Transcript chapter22 - galileo.harvard.edu

Electrical Charges and the Electrical Interaction
What do you know about electrical charges
and their behavior?
What do you know about electrical charges at
the molecular, atomic, and subatomic level?
Electrical Charges and the Electrical Interaction
Charges in nature
nucleon level and larger
sub-nucleon level
questions raised by electrical understanding of atoms
What are charged particles?
They are part of atoms.
The nucleus contains protons (positively charged).
Electrons (negatively charged) orbit the nucleus and
sometimes can hop off and move around freely.
Most materials are neutral.
To give a material net charge, electrons must be
added or removed.
Charge is a conserved quantity.
Electrical Charges and the Electrical Interaction
Charges as the bearers of the electrical interaction
one kind of charge, but positive and negative
Like charges repel each other and opposite charges attract each other.
Scotch tape lab
Four pieces of tape
• Pull two pieces off of table –
bring near each other
• Pull two pieces off of other tape
– bring near each other
• Pull one piece off of table,
another off of a piece of tape,
bring near each other
Demonstrate likes repel,
opposites attract?
http://www.exo.net/~pauld/summer_institute/summer_day14electrostatic/ta
pe_electroscope.html
X-rays from Scotch tape
W hat are some of the differences between gravitational forces and
electrical forces?
1.
Gravitational forces attract, while electrical forces can attract or
repel.
2. Gravitational forces depend on the masses of the interacting
objects, while electrical forces depend on the charges of the
interacting objects.
Similarity: Both decrease as the interacting objects move further
apart.
W hat are some of the differences between gravitational forces and
electrical forces?
1.
Gravitational forces attract, while electrical forces can attract or
repel.
2. Gravitational forces depend on the masses of the interacting
objects, while electrical forces depend on the charges of the
interacting objects.
Similarity: Both decrease as the interacting objects move further
apart.
Electrical Force
(Coulombs Law)
Gravitational Force
Do assignment 5 worksheet on Coulomb’s law
Van de Graff demo
The dielectric breakdown strength of dry air, at Standard
Temperature and Pressure (STP), between spherical electrodes is
approximately 33 kV/cm
The Ability of Materials to Transport Charge
Insulators and conductors
great range of conductivity
Superconductors
Semiconductors
Iron man
Processes of Charging
By friction
By contact
By induction
Processes of Charging
By friction
By contact
By induction
http://phet.colorado.edu/en/simulation/travoltage
Processes of Charging
By friction
By contact
By induction
Electroscope lab
Processes of Charging
By friction
By contact
By induction
If the metal sphere is grounded while the wand induces a charge, the
electrons will flow away to ground and leave the sphere with a net
positive charge.
Electric Field
An electric charge creates an electric field in the space surrounding it. A
second charge placed in this field will experience an electrical force.
Electric field = force per unit charge
The direction of the field lines represents the direction of the force on a
positive test charge. They never cross.
The Electric Field
• The concept of a field compared to action at a distance
• Representation of the electric field by lines of force
What would the electric field lines look like for a charged conducting sphere?
What would the electric field lines look like for a charged conducting plate?
http://web.ncf.ca/ch865/englishdescr/3DEFldIdentCharges.html
http://phet.colorado.edu/en/simulation/electric-hockey
Electric Potential
Definition
distinction from electric potential energy
analogy to gravitational potential
E is force per unit charge
V is Electric potential energy per unit charge
Parallel plate capacitor
Charge separation
An atom can become electrically
polarized if a nearby charge
causes its electron cloud to
become distorted.
Charge separation in atoms can be a
widescale effect.
Review of terms
The electric potential is defined as the electric potential energyper
unit charge.
electric potential 
electric potential energy
charge
The electric field is defined as the electric force per unit charge.
electric field 
electric force
charge
Conceptual questions
1) Why do clothes often cling together after tumbling in a clothes dryer?
2) At some automobile toll-collecting stations, a thin metal wire sticks up from
the road and makes contact with cars before they reach the toll collector.
What is the purpose of this wire?
3) An electroscope is a simple device consisting of a metal ball that is attached
by a conductor to two thin leaves of metal foil protected from air
disturbances in a jar, as shown. When the ball is touched by a charged
body, the leaves that normally hang straight down spread apart. Why?
(Electroscopes are useful not only as charge detectors, but also for
measuring the quantity of charge: the more charge transferred to the ball,
the more the leaves diverge.)
4) Is it necessary for a charged body to actually touch the ball of the
electroscope for the leaves to diverge?
Conceptual questions
5) The five thousand billion, billion freely moving electrons in a penny repel one
another. Why don't they fly out of the penny?
6) How does the magnitude of electric force compare between a pair of
charged particles when they are brought to half their original distance of
separation? To one-quarter their original distance? To four times their
original distance? (What law guides your answers?)
7) The proportionality constant k in Coulomb's Law is huge in ordinary units,
whereas the proportionality constant G in Newton's law of gravitation is tiny.
What does this indicate about the relative strengths of these two forces?
8) If you rub an inflated balloon against your hair and place it against a door, by
what mechanism does it stick? Explain.
Conceptual questions
9) How can a charged atom (an ion) attract a neutral atom?
10) If you place a free electron and a free proton in the same electric field, how
will the forces acting on them compare? Their accelerations? Their
directions of travel?
11) Suppose that a metal file cabinet is charged. How will the charge
concentration at the corners of the cabinet compare with the charge
concentration on the flat parts of the cabinet?