Download Static Electricity

Static Electricity
Physics Classroom Tutorials
Textbook Chapters 32 and 33
Objectives (ch 32)
• Describe the electric forces between charges as
attractive or repulsive
• Describe 3 ways to charge an object
• State Coulomb’s Law and solve problems using this law
• Distinguish between insulators and conductors
• Describe the special properties of superconductors and
semiconductors
• Describe the process of permanently charging an
electroscope by conduction and induction
• Explain how the process of temporary induction causes
an uncharged object may be attracted to a charged
object (ex: sticking a balloon to a blackboard)
Structure of Matter
BASIC TERMINOLOGY AND
CONCEPTS
History of the atom
Application of Atomic Structure
• All materials are composed of atoms
• An atom consists of a nucleus and a vast
region of space outside the nucleus
• The nucleus contains positive protons and
neutral neutrons
CYU
1. _____ are the charged parts of an atom
a)
b)
c)
d)
e)
f)
Only electrons
Only protons
Neutrons only
Electrons and neutrons
Electrons and protons
Protons and neutrons
Neutral vs. Charged Objects and Charge Interactions
BASIC TERMINOLOGY AND
CONCEPTS
Neutral vs Charged
• The number of electrons that surround the
nucleus will determine whether or not an
atom is electrically charged or electrically
neutral
• Protons and electrons have equal but opposite
charges
• Protons = electrons… neutral
• Protons ≠electrons… ion (more p+...cation;
more e-… anion
Charge as an imbalance
• Electrons are always on the move
• In general, for electrons to make a move from
the atoms of one material to the atoms of
another material, there must be an energy
source, a motive, and a low-resistance
pathway
• For now, it is sufficient to say that objects that
are charged contain unequal numbers of
protons and electrons
Charge Interactions
CHARGE AND CHARGE
INTERACTIONS
Charge Interactions
• Electric force is a non-contact force
• Any charged object can exert this force upon
other objects - both charged and uncharged
objects.
• Paula Abdul
– Opposites attract. And likes repel.
Paula Abdul and Newton’s 3rd
Charged and Neutral?
• Positive or negative
charged objects attract
neutral objects
Conductors and Insulators
CHARGE AND CHARGE
INTERACTIONS
Conductors
• Conductors are materials that permit
electrons to flow freely from atom to atom
and molecule to molecule.
– a charged object will always distribute its charge
until the overall repulsive forces between excess
electrons is minimized
– If a charged conductor is touched to another
object, the conductor can even transfer its charge
to that object
Insulators
• insulators are materials that impede the free
flow of electrons from atom to atom and
molecule to molecule.
– If charge is transferred to an insulator at a given
location, the excess charge will remain at the
initial location of charging
– charge is seldom distributed evenly across the
surface of an insulator
The role of Insulators
• Conductive objects are often mounted upon
insulating objects
• This prevents charge from being transferred from
the conductive object to its surroundings.
Calculating the Force Between
Charges
Coulomb’s Law
Coulomb’s Law intro
• Recall Newton's Law of Universal Gravitation:
– F = G (m1m2/d2)
– Describes that the gravitational force between
two objects is proportional to the product of the
masses and inversely proportional to the square
of the distance between them
– G is the universal gravitational constant
Coulomb’s Law
• Electrical force between any two objects
obeys a similar inverse-square relationship
with distance
• States that: the electrical force between two
charged objects is directly proportional to the
product of the quantity of charge on the
objects and inversely proportional to the
square of the separation distance between the
two objects.
Charge as a Quantity
• Charge is measurable, just like mass.
• Unit: Coulomb (C)
– 1 C is very large so microC or nanoC is typically
used
– A total charge of -1 C would need 6.25e18 e-
• The charge on a single e- is -1.6 x 10 -19 C
• The charge on a single p+ is +1.6 x 10 -19 C
Coulomb’s Law Equation
• d is distance between the
charged particles (meters)
• Q1 charge on one particle
• Q2 charge on 2nd particle
• k is proportionality
constant (Coulomb’s law
constant)
 9.0 x 109
 SI unit of charge is the
coulomb, C
The force can attract or repel.
The force is felt by both charges.
Two charges, +3 μC and -5 μC are located 15 mm apart.
Find the force exerted on each charge.
(3E  6)( 5E  6)
F  9.0 E 9
2
(0.015)
F  600 N
Does the negative sign mean attractive force or
repulsive force?
Diff in Gravity and Electrical Forces
• G of attraction between a pair of 1 kg masses
is very small, the electrical force between a
pair of 1C charges is very large.
• Greatest different is that while gravity only
attracts, electrical forces can attract or repel.
Coulomb’s Law ex) from PC
• Suppose that two point charges, each with a charge of +1.00 Coulomb are
separated by a distance of 1.00 meter. Determine the magnitude of the
electrical force of repulsion between them.
– Felect = k • Q1 • Q2 / d2
– Felect = (9.0 x 109 N•m2/C2) • (1.00 C) • (1.00 C) / (1.00 m)2
– Felect = 9.0 x 109 N
• This is a gigantic force which is why objects simply do not acquire charges
on the order of 1.00 Coulomb
• Charge is often expressed in units of microCoulomb (µC) and
nanoCoulomb (nC).
– 1 Coulomb = 106 microCoulomb
– 1 Coulomb = 109 nanoCoulomb
Coulomb’s Law Ex) from PC
• Two balloons are charged with an identical quantity and type of charge:
-6.25 nC. They are held apart at a separation distance of 61.7 cm.
Determine the magnitude of the electrical force of repulsion between
them.
– Given:
• Q1 = -6.25 nC = -6.25 x 10-9 C
• Q2 = -6.25 nC = -6.25 x 10-9 C
• d = 61.7 cm = 0.617 m
– Felect = k • Q1 • Q2 / d2
– Felect = (9.0 x 109 N•m2/C2) • (6.25 x 10-9 C) • (6.25 x 10-9 C) / (0.617 m)2
– Felect = 9.23 x 10-7 N
Coulomb’s Law Ex from PC
• Two balloons with charges of +3.37 µC and -8.21 µC attract each other
with a force of 0.0626 Newton. Determine the separation distance
between the two balloons.
– Given:
• Q1 = +3.37 µC = +3.37 x 10-6 C
• Q2 = -8.21 µC = -8.21 x 10-6 C
• Felect = -0.0626 N (use a - force value since it is repulsive)
– Solve for distance
•
•
•
•
Felect = k • Q1 • Q2 / d2
d2 • Felect = k • Q1 • Q2
d2 = k • Q1 • Q2 / Felect
d = SQRT(k • Q1 • Q2) / Felect
– Substitute givens
• d = SQRT [(9.0 x 109 N•m2/C2) • (-8.21 x 10-6 C) • (+3.37 x 10-6 C) / (-0.0626 N)]
• d = Sqrt [ +3.98 m2 ]
• d = +1.99 m
Lesson 2
METHODS OF CHARGING
Charging by Friction
• Results in a transfer of electrons between
the two objects that are rubbed together
Charging by Friction
-
+
Electron Affinity (wanting e-)
• Different materials have
different affinities for
electrons.
• Tested materials can be
ordered according to their
affinity for electrons
• Materials shown highest on
the table tend to have a
greater affinity for electrons
than those below it
– Which become more
negative?
– Which become more
positive?
Law of Conservation of Charge
• The frictional charging process (as well as any
charging process) involves a transfer of electrons
between two objects
• Charge cannot be created from nothing (just like
mass)
• net charge of the system is 0 units
• When all objects involved are considered prior to
and after a given process, the total amount of
charge amidst the objects is the same before the
process starts as it is after the process ends.
Charging By Induction
Charging by Induction
• In the two cases before, the ultimate charge
on the object is never the result of electron
movement from the charged object to the
originally neutral objects
– The balloon never transfers electrons to or receive
electrons from the spheres
Importance of a Ground
• A ground is simply a large object that serves
as an almost infinite source of electrons or
sink for electrons.
Charging by Conduction
• Involves the contact of a charged object to a
neutral object
• Van de Graaf generator
• Induction does no involve contact
• Because charging by conduction involves
contact, it is often called charging by contact
CYU 1
• A neutral metal sphere is touched by a negatively charged metal rod. As a
result, the sphere will be ____ and the metal rod will be ____. Select the
two answers in their respective order.
–
–
–
–
–
a. positively charged
b. negatively charged
c. neutral
d. much more massive
e. ... not enough information to tell
• BB
– This is a case of charging by conduction. When a charged object is used to charge a
neutral object by conduction, the previously neutral object acquires the same type of
charge as the charged object. The charge object maintains the same type of charge that
it originally had. So in this case, both objects have a negative charge.
CYU 2
• A metal sphere is electrically neutral. It is touched by a
positively charged metal rod. As a result, the metal sphere
becomes charged positively. Which of the following occur
during the process? List all that apply.
–
–
–
–
–
–
a. The metal sphere gains some protons.
b. Electrons are transferred from the sphere to the rod.
c. The metal sphere loses electrons.
d. The overall charge of the system is conserved.
e. Protons are transferred from the rod to the sphere.
f. Positive electrons are moved between the two objects.
• BCD
– In electrostatic activities, protons are never transferred (which rules
out choices a and e). Electrons are not positively charged (ruling out
choice e). Choices B, C and D are all true and explain the essential
nature of the conduction charging process
Grounding
• Grounding is the process of removing the
excess charge on an object by means of the
transfer of electrons between it and another
object of substantial size
• A ground is simply an object that serves as a
seemingly infinite reservoir of electrons
• the ground is capable of transferring electrons
to or receiving electrons from a charged
object in order to neutralize that object
Polarization
CHARGE AND CHARGE
INTERACTIONS
Polarization
• polarization is the process of separating
opposite charges within an object
– The positive charges separate from the negative
ones
• Always involves the use of a charged object to
induce the movement of electrons
Polarization is NOT Charging
• True or False:
– When an object becomes polarized, it acquires a
charge and becomes a charged object.
• False: When an object becomes polarized, its center of
positive charge becomes separated from its center of
negative charge (protons still equal electrons)
– There is a separation of charge, not an imbalance
of charge
Lesson 4 (Chapter 33)
ELECTRIC FIELDS
objectives
• Explain how electric field lines (also called
electric lines of force) can be used to
represent the strength and direction of an
electric field
• Explain how an object can gain or lose electric
potential energy
• Distinguish between electric potential (volts)
and electric potential energy (Joules)
• Describe the function of a capacitor
Force Field
• The altered space around an object
– Examples
• Gravity
• Magnetic fields
• Electric fields
• Action at a Distance
– Example- you can “sense” the force field that
surrounds a charged Van de Graaf generator
• Electric field has a magnitude and a distance
Electric Field Lines
• Just as gravity keeps a satellite orbiting a
planet, the electric field keeps an electron
“orbiting” a proton
• The force one electric charge exerts on
another is the interaction between one charge
and the electric field set up by the other
Electric Fields cont.
• Since there is a magnitude and direction, it’s a
vector quantity.
• Magnitude measured by its effect on charges
in the field
– A positive “test charge” is placed in the field
– Where the force is greatest, the field is strongest
• Direction of the field at any point, is the
direction of the force on a small positive test
charge
Field Lines cont
• Vectors point toward negative, away for
positive
– Always point in the direction of the force that
would act on a positive test charge
• Magnitude is indicated by length of the
vectors
Field Lines (lines of force) cont
• Where the lines are farther apart, the field is
weaker
• For isolated charge, the lines extend to infinity
• For two or more opposite charges, the lines go
from a positive charge and end on a negative
charge
Oppositely charged plates
Equal and opposite
Equal like charges
Electric Shielding
• Charges arrange themselves to ensure a zero
field within a conducting material.
• Faraday cage
Electric Potential Energy (p.523)
• Recall that work is done when a force moves
something in the direction…
• A charged object can have potential energy by
its location in an electric field
– Work is required to push a charged particle
against the electric field of a charged body
– EPE is increased when work is done to push it
against the electric field of something else that is
charged
Electric PE cont.
• Like pushing down on a spring, which requires
work (repulsion)
• Electric Potential Energy- the energy the
charge now possesses by virtue of its location
Electric Potential (33.5)
• Electric potential energy per charge- the total
electric potential energy divided by the
amount of charge
• At any location the PE per charge- whatever
the amount of charge- will be the same
Electric potential =
electric potential energy
charge
• Unit is the volt (V); 1 V = 1 joule/ coulomb
Voltage
• 1 V equals 1 joule of energy per coulomb of
charge; 1000V = 1000 J per coulomb
• EPE sometimes call voltage
– We can speak about the voltages at different locations
in an electric field whether or not any charges occupy
those locations
• Rub a balloon on your hair, the amt of energy
there is very small- about a thousandth of a J
(figure 33.13)
• A high voltage requires great energy only if a
great amt of charge is involved
Electrical Energy Storage
• Electrical energy can be stored in capacitors
• Uses
– Used as on-off switches on computer
motherboards
– Under each key on a keyboard
– Found in a camera’s flash
Basic capacitor (Fig 33.14)
• Two plates separated by a short distance (not
touching) are connected to a charging device
and charge is transferred from one plate to
another
– Positive battery terminal pulls e- from plate
connected to it
– These e- pumped through battery through
negative terminal to the opposite plate
– The plates now have equal and opposite charges,
done charging when potential difference between
plates equals the battery voltage
Capacitors
Charged Capacitors
• A charged capacitor is discharged when a
conducting path is provided between the
plates
– Can be quite shocking
– Can be fatal if voltage is high enough
• The energy stored comes from the work
required to charge it
• The energy is in the form of the electric field
between the plates