, they have this wake behind them.0147

We imagine that the positive charge is that wake. When they’re originally working with the stuff there is no way to tell the difference until we had a better idea of the atomic model so that explains why we’ve got this sort of a little backwards.0162

We’ve got it all set by convention, is what humanity is used to doing. We keep it up just because…make things really confusing to switch over to electrons.0174

We imagine that this sort of imaginary positive charge is what’s sliding around even though the positive charge is actually in the nucleus and the nucleus is fixed in place.0184

Still it’ll work out fine. If we were to put a conductor in an electric field, those charges would get slid around because they have this force pushing on them from that electric field.0195

That might be one way to talk about current. That has a downside; the field varies as we move around. We saw this before that the field couldn’t be totally different from one place to another.0207

To know what current will be supplied by the field, we’d have to know a lot of information about the field. We’d have to know the field at every location along our conductor along with knowing stuff about the conductor.0219

If the conductor was really big and was really conductive in a certain area but later on it really small and tight and so it lowered it conductivity versus we had long length, it’s going to be not easy to do this way.0228

Electric fields isn’t the best way to do this. We don’t really want to have to do that because it’s going to take all this calculation and it’s actually going to require using calculation tools that we don’t have accessible to yet.0241

We’re going to need calculus before we can work that. We need something else. Voltage, what’s way easier to know is voltage.0250

If we know the electric potential at two points we can tell if there will be a current between them. Positive charge flows in the direction of high voltage to low voltage.0257

If we had some voltage source that creates a difference, if we’ve got some voltage source we’re going to have a current between those places.0267

If we’ve got some super positive thing over here and some negative thing here, we’re going to get current flowing between those two.0275

It’s going to depend on how conductive the material is but we’re going to have current flowing between the two.0286

The larger the electric potential the larger the difference there, we’re going to have more current flow. Electric potential difference also called voltage.0292

The more voltage we have between those two points the more voltage, the more current.0300

Given some voltage, how much current flows? That depends on how conductive the material involved is. If it’s a really good conductor it’s really easy for charge to flow, it’s not going to get in the way.0307

So given voltage will put out a lot of current, but if we have a bad conductor we’re going to get some resisting.0316

If it’s a good conductor charge flows very easily but if it’s not a good conductor it will resist some of that charge flow.0325

The less good a conductor that it is the more insulating is. The more of an insulator it is the more resistance it’s going to have. The more it will fight the current, the more it will resist the current.0331

With that idea in mind we’re going to create a new property called resistance which we denote with a capital R.0342

For a given voltage, the high resistance means less current. A low resistance means more current.0347

The more we resist it the less current will make it through. The less we resist it the more current comes through.0353

We define it as the resistance is equal to the voltage divided by the current. One way to look at it is that the current is also equal to the voltage divided by the resistance.0359

A large voltage with a small resistance is going to make a large current. But if we had a small voltage and a large resistance, if this was big and this was small, then we’re going to have the resistance being able to beat it out and we’re going to have very little current able to make it through the line.0372

We measure resistance ohms and it uses the symbol capital Omega from the Greek letters.0390

This relationship is sometimes called Ohms Law. This is actually a misnomer because it’s not always true. It doesn’t hold true for all materials at all voltages.0399

For our purposes it will be true, but an object’s resistance can vary based on the temperature, the amount of current flowing through it, and other variables.0409

It’s not a perfect law. Many materials will for some section of temperature, current and voltage will manage to have this nice linear relationship, but it’s not always true, especially at the far ends.0417

It is called Ohms Law but it’s not actually a law because it doesn’t hold all the time. Still you might hear it referred to as ohms law so I wanted to make sure heard that name for it all.0433

Just in general we’ve got resistances to find as voltage over current. If we know the resistance, we know the voltage, we’ve got the other.0442

If we know any of the two pieces of this puzzle, we know the third.0448

Resistivity, what about if we know….we talk about resistance, it allows to talk about how difficult it is to get current to flow.0453

What if we want to know an objects resistance without having to put a charge across? Without having to put that current down, without having to put that voltage down and see the current and see how much charge comes through then we’d be able to find out what the resistance is experimentally?0460

Instead if we can’t do it experimentally we can talk about its resistivity. Different materials have different conduction levels. Some conduct better, some conduct worse.0475

They’re each going to have a different resistivity, which we use rho once again another Greek letter.0488

If you’re remember…well you might remember that we talked about density using rho, we wind up using the same letter, but since these are not commonly showing up at the same time, we’ll be able to keep it straight. We don’t have to worry about accidentally have two different rhos showing up.0494

We’ve got a resistivity. We’ve going to have to use this resistivity to figure out how much resistance a type of material would have. If we’ve got a line of copper versus a line of gold versus a line of lead versus a line of graphite we’re going to wind up having different resistivities inside of that.0507

They’ll have different resistivities and thus different resistances. We also have to take the shape of the material into account. It’d be harder to push it over a long distance.0530

A 1,000 meters of copper wire, it’s hard to get a current to flow through than 1 millimeter of copper wire. It’s easier to get something to move through a small distance, it will take less force, a less voltage.0538

It’s also going to be easier if have more area to push along. A large pipe, it’s easier to have current flow without much pressure in a large water pipe.0549

If we’ve got a large area, a large cross-sectional area we’re going to be able to have more current flowing more easily. We’ll have less resistance the more area but we’ll have more resistance the longer the length.0558

All these ideas put together we get that the resistance for a uniformed object is going to be resistivity, rho, times its length divided by its cross-sectional area.0573

Rho times l over area if we know what the resistivity for a given material is. Then we can find out what its resistance is without having to just do it experimentally, without having to know the voltage and the current.0584

Energy and power. We know it takes energy to move charge around, to put a charge somewhere because we certainly have to deal with that on our electric bills and the fact that if we want our TV to turn we have to plug it in and we have to give it a source of energy.0597

How much energy does it take to get these charges to move around? We could figure that out if we had a fixed amount of charge. If we wanted to move one coulomb of charge from this place to this place, depending on some electric field in the area, it’s going to take some amount of work.0610

It’ll take some amount of energy. We aren’t dealing with that when we’re dealing with current. We’re dealing with not a fixed amount of charge; we’re talking about amount of charge per unit time.0629

If we’re dealing with current, we’ve got charge continuously flowing. It’s not like we’re picking up a sack of water and moving it to another location. We’ve got a pipe of water moving location…moving water’s location constantly.0643

We’re moving the water to a new location constantly and we’re bringing in a new sack every second or every couple of seconds effectively.0656

We can’t talk about it as one discreet chuck of energy; we have to look at it as a rate of energy.0663

What do we call a rate of energy? We have it as power. The power is the work, the energy, the change in energy divided by the time.0669

If we’re looking at current flowing we want to look at power. Since power equals work over time, and we talked about the fact that we define voltage was equal to the work divided by the charge involved.0679

That was how we defined the idea of voltage. If that’s the case, we can replace that work with qv. We can also slide that dividing by t over and now we’ll have q / t x v.0691

Well q / t was exactly how we defined current so in the end we’ve just got that the power is equal to the current times the voltage.0705

Now we know that power equals current times voltage. We also know that voltage is equal to the current times the resistance or alternately…we could rephrase this in many different ways but remember how we define resistance was resistance is equal to the voltage divided by the current.0715

Voltage is equal to the current times the resistance, all these sorts of things. We’ve got that relationship set up here.0731

If we know that we can also find some other ways to express power by trading things out.0737

First off we can drop in…since voltage equals current times resistance, we start off with our original power equation and then we replace the v with IR and so we simplify it and we get I²R.0743

The current squared times the resistance is another way to look at the power. What would be another way?0757

We could replace the other thing, instead this time we’ll replace current. Since current can be solved for up here by just dividing both sides by resistance, we get the voltage divided by the resistance.0762

We simplify that out some more and we get the voltage squared divided by the resistance is yet another way to express power.0773

At this point we’ve got three different ways to express power. If we want to know the electrical power involved, we have three different ways to find it.0780

Power is equal to the current times the voltage. Power is also equal to the current squared times the resistance. Finally power is equal to the voltage squared divided by the resistance.0787

With lots of different ways to look at energy and power. Well not really energy, power. If we want to find out what energy is, we have to see how long did we let this electric operate. How long did current flow through? How long was this power generating?0797

If we know something is some number of watts per second and multiply by 5 seconds, then we’ve got a solid amount of energy that was used in those 5 seconds.0810

Where does this voltage for a sustained current come from? If we charged up an object to a high voltage, put a lot of electric potential on it and we touched it another object, all that electric potential, the charges would go “Oh man, I want to go to a lower potential,” and they’d all slide over all at once.0820

They’d be this large current, it’d be this big zap as it popped over. It would be very brief. Once the charges rearrange themselves to equilibrium there’s no reason for them to keep going.0835

They’ve put the voltages to the same level by their sliding over. They’ve changed the involved electric fields; they’ve changed the involved potentials. Once they slide over to equilibrium they’re done.0845

Everything is back to how it started before we charged up the objects. This is exactly what happens when you get a static shock. You manage to charge something up to a very high potential, you touch it, it grounds to your body and becomes neutral once again and you experience the current flowing through, which causes your nerves to fire and you experience it as a shock.0854

That’s not a good way to make current. You don’t experience that shock continuously, it’s a brief instant and then it’s over.0875

That’s not a way to have a voltage source. We want something that’s going to be sustained; it’s going to give us a continual source of voltage, a continual source of current.0883

We want current to flow continuously, we need something that can sustain that potential difference over the long term.0891

Something that can keep up a steady pumping, not literally but if we thought about it in terms of water, once again we need something that’s able to keep pumping something up the pipe so we can have that water than go through some system where it does some sort of motion.0896

We need some steady pumping of current as opposed to one brief surge. How do we do that?0910

We get a voltage source. There are two main ways to obtain a voltage source, potential difference.0917

The first is with a generator and the second is by what we commonly call a battery. First a generator is way to convert mechanical rotational energy into electrical energy.0923

An electric motor is just a generator that operates in reverse; it takes in electrical energy and converts it into rotational energy which then through some manner of gears or something manages to normally turn it into linear momentum because rotational energy goes to the wheels if it’s an electric motor in a car.0933

Rotational energy goes to the wheels that rotational energy then through friction is applied to the ground and we get linear energy, linear kinetic energy.0949

In any case we manage to convert some sort of mechanical energy into electrical energy or vice versa for going in backwards, if we’re going backwards to get an electric motor.0957

We’ll explore why precisely this is working when we talk about magnetism but this is one just continual source. We could have a continual source of mechanical rotational energy and so we’re able to have a continual source of electric energy.0967

That’s great. Another great way to do is through a battery. A battery is not technically a battery unless it’s multiple cells. A battery is a collection of wet or dry cells where each cell has a voltage difference between its two ends.0978

If we line up a bunch of them we’re able to get that voltage difference to add up over all of them so we can get a larger voltage than just from a single cell.0994

We call multiple cells put together a battery but in common we also just tend to call them all batteries no matter what.1001

A clever application of chemistry allows us to do this, we won’t get into the why but it’s just a good use of chemistry, a great use of chemistry is chemical battery storage.1008

We can have a way to store electrical energy inside of a battery. Each cell is able to convert some of its chemical energy into electrical energy for us to use.1017

It does so at a steady voltage. It doesn’t just shove all of its energy out at once; it’s able to keep up the conversion over a slow steady thing, so we’ve got this great long term source of potential difference.1025

Speed of electricity. We’ve talked about current, about the amount of charge that flows by a point but we haven’t talked about how fast those electrons move.1037

We haven’t talked about how fast the current is flowing. It turns out that an electron is actually traveling through a conductor around a speed of, it depends on the conductor, it depends on a bunch of things.1048

Normally it’s going to be somewhere around this and in general it’s actually going to be way less than this. But it’s somewhere around 0.0001 meters per second. Keep that in mind, that’s 1 millimeter per second.1058

Or 1 centimeter per hundred seconds. That means it manages to make less than 1 ½ centimeters in 1 ½ minutes.1074

Manages to make less than a centimeter a minute, that’s a tiny, tiny amount of distance every minute. Wait what? That can’t be possible, you flip on a light switch and you know those electrons managed to move to that light like that.1080

You flip on a light switch you before you can say “Bob’s your uncle,” boom there’s light. What’s going on? How is it possible to for the electrons to moving so slowly and yet for us to be able to have effectively instantaneous for our point of view, motion of electricity through the lines?1097

The trick is that it’s not the electron in the light switch the moment you hit the switch, the trick is that it’s the entire column of electrons; it’s the entire wire of electrons moving as one.1114

They all start drifting towards the light as soon as you hit the switch on. It’s like if you had a hose full of water and turned on the water at one end, it’s not going to have to be that you wait for the water from end to get to the other end.1125

Since you’ve already got it full of water, you’ve already got the wire full of electrons just because that’s the nature of it being a metal. As soon as you flip the light switch on, the entire column starts to move at once and so it’s in the water example once again, it’d be that pressure wave.1139

How fast is that pressure wave propagating? The speed of sound in water is super-fast so as soon as you turn on the water at one end, boom; you’ve got water coming out of the other end. It’s the exact same thing with electrons.1152

We flip on the light and boom you’ve immediately got those electrons moving as one unified whole. They’re all handing their electron up to the next guy. It might be that they hand them up relatively slowly but since they’re all handing up simultaneously, the guy who’s already next to the light hands his to the light and we’ve got light the instant we flip on the switch.1163

Ready for some examples? If we’ve got a wire with a current of 0.5 amps flowing in it, how long would it take 30 coulombs of charge to move passed some point on that?1181

If we’ve got some point arbitrarily there, it doesn’t really matter. If we’ve got some wire that has a current of 0.5 amps in it, how long would it be for 30 coulombs of charge to pass that point?1191

We know we defined current to the amount of charge divided by the time. We know what the current is, the current was 0.5 amps.1201

We know how much charge we want to pass that point. We want 30 coulombs of charge. Divide by the time. So time is equal 30 / 0.5 and we’ve got 60 seconds.1211

With a 0.5 amp current it would take 60 seconds for any point to have 30 coulombs of charge pass it.1224

Second example. If we put a voltage, a potential difference of 120 volts across an object and 2 amps of current pass through it, what would be the objects resistance?1232

Here’s some object, we’ll model it as a resister and we’ll talk about circuit diagrams, but this the symbol for a resister.1241

Over here it’s at +120 volts and over it’s at 0, so negative we’d call it normally as its going plus to minus, going down.1247

What would be the objects resistance? We’ve got 2 amps flowing through. Well what was the relationship? It was voltage equals the current times resistance or many other ways to phrase it.1257

This is an easy one to remember. It takes the product of the current times the resistance. More resistance means less current for a given voltage, vice versa more current means less resistance for a given voltage.1268

We drop in 120 volts equals if we’ve got 2 amps flowing through and a resistance of R. We divide it out and we’ve got 60 ohms as our resister.1282

What if we had a slightly different case where we had that same 120 volts going through but we didn’t know the current but we did know the resistance was a 1,000 ohms.1294

We divide both sides by 1,000 and we get 0.12 and it must be in amps because it’s a current, 0.12 amps.1305

Third example. The resistivity for copper is rho of 1.69 x 10^-8 ohms times meters. Seems like a strange unit but it winds up working out to cancel precisely to being ohms at the end of that formula which is exactly what we want because it want it to be resistance.1317

The formula was resistance equals the resistivity times the length divided by the area. What resistance would 10 meters of copper wire with a diameter of 1.628 millimeters have?1333

That’d be about the size of medium duty extension cord. If you wanted to have a 10 meter extension cord and plug it in both end, it’s going to…the actual wire inside of those insulation is going to wind up being 1.628 millimeters.1348

If that’s the case we drop in the numbers we’ve got 1.69 x 10^-8 times the length, 10 meters, divided by the area, of well what’s the area?1366

Its diameter was 1.628 millimeters so diameter equals 1.628 millimeters. What’s its radius? Its radius is half of that. It’s going to be 0.814 millimeters. That’s great, but what are we working in? We’re not working in millimeters we’re working in meters.1379

We can convert that out and that’s going to be times 10^-3, since we’re already over 1 it becomes 8.14 x 10^-4 meters. We’ve got that, how do we find the area for a circle?1401

An area for a circle, πr². We toss that in down here, we’ve got π x 8.14 x 10^-4². Punch that all into a calculator and we get 0.081 ohms per 10 meters.1416

That’s actually a really small resistance. That’s a really small resistance and that’s 10 meters. That means we can cover an entire football field, we can cover an entire soccer field for only less than an ohm.1434

We haven’t really worked with the stuff much but a 1 ohm resistance is incredibly small. That’s going to allow for a massive amount of current to still flow through, that’s effectively negligible for what we’re getting here.1448

In over really, really long distance, if we were to lay a state wide, something that’s able to cross huge distance from city to city from power plant to the city it’s operating for, that’s going to start to be an issue.1459

Then that becomes more complicated and there have to be ways to step up the voltage so you have less resistance…the resistance of the line will have less of an effect in the power loss.1473

That’s more of complicated thing; we’re not going to get into that right now. It does mean for our purposes the extension cords, all the wires inside of a home are effectively no resistance.1483

If we can get that 120 volts, that 220, 240 volts, whatever voltage we manage to get into the home, we’re going to wind up having effectively no resistance inside of the home.1492

It’s going to be another issue that has to be worked out by the electric companies to be able to get the stuff there but there are clever ways to make sure that there’s not much resistance effectively in those lines, not much energy is lost to the resistance of those lines.1505

Final example. If we’ve got a voltage of 120 volts and we put it over a light bulb and that light bulb takes 100 watts of power, which is the standard incandescent light bulb, what resistance must the light bulb have?1517

Voltage equals current times resistance, but we don’t want voltage equals current times resistance. What we want know is power. What is power? Power was equal to current times voltage which is equal to current squared times resistance.1530

Which is equal to the voltage squared over the resistance. Which one of these would be the best choice to pick?1548

We know the voltage; we want to know the resistance. We know the power. This is the best one to choose. Power equals v² divided by resistance.1554

We plug in the numbers we know, 100 watts is equal to a 120² divided by a resistance of unknown, so the resistance winds up equaling 144 ohms.1564

144 ohms. So compare that 144 ohms is a fairly normal object, a light bulb. That’s resistance in a light bulb compared to what we’ve got when we’re dealing with that extension cord that we were just talking about.1580

That extension cord is practically no resistance for what it’s doing. Very, very little resistance compared to everything else that electric cord is going to wind up interacting with.1595

The current is going to not wind up noticing its resistance; it’s going to notice resistance of what it’s going to.1604

For our proposes we’ll be able to treat that electric current as if it’s one echo potential surface as if it all has no voltage drop over that wire.1610

The other half of this, if electricity costs .20 cents for kilowatt hour, a reasonable price for electricity, little high in some places, a little less than what it is in other places, how much would it cost to run that light bulb for 10 hours a day over the course of a month?1619

If its .20 cents per kilowatt hour and it’s a 100 watt bulb, then 100 watt for 1 hour, well how much energy does that wind up giving us?1634

Remember, watt is power so we have to multiply it some amount of time to turn that into an amount of energy. They sell us energy from the energy company, not power. They give us power through the lines but we’re going to buy energy.1644

100 watts for 1 hour, that’s going to wind up being 100 watt hours. If we’re going to have that run for 10 hours in a day, then that’s going to wind converting to 100 x 10, a 1,000 or 1 kilowatt hour.1659

We’ve got 1 kilowatt hour per day. If we run that for 30 days then we’ve got 30 kilowatt hours.1682

Then if we’ve got 30 kilowatt hours used over the course of our month of 30 days, reasonable length for a month. Then how much would it cost?1694

Well 30 kilowatt hours times .20 cents per kilowatt hour, we wind up getting, it costs $6.00. Running a 100 watt light bulb for the entire course of the night is a nice convenient thing, it might be useful to have a hall light on at all times.1701

That costs you $6.00 to have that convenience, something to think about. All of the lights you’ve got on, if you leave a light on all day, that’s costing real money and there’s actually some reasonable things.1725

It doesn’t turn up that much but over the course of a month or a year it totals up to something you can really care about, so it’s a good reason to keep your lights off when you aren’t using them.1735

Hope that was interesting, hope you learned a lot and we’re ready to hit electric circuits where we’ll really get the chance to start understanding something about how technology is working.1744

Alright see you at educator.com later.1751

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