Transcript Powerpoint

Lecture
Presentation
Chapter 20
Electric Fields
and Forces
© 2015 Pearson Education, Inc.
Suggested Videos for Chapter 20
• Prelecture Videos
• Charges and Forces
• Coulomb’s Law
• Electric Fields
• Class Videos
• Charges and Forces:
Demonstrations
• Charges and Forces:
Warm-Ups
• Electric Field
© 2015 Pearson Education, Inc.
• Video Tutor Solutions
• Electric Fields and Forces
• Video Tutor Demos
• Charged Rod and
Aluminum Can
• Electroscope in
Conducting Shell
Slide 20-2
Suggested Simulations for Chapter 20
• ActivPhysics
• 11.1–11.6, 11.9, 11.10
© 2015 Pearson Education, Inc.
• PhETs
• John Travoltage
• Balloons and Static
Electricity
• Conductivity
• Charges and Fields
• Electric Field Hockey
• Microwaves
• Optical Tweezers and
Applications
• Electric Field of Dreams
Slide 20-3
Chapter 20 Preview
Looking Back: Vectors and Components
• In Sections 3.1–3.3, you studied
how a vector could be resolved into
its component vectors.
• Electric forces and electric fields
are vectors, so you will need to use
vector components to solve electric
force and field problems.
• You learned that a vector can be
represented as the sum of its
component vectors and .
© 2015 Pearson Education, Inc.
Slide 20-4
Chapter 20 Preview
Stop to Think
The tension in the rope is 100 N. Given that sin 30° = 0.50
and cos 30° = 0.87, the x- and y-components of the tension
are
A.
B.
C.
D.
E.
–87 N, 50 N
87 N, 50 N
–50 N, 87 N
50 N, –87 N
87 N, –50 N
© 2015 Pearson Education, Inc.
Slide 20-5
General Force Model
Newton 0th Law
Objects are dumb - They have no memory of the past and cannot predict the
future. Objects only know what is acting directly on them right now
Newton's 1st Law
An object that is at rest will remain at rest and an object that is moving will continue
to move in a straight line with constant speed, if and only if the sum of the forces
acting on that object is zero.
ax = 0 m/s2 IFF Sum Fx = 0 N
(IFF => if and only if)
Newton's 3rd Law
Recall that a force is an interaction between two objects. If object A exerts a force
on object B then object B exerts a force on object that is in the opposite direction,
equal in magnitude, and of the same type.
Visualizations:
• Force Diagrams
• System Schema
Slide 20-6
Net Force Model
Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the
object
ax = 1/m * (Sum Fx)
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20-74-19
Lists types of forces you used in Physics 1
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Section 20.1 Charges and Forces
© 2015 Pearson Education, Inc.
Experimenting with Charges
• The major tools in a modest laboratory for studying
electricity include:
• A number of plastic and glass rods, each several inches
long. These can be held in your hand or suspended by
threads from a support.
• Pieces of wool and silk
• Small metal spheres, an inch or two in diameter, on
wood stands
© 2015 Pearson Education, Inc.
Slide 20-10
X-force Demonstrations
• Balloon and wall/ceiling
• Rubbing the rod
• Rod and Paper
• Rod and Rod
• Attraction
• Repulsion
• Comb and Hair
• Transfer of Charge
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Slide 20-11
X-force Demonstrations
• What is the nature of this force?
• Is it familiar or a new type of force?
• How does magnitude of x-force compare to gravity?
• Do magnets have x-property?
• How do we tell if an object is charged?
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Slide 20-12
Results of Sticky Tape Warm-up: Part 1
Top Tape
Bottom Tape
Paper
Foil
Top Tape
Repulsion
Attraction
Attraction
Attraction
Bottom Tape
Attraction
Repulsion
Attraction
Attraction
Paper
Foil
No interaction No Interaction
No Interaction
Table of interactions between Top Tape, Bottom Tape, Foil strips, and Paper
strips
• Also, the effect is stronger if there is more charge and the interaction, i.e. the
force gets weaker as distance between the objects increases.
• The interaction between foil and the charged tapes was stronger than the
interaction of paper with the charged tapes.
Slide 20-13
Define property and type of force
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Summarize what we know about
Electric Forces and Charge
Write ideas on Board
Identify
• Which are correct?
• Which are incorrect?
• Which are duplicates?
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Historical models for electric charge
• Fluid models of electric charge
• Too much electric fluid in an object makes it positively
charged and too little makes it negatively charged.
• Particle model of electric charge
• Electric charge is carried by microscopic particles.
Removing or adding these particles to an object changes the
charge of the object.
• How could we test these models?
© 2014 Pearson Education, Inc.
Slide 20-16
Contemporary model for electric charge
• Two objects start as neutral—
the total electric charge of
each is zero.
• During rubbing, one object
gains electrons and becomes
negatively charged.
• The other object loses an
equal number of electrons and
with this deficiency of
electrons becomes positively
charged.
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Slide 20-17
Charge Conservation
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Slide 20-18
Charges, Atoms, and Molecules
• An atom has a dense, positively
charged nucleus, containing
positively charged protons and
neutral neutrons.
• The nucleus is surrounded by
the much-less-massive orbiting
negatively charged electrons
that form an electron cloud.
• Charge, like mass, is an
inherent property of electrons
and protons.
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Slide 20-19
An Atomic View of Charging
• Electrons and protons are the basic charges in ordinary
matter.
• There are no other sources of charge.
• An object is charged if it has an unequal number of
protons and electrons.
• Most macroscopic objects have an equal number of
protons and electrons. Such objects are electrically
neutral.
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Slide 20-20
An Atomic View of Charging
• Objects gain a positive charge not by gaining protons, but
by losing electrons.
• Protons are extremely tightly bound within the nucleus, but
electrons are bound much more loosely.
• Rubbing causes charge separation – takes energy to
separate charges
© 2015 Pearson Education, Inc.
Slide 20-21
Conceptual Exercise 14.2
• You pull your sweater and shirt off together, and then pull them
apart. You notice that they attract each other—a phenomenon
called "static" in everyday life. Explain the mechanism behind
this attraction and suggest an experiment to test your
explanation.
© 2014 Pearson Education, Inc.
Slide 20-22
How do charged objects and neutral objects interact?
Part A
A small ball with zero net charge is positively charged on one side, and equally
negatively charged on the other side. The ball is placed near a positive point
charge as shown.
• Would the ball be attracted toward, repelled from, or unaffected by the
positive point charge? Explain.
• Is your answer consistent with what you observed in the tape activity?
Explain.
Through careful observations of physical phenomenon, scientists develop
models or mental pictures to account for what is observed. These scientific
models can also be used to predict physical behavior. From observations of
electrical we can develop a model for electric charge.
Slide 20-23
How do charged objects and neutral objects interact?
Part B
Use your model for electric charge to account for the electrical attraction between
a charged tape and an uncharged metal ball. As part of your answer, draw a
sketch of the charge distribution on the tape and ball both before and after they
are brought near one another.
Slide 20-24
Charge Model:
• Electric forces can be attractive or repulsive
•
Objects with the same sign of charge repel each other
•
Objects with the opposite sign of charge attract each other
•
Neutral objects are polarized by charged objects which creates attractive
forces between them
• There are two kinds of charges, positive (protons) and negative
(electrons). In solids, electrons are charge carriers (protons are
2000 time more massive).
• A charged object has a deficit of electrons (+) or a surplus of
electrons (-). Neutral objects have equal numbers of + and –
charges
• Fe gets weaker with distance: Fe α 1/r2
• Fe between charged tapes are > Fe between charged tapes &
neutral objects
• Rubbing causes some objects to be charged by charge
separation
• Charge can be transferred by contact
Slide 20-25
Charging objects with insulators and conductors
• Demonstrations
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Polarization model
• Insulator and Conductor simulation
http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm
•
Colorado Phet
http://phet.colorado.edu/simulations/index.php?cat=Electricity_Magnets_
and_Circuits
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Charge Diagrams:
Polarization, Grounding, and Charging
• Insulator and Conductor simulation
http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm
•
Grounding Simulation
•
Shocked by the door – John Travoltage
•
Grounding a Positively charged-electroscope
•
Grounding a negatively charged-electroscopes
•
Balloons and Charge
•
http://phet.colorado.edu/en/simulation/balloons
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Triboelectric series => Triboelectric Effect
When two of the following materials are rubbed together under ordinary circumstances, the
top listed material becomes positively charged and the lower listed material becomes
negatively charged.
MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk
paper cotton wood acrylic cellophane tape polystyrene polyethylene
rubber balloon saran wrap MORE NEGATIVE
Slide 20-29
A Question of Charge (Warm-up for next class)
•
If I give you an object, how can you tell if it is positively or negatively
charged? (assume a top tape is + charged.)
•
Suppose a newspaper article claims that scientists in California have
found a third type of charged object.
What experiments would you perform to check their claim?
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