Transcript Electrostatics PP

Bellringer
• How do you find the
area of a triangle?
• This was the most
missed question on
the test…
DO
STOP
WORK
Objectives
• Investigate the majesty of electricity so you can
soon control lightning like Zeus.
• Also so you can solve Regents questions…
Updates
• Midterm next week (Feb 3rd)
• Tests
• Create your own labs
• Power Labs
Notes
• Remember that the second marking period ends
this Friday
• So if you have any missing labs, or homework
assignments you need to turn them in ASAP
otherwise they will be zeros forever.
Congratulations!
• You’ve made it through mechanics!!
• Now onto the cool stuff!
Electrostatics
Electrostatics
• Electrostatics is the study of electric charges that
can be collected and held in one place.
• Electrostatics is the study of static electricity!
Electrostatics
• The effects of electrostatics are observable over a
vast range of scales.
What is an “Electrical” charge?
• The electrical charge of an object refers to the
amount of extra electrons it does or doesn’t have.
• There are two types of electrical charges.
▫ Positive (+)
▫ Negative(-)
• If an object does not have an electrical charge it is
said to be neutral, or have zero charge.
Electric Charges
• Atoms only care about the electrons in their filled
shells.
• Atoms can lose or gain extra electrons causing the
atom to no longer be neutral
▫ Extra electrons = negative charge
▫ Deficit of electrons = positive
• Protons don’t move!!!!
Where do electric charges come from?
• Electrical charges come from any kind of
transfer of electrons.
• Balloon Demo
▫ How does the balloon get its charge?
▫ What can it do with a charge?
Can electric charges produce a force?
• Based on the balloon activity what do you think?
• Yes they can! It is called the “Electrostatic force”
• What variables do you think effect it?
The Electrostatic Force
• 𝐹𝑒 =
𝑘𝑞1 𝑞2
𝑟2
• 𝐹𝑒 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐹𝑜𝑟𝑐𝑒 (Newtons)
• k= electrostatic constant
𝑁𝑚2
( 2 )
𝐶
• q = charge (Coulombs C)
• r = distance between centers (Meters m)
The Electrostatic Force
• Does this force always attract like gravity?
• How could we find out?
• Electroscope/Wimshurst Demo
Bellringer 3 QUESTIONS!!!!!
1. Opposite charges
______.
2. Like charges _____.
3. Can protons flow
like electrons?
DO
STOP
WORK
Objectives
• Continue to investigate the wonders of electricity
• Solve some problems with your new found
knowledge
The Electrostatic Force
• The electrostatic force can be both attractive and
repulsive.
• Like charges repel
• Opposite charges attract
Electrostatic Vocab
• Conductor: A material that allows charges to
move about easily
▫ Usually metals
• Insulator: A material through which a charge
will not move easily.
▫ Usually plastics
Wimshurst Machine Demos
• Wimshurst Machine Demos
▫ Sparks, plates, bowl, ring
• Defibulators
▫ About 1,000 volts sent across your heart muscle
Where do sparks come from?
• Sparks are a continuous discharge of electrons.
• Is air a conductor or insulator?
▫ Air is generally considered an insulator, but under
certain conditions it can become a conductor
(sparks fly)
Where do the electrons go on a
charged object?
• Electrons are all negatively charged so they repel
each other.
• So when an object is holding a charge it is evenly
distributed throughout it
Van Der Graaf Generator
• Demos
Bellringer 3 QUESTIONS!!!!!
1. Opposite charges
______.
2. Like charges _____.
3. Can protons flow
like electrons?
DO
STOP
WORK
Objectives
• Continue to investigate the wonders of electricity
• Solve some problems with your new found
knowledge
Reminders
• 80 and below = SLC
• Physics Club today after school
• Your Midterm is a week from today. STUDY UP!
Conservation of Charge
• The charge of a system must be conserved
• A system can only gain or lose electrons by
interacting with another system.
Ways to charge
1. Charging by Conduction: Charging a neutral
object by touching that object with a charged
object.
2. Charging by Induction: Charging a neutral
object by bringing a charged object near it.
Charging By Conduction
• Many of the demos we’ve done already.
• Electroscope
Charging by Induction
• Electroscope Demo
Induction Charging People Demo
Conductive vs. Inductive
• Conductive
▫ Needs contact
▫ The object that gets charged is the same charge as
the charger
• Inductive
▫ No contact
▫ The object that gets charged is the opposite charge
as the charger
Checkpoint
1. What are two differences between charging by
conduction vs induction?
Example
• If a positively charged rod conductively charges
an electroscope. What charge will the
electroscope have?
• Positive
Example
• What charge will the
leafs of the electroscope
have after it is charged
inductively?
• Positive
The Coulomb
• What is a “Coulomb”?
• It is the standard unit for electrical charge (C)
• Named after Charles-Augustin de Coulomb, a
French physicist who developed Coulomb’s Law
The Coulomb
• One Coulomb is 6.25x10^18 elementary charges
• One elementary charge is the charge of one
proton, or one electron
▫ One elementary charge (e) is 1.60x10^-19C
The Coulomb
• Static electric shocks from doorknobs, socks, etc.
are typically only a few microcoulombs
• Lightning bolts are around 15C
• The amount of charge that travels through a AA
battery is about 5,000C
Coulomb Questions
•
•
•
•
•
Is it possible to have a charge of 2.8x10^-19C?
Nope
What about 3.2x10^-19C?
Yup
How much charge is gained when something
gains 4 electrons?
• -6.4x10^-19C
The Elementary Charge
• The elementary charge is the charge of a single
electron
• It was discovered by American physicist Robert
Andrews Millikan in his famous Millikan oil
drop experiment in 1909.
▫ His measurement has been updated
Millikan Oil Drop
• He sprayed drops of
oil between to
differently charged
plates and balanced
the downward
gravitational force
with the upward
electrical force.
Coulomb’s Law
• 𝐹𝑒 =
𝑘𝑞1 𝑞2
𝑟2
• 𝐹𝑒 = 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐹𝑜𝑟𝑐𝑒 (Newtons N)
• k= electrostatic constant
𝑁𝑚2
( 2 )
𝐶
• q = charge (Coulombs C)
• r = distance between centers (Meters m)
Example
• What is the electrostatic force between two
objects that both have a charge of +2.0x10^-4C
and are 2.0m apart?
• 𝐹𝑒 =
• 𝐹𝑒 =
𝑘𝑞1 𝑞2
𝑟2
(8.99𝑥109 𝑁𝑚𝑚/𝐶𝐶)(2.0𝑥10−4 𝐶)(2.0𝑥10−4 𝐶)
• 𝐹𝑒 =89.9N
(2.0𝑚)2
You try
• What is the electrostatic force between a
+2.0x10^-4C point and a -4.0x10^-4C point that
are 4.0m apart?
• 𝐹𝑒 =
• 𝐹𝑒 =
𝑘𝑞1 𝑞2
𝑟2
(8.99𝑥109 𝑁𝑚𝑚/𝐶𝐶)(2.0𝑥10−4 𝐶)(−4.0𝑥10−4 𝐶)
• 𝐹𝑒 = −4.495x10^1N
(4.0𝑚)2
Bellringer
• What is the
electrostatic force
between an electron
and a proton that are
5.26x10^-11m apart?
• -8.32x10^-8N
DO
STOP
WORK
Objectives
• Solve some problems with your new found
knowledge of the universe
• Learn what force field is and how to use it
Updates
• Below an 80 = SLC
• Only a few days left in this quarter
▫ #checkyogrades
• Midterm next Tuesday
▫ All Regents Questions
Example Problem
• Page 559
Practice
• Page 560 of the textbook, questions: 9-14
Answers
9. 1.6x10^4N attractive
10. 3.0x10^-6 C
11. The force diagram is reflected about the y-axis
with respect to the diagram shown in Example
Problem 1. Magnitudes of all forces remain the
same. The direction changes 42 degrees above
the negative x-axis, or 138 degrees
counterclockwise from the positive x-axis
Answers
12. The electrostatic force between the two charges
decreases by a factor of 9 (3 squared)
13. 0.068 N towards the right
14. 3.1 N toward the right
Simulation
• https://phet.colorado.edu/en/simulation/balloo
ns
Homework
• Page 561 15-23
▫ Will be collected.
▫ You may need to read the previous
chapter to answer the questions.
Bellringer – Take out your HW
• How many more
electrons are there
than protons in a -1.5
Coulomb charge?
• 9.38x10^18 electrons
DO
STOP
WORK
Objectives
• Go over homework
• Learn what force field is and how to use it
Updates
• Below an 80 = SLC
• Only a two days left in this quarter
▫ #checkyogrades
• Midterm next Tuesday
▫ All Regents Questions
Homework
• Page 561 15-23
▫ Will be collected.
▫ You may need to read the previous
chapter to answer the questions.
Simulation
• https://phet.colorado.edu/en/simulation/balloo
ns
Force Fields
• How would you define a “force field”?
• Have you ever been in a force field before?
• Can gravity be a force field?
Gravitational Field
• Draw a few vectors around the Earth to show
which way Earth’s Gravitational Field pulls at an
given point.
Electric Field
• Just like the gravitational force, the electrostatic
force produces a force field too.
• An Electric Field is a property of the space
around a charged object that exerts forces on
other charged objects.
Electric Field
• Since the electrostatic force can both attract and
repel the field lines are not always pointed in
Electric Field
• Those vectors are called “Electric Field Lines”
• An “Electric Field Line” indicates the direction of the
force due to the electric field on a positive test
charge
• They started using positive “test” charges before
they realized that only electrons (negative) move.
Checkpoint
1. What is an “electric field”?
2. What is an “electric field line”?
3. Why do they use positive test charges if
protons don’t flow?
• Try to visualize what you think the electric field lines
will look like between a positive and a negative point
charge.
• Note: You can never cross electric field lines
• What about the electric field lines between two
positive charges?
• What about the electric field lines between two
negative charges?
This is like the two oppositely charged plates with
the pieces of foil demo.
Electric Field Lines
• Electric Field Simulation
▫ https://phet.colorado.edu/en/simulation/charges
-and-fields
• Electric Field Demonstration
▫ http://www.youtube.com/watch?v=7vnmL85378
4
Bellringer – Take out your HW
• How many more
electrons are there
than protons in a -1.5
Coulomb charge?
• 9.38x10^18 electrons
DO
STOP
WORK
Objectives
• Go over homework
• Learn what force field is and how to use it
Updates
• Below an 80 = SLC
• Only a two days left in this quarter
▫ #checkyogrades
• Midterm next Tuesday
▫ All Regents Questions
Electric Field Strength
• The strength of every electric field is not the
same. Just like every gravitational field is not
the same.
• It depends on the electrostatic force and the
charge of the object in the field.
Electric Field Strength
•𝐸=
𝐹𝑒
𝑞
• E is the electric field strength in N/C
• F is the electrostatic force in N
• q is the charge of the object in the field in C
• 𝐸=
𝐹𝑒
𝑞
=
𝑘𝑞1 𝑞2
𝑟2
𝑞2
=
𝑘𝑞1
𝑟2
where 𝑞1 is the charge of the object
producing the field
Example
• Suppose that you are measuring an electric field
using a positive test charge of 3.0x10^-6C. This
test charge experiences a force of 0.12N. What is
the magnitude of the electric field strength at the
location of the test charge?
Example Solution
•𝐸=
𝐹𝑒
𝑞
•𝐸=
0.12𝑁
3.0𝑥10−6 𝐶
4𝑁
4.0𝑥10
𝐶
•𝐸=
You Try
• What is the electric field acting on a positive test
charge of 5.0x10^-5C, if this test charge
experiences a force of 0.25N?
•𝐸=
𝐹𝑒
𝑞
•𝐸=
0.25𝑁
5.0𝑥10−5 𝐶
𝑁
5
0.05𝑥10
𝐶
•𝐸=
Bellringer
• Draw at least 6
electric field lines
between these three
charges.
DO
STOP
WORK
Objectives
• Solve some problems with your new found
knowledge of the universe
• Learn what force field is and how to use it
Reminder
• All missing labs and homework assignments are
due tomorrow
• If you don’t hand them in by then they will be
zeros forever
▫ *Zeros bring your average down big time.
Example
• What is the magnitude of the electric field at a point
that is 0.30m to the right of a small sphere with a
net charge of -4.0x10^-6C?
•𝐸=
•𝐸=
•𝐸=
𝑘𝑞
𝑟2
9.00𝑥109 𝑁𝑚2 /𝐶 2 (−4.0𝑥10−6 𝐶)
(0.30𝑚)2
𝑁
𝑁
−4.0𝑥105 𝑜𝑟 4.0𝑥105
𝐶
𝐶
𝑡𝑜 𝑡𝑜𝑤𝑎𝑟𝑑 𝑡ℎ𝑒 𝑠𝑝ℎ𝑒𝑟𝑒
Practice
• Page 572-573, numbers 1, 2, 4, 5, 6, 8, 9, 10
Answers
1.
2.
4.
5.
6.
40 N/C
3.0x10^6 N/C
3.0x10^5 N/C , 0.65 N , 3.0x10^-6 C
8.1x10^-6 N south
1.6x10^4 N/C toward q
Answers
8. 2.6x10^4 N/C
9. 6.5x10^3 N/C
10. 2.5x10^4 N/C east
Homework
• Page 590 Numbers 50, 51, 53a,
53b, 53d, 54, 55, 56
Bellringer
• What is the charge of
a particle that is in an
electric field of 35N/C
and is being acted on
by a 20N force?
•𝐸=
𝐹
𝑞
• 0.57C
DO
STOP
WORK
Objectives
• Go over midterm
Updates
• SLC today! 79 or below on the Energy Test
• Midterm…
▫ 63.8% average
▫ 7.1 points behind last year
Midterm Analysis
• Must be completed by everyone.
• Corrections
▫ Show all work for every question you lost any
credit on.
• Due Friday the 13th of February 2015
Work done on a charge
• You know from mechanics that the work done
on an object is W=Fd
• The same relationship is used to find the work
done to move a charge
• 𝑊𝑞 = 𝐹𝑑
Electric Potential Difference
• Small difference…
• The work done on a charge is expressed as work
done per unit charge and it is called the “electric
potential difference”
• Often just called the potential difference
Electric Potential Difference
• Electric potential difference is the work needed
to move a positive test charge from one point to
another, divided by the magnitude of the test
charge.
• You can also think of electric potential difference
as the change in electric potential energy per
unit charge.
Electric Potential Difference
•𝑉=
𝑊
𝑞
• V is potential difference which is measured in Volts (V).
1 volt = 1 joule per Coulomb
• W is the work done to a charge measured in Joules
• q is the charge of the object being worked on measured
in C
Positive Electric Potential Difference
A
B
PE
Work
PE
A positive force is required to move a positive charge a distance away
from a negative charge, so positive work is done.
Negative Electric Potential Difference
A
B
PE
Work
PE
A negative force is required to move a positive charge a distance
toward a negative charge, so negative work is done.
Checkpoint
1. What are the units of potential difference?
2. Do you do negative or positive work to
separate a positive and negative charge?
Example
• Two large plates have a potential difference of 100
volts. How much work is needed to move 1 Coulomb
of negative charge from the positive plate to the
negative plate?
•𝑉=
𝑊
𝑞
𝑊
1𝐶
• 100𝑉 =
• W=100J
You Try
• Two large plates have a potential difference of 200
volts. How much work is needed to move 4 Coulombs
of negative charge from the positive plate to the
negative plate?
•𝑉=
𝑊
𝑞
𝑊
4𝐶
• 200𝑉 =
• W=800J
Parallel Plates – Uniform Electric Field
Electric Potential Difference in a
Uniform Electric Field
•𝑉=
𝑊
𝑞
=
𝐹𝑑
𝑞
=
𝐹
𝑑
𝑞
= 𝐸𝑑
• V is the potential difference (V)
• E is the electric field intensity of a uniform field
• d is the distance between the two plates
• This equation only works in a uniform electric field
▫ Like one found between two parallel plates oppositely
charged plates.
Example
• What is the electric field between two parallel
plates that have a potential difference of 200
volts and are 0.20m apart?
• V=Ed
• 200V=E(0.20m)
• E=1,000N/C
Practice
• The electric field intensity between two large
charged parallel metal plates in 6,000N/C. The
plates are 0.05m apart. What is the potential
difference between them?
• 300V
• A voltmeter reads 400V across two charged, parallel
plates that are 0.020m apart. What is the
magnitude of the electric field between them?
• 20,000N/C
Homework
• Electrostatics multiple choice practice
homework numbers 1 through 25
Bellringer
• What is the electrostatic force
between a +2.5 C charge and a
-3.0 C charge that are 0.2m
apart?
• 𝐹𝑒 =
𝑘𝑞1 𝑞2
𝑟2
• 𝐹𝑒 = −1.69𝑥1012 𝑁
• Why is this force so large?
DO
STOP
WORK
Objectives
• Scantrons
• Finish notes on electrostatics
• Practice everything we’ve learned about
electrostatics so far
Bellringer
• Draw the electric field lines for the charges (you
have 2 mins)
• Take out your HW
Objectives
• Review last night’s homework
• Review the midterm and a way to get points
back…
Electronvolt
• An electronvolt is a unit of energy commonly used in
electrostatics (unit eV)
• One electronvolt is 1.6x10^-19 joules
• It is the amount of energy gained or lost by the
charge of a single electron moved across an electric
potential difference of one volt.
Electronvolt: the super unit!
Measurement
Unit
Energy
eV
Mass
eV/c^2
Momentum
eV/c
Temperature
eV/k(b)
Time
h(bar)/eV
Distance
h(bar)c/eV
Practice
• What is the work done, in eV, on 1 electron that
is moved across a potential difference of 9 volts?
•𝑉=
𝑊
𝑞
𝑠𝑜
9𝑉 =
𝑊
−1.60𝑥10−19 𝐶
• 𝑊 = −1.44𝑥10−18 𝐽𝑜𝑢𝑙𝑒𝑠
•𝑊=
−1.44𝑥10−18 𝐽
• 𝑊 = −9𝑒𝑉
∗
1𝑒𝑉
1.60𝑥10−19 𝐽
You Try
• What is the work done, in eV, on 2 electrons that are
moved across a potential difference of 200 volts?
•𝑉=
𝑊
𝑞
𝑠𝑜
200𝑉 =
𝑊
−3.20𝑥10−19 𝐶
• 𝑊 = −6.00𝑥10−17 𝐽𝑜𝑢𝑙𝑒𝑠
•𝑊=
−6.00𝑥10−1 𝐽
• 𝑊 = −400𝑒𝑉
1𝑒𝑉
∗
1.60𝑥10−19 𝐽
Shortcut
• Just multiple the number of electrons by the
voltage!
▫ (Electronvolt)
• How much energy is gained by moving 3
electrons across 12 volts?
• 36eV
Why use electronvolts?
• When you are just working with a single
electron, or a beam of electrons, it makes math
way easier by using eVs.
• This way the voltage and the energy are the same
number!
Electron Gun
http://www.mrwaynesclass.com/electro/reading/
index06.html
Electron Beam
• Demo
• Old TV’s
Particle Accelerator
• http://www.youtube.com/watch?v=b5Q5rFf-aZI
Homework
• Electrostatics multiple choice practice
homework numbers 1 through 25
Bellringer
• Draw the electric field lines for the charges (you
have 2 mins)
• Take out your HW
Objectives
• Review last night’s homework
• Review the midterm and a way to get points
back…
Updates
• Skipping SLC
• Midterm analysis due Friday the 13th
• Electrostatic test next week (Wednesday?)
Homework
• https://www.youtube.com/watch?v=JsVZwc1dOo
• Any questions?
Classwork
• Questions 26-43 (15mins)
• Go over Midterm Part 2
• Homework (Finish Electrostatics MC Questions)
Bellringer
• What is the energy (in
electronvolts) gained or lost
when one electron is moved
across a 1 volt potential
difference?
• 1eV
DO
STOP
WORK
Objectives
• Review last weekend’s homework
• Practice part twos for the Electrostatics Test
Updates
• SLC anytime this week
• Test on tomorrow or Thursday
• Midterm analysis due Friday
Homework
• Homework Questions
• https://www.youtube.com/watch?v=JsVZwc1dOo
• Practice Problem
Electrostatics Test
• Wednesday or Thursday?
• 25 multiple choice questions
• 6 written response
• See me if you have questions
Homework
• Any questions?
• Tonight’s Homework:
1. Study for your electrostatics test
2. Work on/complete your test corrections
Video
• http://www.youtube.com/watch?v=VhWQr1LYXY
Midterm
• Let’s go over it…
Bellringer
• Is it dangerous to ride on a train that is traveling
through a lightning storm?
• Well it depends on the conductor!
Objective
• Review for tomorrow’s test
Electrostatics Test
• 25 multiple choice
• 14 long answer parts (6 questions)
Extra Credit Assignment
• Go to
http://www.nsf.gov/news/special_reports/olym
pics/ and watch one of the videos
• Write a summary of a the video or write a
summary of physics in a different winter
Olympic sport
Midterm Corrections
• Remember they are due this Thursday before I
leave the building.
Sample Part Two Questions